Showing posts with label Cisco CCNA: Local Area Networking. Show all posts
Showing posts with label Cisco CCNA: Local Area Networking. Show all posts

Saturday, March 24, 2012

Cisco CCNA: Using A Router

Cisco CCNA: Using A Router


Unit 1. Routing



In this course, you learn how routers determine which path a packet will take. You will see that there are many different types of routing protocols and learn the advantages and disadvantages of each. You also learn the parts of a router.
In this unit, you learn the difference between a routed and a routing protocol. You also learn about static, default, and dynamic routes. You see the advantages and disadvantages of using each type of route. In addition, you learn how metrics are used in path determination.

After completing this unit, you should be able to:
  • Discuss the routing process

  • Explain the difference between routed and routing protocols

  • Describe how multiprotocol routing works

  • List the differences between a static and a dynamic route

  • Define a default route

  • Name three metrics used in path determination


This unit provides information that is relevant to the following CCNA exam objectives:
  • List the key internetworking functions for the OSI Network layer


Topic 1.1: The Routing Process

*Delivering Packets
To deliver a packet from one network to another, routers use two basic functions:
  • Path determination

  • Packet switching


*Path Determination
Path determination is the means by which all possible routes to a chosen destination are evaluated, and a single route is chosen.

*Routers in Action
Routers operate at the Network layer of the OSI reference model. Using the network address, routers forward a packet to the correct network on the path. Once the packet arrives at the correct network, the router's function is complete.

*Packet Switching
Packet switching is the process of receiving a packet on one port and forwarding the packet to another port.

*Switching between Interfaces
Routers can receive a packet on one interface and send it out on a different one. The packet switching function allows the router to switch packets between interfaces, while the path determination function determines the best pathway for a packet to travel on the network.

Topic 1.2: Packet Switching

*Examining Address Information
When a router receives a packet, it strips off the Data Link frame header and examines the network address information to determine if it knows how to forward the packet.

*Forwarding the Packet
If the router knows how to forward the packet, the packet is re-encapsulated with a Data Link header for the chosen interface and forwarded to its next stop.

*Dropping the Packet
If the router does not know how to forward the packet, the packet is dropped.

*Moving between Routers
Thus, as packets move between routers, they are decapsulated and re-encapsulated at each stop.

Question 1

Topic 1.3: Routed vs. Routing

*Protocol Distinctions
Routing protocols and routed protocols each provide distinct services, though the terms are often confused. Packets are handled by routed protocols, while routing information is exchanged between routers using routing protocols.

*Routed Protocols
Routers use routed protocols to deliver packets from one network to another. These protocols define the network address fields of a packet. IP and IPX are examples of routed protocols.

*Routing Protocols
Routing protocols, on the other hand, allow routing information to be shared between routers. Routing information is stored in routing tables which contain information about available routes. IGRP and OSPF are examples of routing protocols.

*Routing Tables
Routing protocols allow routing information in the form of routing table updates to be exchanged between routers. Routing Information Protocol (RIP), Interior Gateway Routing Protocol (IGRP), Enhanced IGRP, Open Shortest Path First (OSPF), and NetWare Link Services Protocol (NLSP) are routing protocols that work with TCP/IP.

*Multiprotocol Routing
Multiple routed protocols and routing tables can be supported by a single router. Each protocol has no knowledge of the existence or operations of the other protocols. This type of separated multiprotocol routing is called "ships-in-the-night" routing.

Question 2

Question 3

Topic 1.4: Static Routes

*Types of Routes
As routers exchange network information and forward packets of user data, they use three basic types of routes: static, default, and dynamic.

*Static Routes
Static routes are routes that are defined manually by the network administrator. The problem with static routes is that each time the topology of the network changes, the router's information must be updated manually.

*Advantages of Static Routes
While static routes require manual input and updates, there are advantages to using this type of routing system. The first is security. Since updates are completed manually, network information is not automatically shared with other routers to keep this information private.

*Other Uses for Static Routing
Another instance where static routing is a good choice is when you have a stub network which is only accessible by a single path. Static routing is also a good choice for dial-on-demand networks, since the links are not up all of the time.

*Disadvantages of Static Routing
Static routing has its disadvantages as well. If a link goes down between Router A and Router B, communication between these routers is impossible until Router A's information about the downed link is manually updated, even if there is another viable path to Router B.

*Not for Large Networks
Since static routing does not allow flexibility in path determination and requires a large time investment to update routing tables, it is not a good solution for large or complex networks.

Question 4

Topic 1.5: Default Routes

*Default Routes
Default routes are routes used by a router when it does not know how to forward the packet. For example, if Router A receives a packet that it does not know how to forward, it sends it along its default route to Router B. If Router B doesn't know how to forward the packet, it sends it to its default route, and so on.

*Moving between Routers
As the packet is moved between routers along default routes, it will eventually arrive at a router that knows how to forward it. Default routes keep routers from having to know the topology of all other networks, which is impossible.

*Using Default Routes
Default routes should be used when a router has specific internetwork knowledge, but doesn't need to maintain specific knowledge of other networks. Default routes can be used with either static routes or dynamic routes, which you will learn about later.

Question 5

Question 6

Topic 1.6: Dynamic Routes

*Dynamic Routing
Static routing requires manual updates to the routing table, however dynamic routing allows route information to be automatically updated and shared between routers.

*Complex Networks
Because network topology information is automatically shared between routers, network information is not kept private. However, dynamic routing is a good choice for complex networks or when the network administrator doesn't have time to statically configure the routing tables.

*Advantage of Dynamic Routing
The primary advantage of using dynamic routing is its flexibility. In this case, if the link between Router A and Router B goes down and there is another viable path to Router B, Router A will automatically re-route packets to the viable path.

*Basic Router Functions
Two basic router functions allow for the success of dynamic routing. They are the router's maintenance of the routing table and the timely distribution of routing updates to other routers.

*Routing Protocols
Dynamic routing relies on routing protocols, which are configured to control how routing updates are conveyed, including the type of information exchanged, the frequency of the exchange, and how to find the recipients.


* Exercise 1
Try working with static and dynamic routing systems.


Examine the following table
Step Action
1 Draw a square.
2 At each corner of the square place a router.
3 Label each router A, B, C, or D.
4 Assume that you are using a static routing system. According to the routing table, Router A's best path to Router B is a direct one.
What happens if the connection between Router A and Router B fails?
What actions will have to be taken to solve this problem?
5 Now assume that you are using a dynamic routing system. According to the routing table, Router A's best path to Router B is a direct one.
What happens if the connection between Router A and Router B fails?
What actions will have to be taken to solve this problem?
6 Now that you have examined the difference between static and dynamic routing systems, list two instances where static routing should be used.
7 List two instances where dynamic routing should be used.
8 Describe how default routes work.
Can default routes be used in conjunction with static and dynamic routing?
Why are default routes used?


Topic 1.7: Metrics

*Metrics and Routing Algorithms
Both static and dynamic routing use routing tables to determine which path is best for a packet. However, since dynamic routing automatically updates routing tables, routing algorithms have to determine the best path. Metrics are the methods used by routing algorithms to determine the best path through a network. As a rule, the smaller the metric, the better the path.

*Types of Metrics
Each metric uses a different standard to determine what the best path might be. Metrics commonly used by routing algorithms include hop count, ticks, path cost, bandwidth, link delay, load, and reliability.

*Path Cost
Path cost can be automatically or manually assigned, and refers to the speed of each link in a path. A slower network will have a higher cost than a faster network.
*Tick
A tick is an 1/18th of a second. Novell's IPX RIP uses time measurement to determine the best path. The lower the number of ticks, the better the path.

*Hop Count
Hop count is the number of routers on a path. The lower the count, the better the path.
*Link Delay
Link delay considers factors such as bandwidth, queue length at each router, network congestion, and physical distance. The lower the delay, the better the path.

*Bandwidth
Bandwidth is another commonly used metric. The bandwidth rating is simply an assessment of the link's maximum throughput. Keep in mind that a high-bandwidth, high-traffic site may not be the optimal path.
*Reliability
Reliability ratings can be dynamically assigned by a routing protocol, but are typically assigned by a network administrator. This metric reflects the rate of failure of network links and the speed with which they are repaired.

*MTU
MTU refers to the maximum transmission unit of a link. This value is the maximum octet length of a message that all links in a path will accept. Routes that allow larger MTUs are considered to be faster.
*Calculating Load
CPU use and the number of packets processed in a second are some of the factors used to calculate load, which is the amount of traffic on a network. Load is dynamically calculated, and continually monitoring these factors can be resource intensive.

*Least Costly Path
While expense is not a metric, many organizations make routing decisions based upon the least costly path. For example, a company may choose to send information over public lines, even though the throughput is lower than a costly leased line.

Question 7

Topic 1.8: Unit 1 Summary

In this unit, you reviewed the routing process and packet switching. You also learned the distinction between routing protocols and routed protocols. Routed protocols handle user data, while routing protocols are used to share network information between routers.
You also learned about static, default, and dynamic routes. You learned the strengths and weaknesses of each, as well as how each learns about network topology. You also saw how routing metrics are used in path determination, and now know what some common routing metrics are.
In the next unit, you'll learn about distance vector routing.

Unit 2. Distance Vector Routing



In this unit, you learn about distance vector routing. As you learned previously, routing algorithms are fundamental when using a dynamic routing system.
You see how network discovery works in distance vector routing. You also learn the problems which can arise, and several ways to solve those problems.
In addition, you learn about several routing protocols which use distance vector algorithms.

After completing this unit, you should be able to:
  • Define convergence

  • Explain how distance vector routing works

  • Describe network discovery in distance vector routing

  • Name one problem with distance vector routing

  • List three solutions for the problem in distance vector routing


This unit provides information that is relevant to the following CCNA exam objectives:
  • List the key internetworking functions for the OSI Network layer


Topic 2.1: Convergence

*Dynamic Routing
Dynamic routing depends on all of the routers in a network having similar information in their routing tables. The faster a routing algorithm can distribute information between routers and have all of the router's information be consistent, the better its performance.

*Convergence
The routers in a network approach a state of convergence when all of them have an accurate, consistent view of the network.

*Fast Convergence
Until network routers reach a state of convergence, their routing tables may contain incorrect information about network topology. Faster convergence times allow routers to quickly adapt to a new network topology.

Topic 2.2: Distance and Direction

*Dynamic Routing Protocols
Dynamic routing protocols fall into two basic divisions — distance vector algorithms and link-state algorithms. We'll be looking at distance vector algorithms in the rest of this unit.

*Distance Vector Algorithms
Distance vector algorithms determine network topology and data paths using information from routing tables. From the routing tables, distance vector algorithms determine the distance and direction of each link in the network.

*Using Hop Count
Distance vector algorithms typically use hop count to determine the distance of a path. Hop count is a metric based on the number of routers that a packet passes through on a given path. Direction is determined by deciding which of the router's ports the packet will be sent through.

*Routing Table Updates
Routers using a distance vector algorithm update their knowledge of the network by exchanging routing table information with neighboring routers. These information exchanges are referred to as routing table updates.

*Sending Table Updates
Routing table updates are sent immediately if the topology of the network changes. Otherwise, routing table updates are sent on a periodic basis determined by the particular routing protocol being used.

*Knowledge of the Router
Because routing table information is exchanged only between neighbors, the router does not know the exact topology of the network. It knows only how many hops a destination is from a neighboring router.

*Updating the Router Table
For example, let's say Router A has received information from Router B stating that Network F is four hops away. Router A takes this information and updates its routing table to reflect that Network F is five hops away from it.

*Distance Vector Routing
In distance vector routing, as long as the distance to a destination is getting progressively shorter, it is considered to be the best path. The distance is considered to be shorter if the metric for the destination decreases at each hop.

*Distance Vector Routing Protocols
Routing Information Protocol (RIP) and Interior Gateway Routing Protocol (IGRP) are both examples of a distance vector routing protocol.

Question 8

Question 9

Topic 2.3: Routing Loops

*Routing Loops
Distance vector routing algorithms develop routing loops if a change in network topology or slow convergence causes inconsistent routing table information between routers. A routing loop causes a packet to bounce between routers instead of being sent to its proper destination. Let's look at an example of a routing loop.

*A Routing Example
The network shown has reached convergence. However, let's assume network 27.7.0.0 fails. Since Router C is directly connected to the network, it senses the failure of 27.7.0.0, and stops routing packets to that network.

*Router A
However, Router A's routing table shows a viable path to 27.7.0.0 through Router B, so router A continues to route packets for network 27.7.0.0 to Router B.

*Router B and Router C
Router B and Router C now exchange a regular routing table update. Since Router B's routing table still indicates a path to 27.7.0.0, Router C believes it has a path to 27.7.0.0 through Router B and updates its forwarding interface and hop count for 27.7.0.0 accordingly.

*Router C
Router C's inaccurate routing information is now passed to the other routers in the next routing table update. The other routers increase the hop count for 27.7.0.0 by one. Since each router believes a viable path to 27.7.0.0 exists with one of the others, the packet is simply passed back and forth between the routers.

*Counting to Infinity
As the packet is passed between the three routers, routing table updates are exchanged, with the hop count for 27.7.0.0 increasing each time. This type of routing loop is called counting to infinity.

Question 10

Question 11

Topic 2.4: Routing Loop Solutions

*Addressing Routing Loops
There are several ways to address routing loops. Solutions for routing loops include a defined maximum, Time-to-Live (TTL) parameters, split horizon, poison reverse, hold-down timers, and triggered updates. Let's look at each solution.

*Defining the Maximum
System administrators can set a maximum hop count in a process called defining infinity. When the maximum hop count is reached, routers update their routing tables to show the network is unreachable. RIP defines this number as 16 hops.

*Time-to-Live Parameter
IP uses a TTL (time-to-live) parameter to control routing loops. The TTL parameter is a value placed on a packet that decreases by 1 each time a packet is processed by a router. When it reaches zero, the packet is dropped.

*Split Horizon
Split horizon prevents routing loops by never sending update information in its originating direction. For example, if Router B has received routing table information from Router A, it will not send any information on the same networks back to Router A.

*Poison Reverse
A variation of split horizon is called poison reverse. In this scheme, when a router discovers a network link is down, it poisons the route by assigning it an infinite hop count. Poison reverse is commonly used with hold-down timers, which you'll learn about next.

*Hold-Down Timers
Hold-down timers accept routing table updates that show a better metric for a route, while updates showing a poorer metric are ignored. Routing loops are prevented, but routes are reinstated when they are again accessible. Hold-down timers are normally set to be just over the amount of time it takes the network converge.

*Triggered Updates
Some routing protocols support triggered updates, which allow routing table updates to be circulated if a router detects a change in network topology. Triggered updates are frequently used with hold-down timers to prevent a regular routing update from incorrectly reinstating an unavailable route.

*Combining Solutions
Each of these solutions can prevent routing loops in simple networks, but more complex networks may require a combination of solutions. For example, if a router has multiple paths to the same destination, a combination of triggered updates, poison reverse, and hold-down timers can be used to prevent a routing loop.

Question 12

Question 13


* Exercise 1
Try working with distance vector routing.

Examine the following table
Step Action
1 Sketch two networks connected by a cloud of routers.
2 Label each of the networks 1.1.1.2 and 1.1.1.3. Label the routers alphabetically.
3 Below the sketch, list at least two distance vector routing algorithms.
4 Assume the link to 1.1.1.3 has gone down.
What will happen using a distance vector routing algorithm?
5 Explain how data transfer will be impacted by the loss of the link to 1.1.1.3.
6 What methods can be used to maintain correct data flow?
List at least three.
7 Explain how each method works.
8 In the network you have drawn, will a single solution be enough? Why or why not?



* Exercise 2
Try applying distance vector routing protocols and solutions.

Examine the following table
Step Action
1 Contact a network administrator who uses distance vector routing in his network.
2 Discuss the advantages of using distance vector routing.
Why was distance vector routing chosen? What advantages does it offer?
3 Discuss the disadvantages of using distance vector routing.
What problems have arisen in this particular network? What solutions were used? Why?


Topic 2.5: Unit 2 Summary

In this unit, you learned how distance vector routing algorithms, such as RIP and IGRP, work. You saw that hop count is a common metric used by these types of algorithms.
In addition, you learned about routing loops, which are the biggest problem with distance vector routing algorithms. You saw how these loops occur, and how solutions such as triggered updates, split horizon, hold-down timers, and triggered updates can be used to prevent them.
In the next unit, you'll learn about link-state routing algorithms.

Unit 3. Link-State Routing



In this unit, you learn about link-state routing algorithms. You learn how network discovery works with link-state routing algorithms. You see the advantages and disadvantages of using a link-state routing algorithm. You also learn the difference between link-state algorithms and distance vector algorithms.
In addition, you learn about hybrid routing algorithms, which combine aspects of both link-state and distance vector algorithms.

After completing this unit, you should be able to:
  • Explain how link-state vector routing works

  • Describe network discovery in link-state routing

  • Name a problem with link-state routing

  • Name a solution for problems in link-state routing

  • Identify a balanced hybrid protocol


This unit provides information that is relevant to the following CCNA exam objective:
  • List problems that each routing type encounters when dealing with topology changes, and describe techniques to reduce the number of these problems


Topic 3.1: Shortest Path First

*Distance Vector Routing
In distance vector routing, each router receives routing table updates from neighboring routers. In distance vector routing, no single router has complete knowledge of network topology, but each router knows the distance and direction of routes relative to neighboring routers.

*Link-State Routing
In link-state routing, each router maintains complete knowledge of network topology, including all routers in the network and the connections between them.

*Learning through Neighbor Notification
Link-state routers start to learn about the network through neighbor notification. Neighbor notification is the process by which routers identify their neighbors.

*Link-State Packet
Once their neighbors have been identified, each router broadcasts an LSP (link-state packet). Each LSP contains information about the routers and networks to which an individual router is connected.

*Receiving the LSP
Routers receiving the LSP use its information to build a topological database of the network. The database contains information from all of the LSPs which have been broadcast.

*Constructing an SPF Tree
Link-state algorithms, also called shortest path first (SPF) algorithms, use the information in the topological database to calculate the shortest path from a router to each network in the database. The router then constructs an SPF tree containing all of the shortest paths, with itself as the root.

*Updating Routing Tables
Routers then update their routing tables with port and metric information, based on the SPF tree, which the router has constructed.

*Link-State Algorithms
Link-state algorithms are very useful. NetWare Link Services Protocol (NLSP) and Open Shortest Path First (OSPF) are examples of link-state routing protocols.

Question 14

Question 15

Question 16

Topic 3.2: Resources and Network Changes

*Adapting to Topology Changes
In link-state routing, LSPs are not just used to learn network topology, they are also used to adapt to network topology changes. When a router notices a status change in one of its neighbors, it broadcasts a new LSP onto the network.

*Calculating the Best Route
Each time a router receives a LSP, it uses the SPF algorithm to recalculate the best routes. The router then uses this information to update the routing table with accurate information.

*Routing Loops
Because link-state protocols recalculate the best route each time an update is received, any routing loops which may have been created are automatically terminated when the routing table is updated.

*Broadcasts and Bandwidth
Routers using link-state routing protocols broadcast LSPs and then use that information to perform route calculations. Upon initialization, these broadcasts are very heavy. This causes link-state routing protocols to consume more bandwidth, memory, and processor resources upon initialization than distance vector routing protocols.

*LSP Flooding
Bandwidth is primarily consumed in the initial LSP flooding as newly connected routers learn about the network. Once the initial process of network discovery is complete, LSPs are only broadcast at configured intervals or to notify a change in network topology.

*Processor and Memory Requirements
Processor and memory requirements are proportional to the size and complexity of the network. The more paths a router has to calculate, the more processor and memory resources will be consumed.

Question 17

Topic 3.3: LSP Updates

*An Accurate Picture of the Network
Routers that run link-state routing algorithms use LSPs to learn about and update network topology. Because of their importance, it is vital that each router in a network receives all of the LSPs needed to maintain an accurate picture of the network.

*Varying Information in the Routing Tables
If every router does not receive the most recent LSP, then the routing tables will vary between routers, and routes will become unreachable.

*Unsynchronized LSPs
Routes can also become unreachable if LSPs are unsynchronized. Let's say Network A between Router 1 and Router 2 goes down, and then comes back up shortly afterward. Router 3 may receive the message from Router 1 that the network is down after the update from Router 2 that the network is back up.

*Preventing Faulty LSP Information
The larger the network, the more likely it is that faulty LSP information will cause routes to become unreachable.
The next few pages show a number of ways to help prevent faulty LSP information and ease resource requirements.

*Periodic Updates
Resource requirements can be lessened by lengthening the period of time between regular LSP updates. This change in periodic updates will not interfere with updates triggered by changes in topology.

*Multicast Routers
The network administrator can designate certain routers to be focal points for topology information by placing them in the same multicast group. LSPs will only go to the routers in the group, preventing LSPs from flooding the network. Other routers can then refer to the designated routers as a source of information about network topology. This solution eases resource requirements and helps prevent faulty LSP information.

*Network Hierarchy
In large networks, it may be necessary to set up a hierarchy for the network, so that LSPs in one area will not be passed into another area. Breaking the network into smaller, more manageable parts reduces the likelihood of faulty LSP information and reduces resource requirements.

*Marking LSP Packets
The LSP packet can be marked so that routers will know how old the packet is. These markers can include time stamps, sequence numbers, and aging schemes. These markers help to prevent the distribution of faulty LSPs and to coordinate updates.

Question 18

Question 19

Question 20

Topic 3.4: Balanced Hybrid

*Balanced Hybrid Routing Protocol
There is another type of routing protocol called the balanced hybrid. It combines aspects of both link-state and distance vector routing.

*Balanced Hybrids in Action
The balanced hybrid protocol uses distance vectors with more accurate metrics for path determination. However, balanced hybrids broadcast updates when triggered by topology changes instead of using periodic updates. Balanced hybrids tend to converge quickly and use network resources efficiently.

*Examples of Balanced Hybrids
Intermediate System-to-Intermediate System (IS-IS) and Enhanced Interior Gateway Routing Protocol (Enhanced IGRP) are examples of balanced hybrid routing protocols.

Question 21


* Exercise 1
Try working with link-state routing.

Examine the following table
Step Action
1 Contact a network administrator who uses link-state routing in her network.
2 Discuss the advantages of using link-state routing.
Why was link-state routing chosen? What advantages does it offer?
3 Discuss the disadvantages of using link-state routing.
What problems have arisen in this particular network? What solutions were used? Why?


Topic 3.5: Unit 3 Summary

In this unit, you learned about link-state routing protocols. You saw how link-state routing algorithms use LSPs, SPF algorithms, and SPF trees to build a database of the best paths.
In addition, you learned how link-state algorithms are resource intensive and prone to faulty LSP information. You also saw how multicasting, hierarchies, and time stamps can be used to prevent these problems. Finally, you learned about balanced hybrid routing.
In the next unit, you'll learn more about particular routing protocols, as well as the parts of a router.

Unit 4. Understanding Routers



In this unit, you learn about autonomous systems. You see how autonomous systems work. You also learn about the protocols which are used to route within and between autonomous systems.
In addition, you learn about the parts of a router. You see the functions performed by each part, and how that function contributes to the router's ability to perform its routing function.

After completing this unit, you should be able to:
  • Define an autonomous system

  • List three Interior Gateway Routing Protocols

  • List one Exterior Gateway Routing Protocol

  • Name the parts of a router

  • Identify the function of each part of a router


This unit provides information that is relevant to the following CCNA exam objectives:
  • Examine router elements

  • List problems that each routing type encounters when dealing with topology changes, and describe techniques to reduce the number of these problems


Topic 4.1: Autonomous Systems

*Autonomous Systems
An autonomous system is the basic unit in routing. It is a set of routers and networks under the same administration. Each router in an autonomous system must be interconnected and run the same routing protocol.

*Autonomous System Numbers
Autonomous systems are assigned unique numbers by the Network Information Center (InterNIC). These numbers are called Autonomous System Numbers (ASN).

*Routing Domains
Autonomous systems are also referred to as routing domains or broadcast domains.

*Interior Gateway Protocol
An interior gateway protocol (IGP) is what networks and routers in an autonomous system use to share routing table information. The routing protocols you learned about in the last unit, such as RIP, IGRP, and OSPF are examples of Interior Gateway Protocols.

*Exterior Gateway Protocols
Exterior gateway protocols are used to share routing information between autonomous systems. Two commonly used exterior gateway protocols are the Border Gateway Protocol (BGP) and the Exterior Gateway Protocol (EGP).

*Gateways
Interior and exterior gateway protocols use the gateway of the autonomous system to define their area of operation. A gateway is simply the point that acts as an entrance to a network. Routers are commonly used as gateways.

Question 22

Question 23

Question 24

Topic 4.2: Internal Components

*Routers and Memory
A router's hardware components are as important to a router's function as its software protocols. Cisco routers use several different types of memory which play an important role in the router's startup and performance.

*Read Only Memory
Read Only Memory (ROM) stores the bootstrap startup program and Power-On-Self-Test (POST). Some routers also contain Cisco's Internetwork Operating System (IOS) in ROM. Series such as the 2500 and 4500 have a scaled-down version of the software, while series such as the 7500 contain the full IOS.

*ROM Chips
ROM chips are plugged into sockets on the router's motherboard so that they can be easily replaced or upgraded.

*Random Access Memory
Random Access Memory (RAM)/Dynamic Random Access Memory (DRAM) is the working storage area for the router. RAM/DRAM provides services such as packet buffering and caching, and dynamic configuration information, such as routing tables.

*RAM/DRAM
RAM/DRAM handles the IOS when the router is on, and is cleared when the router is off.

*Nonvolatile RAM
Nonvolatile RAM (NVRAM) stores a backup copy of the router's startup configuration file. This information is retained in the event of a power failure, or if the router is powered down.

*Flash Memory
Flash memory is an erasable and reprogrammable type of ROM. A copy of the IOS is held in flash memory. In fact, multiple copies of the IOS can be held in flash memory, which allows an administrator to load IOS upgrades to different routers individually and then run the upgrade after all of the routers are loaded. Flash memory is not cleared when the router is powered down.

Question 25

Question 26

Question 27

Topic 4.3: Other Router Elements

*Network Interfaces
In addition to memory components, routers can support many different types of network interfaces, including Token ring, Ethernet, ATM, FDDI, BRI, serial, and Channel Interface Processor (CIP) for SNA support.

*Routers and External Ports
Routers can be configured through a variety of external ports, including the console port, auxiliary port, virtual terminals, a TFTP server, and a Network Management Station. Let's examine each of these.

*Console Port
The console port connects the router to a console terminal which can be used to configure the router after installation.

*Auxiliary Port
The auxiliary port can also be connected to a terminal that can be used to configure the router.

*Virtual Terminals
Virtual terminals 0 through 4 can also be used to configure a router. These virtual terminals can be accessed via Telnet.

*Configuration Information
Configuration information can be stored on a TFTP server and can then be downloaded to a router. The TFTP server can be either a PC or a Unix workstation.

*Remote Management of Routers
Routers can also be managed remotely from a Network Management Station with network management software.

*Cisco Discovery Protocol
Cisco Discovery Protocol (CDP) is used to learn configuration information about other routers, such as whether the router is running IP or IPX.

Question 28


* Exercise 1
Try identifying parts of a router.

Examine the following table
Step Action
1 Contact a system administrator who is using a Cisco router.
Ask if you can see the router.
2 Ask the system administrator to identify the different ports of the router.
Which port does the administrator typically use to configure the network? Why?
3 Is CDP utilized in this network? Why or why not?
4 Which version of Cisco IOS is being used?
5 List the four types of memory in a Cisco router.
6 Name the function of each type.
Is the information retained if the router is powered down?


Topic 4.4: Unit 4 Summary

In this unit, you learned about autonomous systems. You saw how gateway protocols are used to exchange information within and between autonomous systems.
In addition, you learned the parts of a router. You were introduced to the function of each part. You also saw how CDP can be used to learn configuration information from other routers in a network.
In this course, you have become familiar with the basic concepts of router operation. You saw how routers support a wide variety of protocols to accomplish data transfer across a variety of networks.
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Cisco CCNA: Local Area Networking

Cisco CCNA: Local Area Networking


Unit 1. Introduction to LANs



In this course, you will learn many aspects of local area networks, or LANs. You will learn to identify LAN components and configurations. You will also learn about the three most common types of LANs: Ethernet, Token Rings, and FDDI. These LAN types differ in the way they encapsulate data, gain access to the media, and handle errors.
In this unit, you will learn about basic LAN components and standards. Then you will learn the standards of Ethernet, Token Ring, and FDDI, as well as how these LAN types operate.

After completing this unit, you should be able to:
  • Identify the different LAN topologies

  • Identify various cables used in LAN connections

  • Recognize the organizations governing the different network types

  • Distinguish between physical and logical addresses

  • List the components of a MAC address


This unit provides information that is relevant to the following CCNA exam objectives:
  • Define and describe the function of a MAC address

Topic 1.1: LAN Basics

*Defining a Local Area Network
A Local Area Network (LAN) is a collection of computers in a limited geographical area that are connected in order to share resources. A LAN consists of nodes (workstations, servers, peripherals), their network cards, and the cables connecting everything together.

*Sharing and Communication
One of the many benefits of using LANs is that it allows users to share data. LANs also allow many different users to share expensive peripherals, such as printers, scanners, and plotters. One of the most popular LAN benefits is that they make communication faster and easier.

*Topologies and Cable
In this course you will learn about the different types of LANs, such as Ethernet, Token Ring, and Fiber Distributed Data Interface (FDDI). On the Physical layer of the OSI model, these types of LANs can be distinguished by their topologies and choice of cable.

Topic 1.1.1: LAN Topology

*LAN Topology
LAN topology comes in three basic physical forms: bus, ring, and star. Large networks may combine different LAN topologies into a more complex network.

*The Bus Topology
A bus is a series of computers in a linear arrangement. A terminator placed at each end-point of a bus network absorbs signals sent out by network devices so that the signals do not reflect back down the line. This is usually the easiest and least expensive topology to implement, and therefore very common.

*The Ring Topology
A ring is a series of computers connected in a ring arrangement. Each computer is connected to two adjacent computers to form a closed loop. A ring topology eliminates the need for terminators, but it is not easily administered.

*The Star Topology
A star is a group of computers connected to a central hub. This arrangement allows for efficient central administration and cable troubleshooting. The star topology also has the ability to mimic a ring topology in what is known as a logical ring.

*A Bus Star Topology
A cascading star (or bus star) is a number of stars connected to a bus backbone. Cascading stars are very common in Ethernet implementations and they give network administrators greater control of the network.

Topic 1.1.2: LAN Cabling

*Choosing Cable
The choice of cable is a very important aspect in LAN operation. The most popular cables in use today are copper (coaxial, unshielded twisted pair, shielded twisted pair) and fiber optic. Each of these cable types has advantages and disadvantages, and network architects need to know which cables are best suited for specific tasks.

*Coaxial Cable
Coaxial cable has an inner conductor surrounded by insulation, a metal screen around the insulation, and an outer plastic cover. Coaxial cable has the advantages of carrying a large bandwidth, relative immunity to electromagnetic interference, the ability to carry signals over a significant distance, and the fact that many cable installers are familiar with it.

*Unshielded Twisted Pair Cable
Unshielded twisted pair cable (UTP) is lighter, thinner, more flexible, and easier to install than coaxial or fiber-optic cable. It is also inexpensive. It is the cable of choice for most locations, but is subject to interference from electrical devices. There are five categories or grades of UTP (Categories 1-5) with higher categories being better choices for data transmission.

*Shielded Twisted Pair Cable
Shielded twisted pair cable (STP) is a twisted pair cable surrounded by an RF-insulating material, making it immune to electrical interference. It is more expensive than UTP and must be grounded correctly to work properly.

*Fiber Optic Cable
Fiber optic cable transmits light pulses along a core fiber surrounded by cladding, which is a shield that reflects signals back to the fiber to reduce signal loss, and a protective cover. It has enormous bandwidth and can carry signals for extremely long distances, it is immune to electromagnetic interference, and it is more secure than copper cable. It is also expensive and can be difficult to install and maintain.

*Distance Constraints
There is a physical relationship, known as the distance constraint, between the length of a cable and the speed of data transmission. Faster transmission rates reduce the distance that data can travel on a given cable. A device known as a repeater may extend these distance constraints, but there is an upper limit to cable length.

*Using Repeaters
Repeaters receive a signal from an input port and transmit the signal to its output port. This amplifies the signal and increases the effective segment distance of a network. Thus, repeaters are considered a part of the overall cabling scheme of a network.

Topic 1.1.3: Types of LANs

*LAN Architecture
Four types of LAN architecture in use today are Ethernets, Token Buses, Token Rings, and FDDIs. The standards for Ethernet, Token Bus, and Token Ring are governed by the IEEE (Institute of Electrical and Electronic Engineers) 802 committee. The IEEE 802 committee has five subcommittees covering the different types of LANs and LAN issues. FDDIs are governed by ANSI (American National Standards Institute).

*Governing the Logical Link Control Sublayer
IEEE 802.1 covers issues shared by all LANs. IEEE 802.2 governs the Logical Link Control sublayer of the OSI model. The other subcommittees each control standards for specific LAN types.

*Ethernet Standards
Ethernet standards are governed by IEEE 802.3. An Ethernet is a logical bus network with a backbone attached to the LAN nodes and with a terminator at each end. Ethernets may be physically configured in a star formation, but retain the characteristics of a logical bus. Ethernets control media access by a method known as CSMA/CD (Carrier Sense Multiple Access/Collision Detection).

*Token-Passing Networks
Token-passing networks use an electronic token to determine media access. These networks will be discussed more thoroughly in a later unit of this course.

*A Logical Ring of Computers
Token Ring networks use standards controlled by IEEE 802.5. A Token Ring is a logical ring of computers that take turns broadcasting. An electronic token is used to determine which node will broadcast. Token rings may be configured in a star formation, but retain the characteristics of a logical ring.

*Token Bus Standards
Token Bus standards are controlled by IEEE 802.4. Token Bus networks, developed by General Motors, are not very common. They were developed for computer-controlled manufacturing applications. Media access is controlled by the possession of an electronic token.

*Fiber Distributed Data Interface
FDDI, or Fiber Distributed Data Interface, is a token-passing network connected by fiber optic cables. These networks are very fast, with data rates of up to 100 Mbps.
FDDI LANs are configured as dual token rings for higher performance and also as backup in case of a failure in one ring. FDDI standards are governed by ANSI.

Topic 1.2: LAN Addressing

*Using Addresses
Addresses are used in LANs to insure that information gets from the sending node to the receiving node. There are two types of addresses: physical addresses and logical addresses. These two address types operate on separate layers of the OSI model.

*Physical Addresses Are Permanent
Physical addresses (also known as link-layer, hardware, BIA, or burned-in addresses) are addresses that are an integrated part of the network card. They are burned into ROM by the manufacturers and will not change over time or when a LAN is reconfigured.

*Unique Physical Addresses
Physical addresses are unique for each network connection. They reside in what is known as a flat address space. A physical address is to a network connection as a social security number (SSN) is to an American. Just as no two people can have the same SSN, no two network cards can have the same physical address.

*Physical Addresses in the OSI Model
In the OSI model, physical addresses reside in the Media Access Control (MAC) sublayer of the Data Link layer. Therefore physical addresses are also known as MAC addresses.
Defining unique MAC addresses is the most important function of the MAC sublayer.

*MAC Addresses
Physical addresses, or MAC addresses, are only used within a local domain, which is a group of network devices using the same protocols and administered as a unit. Physical addresses are attached to a frame to deliver data to another network component within the local domain. To send data outside the local domain, a logical address is needed.

*Logical Addresses
Logical addresses (also called virtual addresses or Network-layer addresses) are used to transport data to a destination outside the local domain. Data traveling outside the local domain is attached to packets (or datagrams) containing the logical address of the originating node and the destination node.

*Following a Logical Address
Logical addresses are hierarchical, like mail addresses. Just as mail would have to travel to a city, a street, a building, and a mailbox to get to a person, a data packet may need to travel through hub A, bridge B, router C, and hub D to get to destination E.

*Resolving the Logical Address
When a packet arrives at the local domain of the destination, the logical address must be resolved to the physical address of the destination node. ARP, or Address Resolution Protocol, is a commonly used protocol that maps logical addresses to physical addresses. This will be covered in a later course in this series.

Question 1

Topic 1.3: The MAC Address

*The Hardware Address
The MAC, or physical, address is the hardware address of a network node. These addresses are used to route data frames within a local domain. MAC addresses are 6 bytes (48-bits) long and are expressed in 12 hexadecimal digits. Hexadecimal numbers are covered in another course in this series.

*The Organizational Unique Identifier
The first 3 bytes (6 hexadecimal digits) of the MAC address contains the OUI (Organizational Unique Identifier) which is the manufacturer's identification or vendor code. OUIs are administered by the IEEE to make sure each manufacturer has a unique OUI.

*The Interface Serial Number
The last 3 bytes (6 hexadecimal digits) of the MAC address usually represent the interface serial number. There are no current standards for this part of the MAC address, and each vendor has its own method of determining these numbers.

*Transmitting a Frame
When a node is ready to transmit a data frame within the local domain, it places the MAC address of both the sending node and the receiving node onto the frame. The frame is then transmitted over the local network. The receiving node will process the frame when it recognizes its own MAC address on the frame.

Question 2


* Exercise 1
Try using the World Wide Web to find out more about local area networks.


Examine the following table
Step Action
1 Use your browser to navigate to your favorite search engine (i.e., Yahoo!, LookSmart, Excite, etc.)
2 Search for terms you found in this unit.
3 Follow the links to find more information on the terms you selected.


Topic 1.4: Unit 1 Summary

In this unit you learned how to identify the different LAN topologies and the different types of cable used to connect LAN components together. You also learned about different types of LANs and the agencies that govern the standards for each type.
In addition, you learned to distinguish between physical and logical addresses. You learned about the MAC address, and how the MAC address uniquely identifies each component in a LAN.
In the next unit of this course, you will learn much more on Ethernet.

Unit 2. Ethernet



In this unit, you will learn about Ethernets. You will learn about the different types of Ethernet and how Ethernet networks package and transmit data. You will also learn how Ethernets control access to the media and handles transmission errors.
In addition, you will learn about the requirements and capabilities of half-duplex and full-duplex Ethernet operation.

After completing this unit, you should be able to:
  • Identify the different Ethernet types

  • Identify various cables used in Ethernet connections

  • Distinguish between Ethernet and IEEE 802.3 frames

  • List the methods of CSMA/CD

  • Distinguish between half-duplex and full-duplex Ethernet


This unit provides information that is relevant to the following CCNA exam objectives:
  • Describe full- and half-duplex Ethernet operation


Topic 2.1: Ethernet Basics

*Understanding Ethernet
Ethernet is a type of LAN developed by Xerox in cooperation with Digital Equipment Corporation (DEC) and Intel. Ethernet LANs are very sturdy and reliable, and Ethernets are the most popular type of LAN in use today.

*Data Link Differences
Ethernet is almost identical to the IEEE 802.3 standard and are both governed by the IEEE 802.3 subcommittee. However, they have differences in the Data Link layer of the OSI model. IEEE 802.3 splits the Data Link layer into the MAC and LLC sublayers. Ethernet does not support LLC services, but conforms to the rest of the IEEE 802.3 standard.

*Ensuring Data Transfer
Ethernet and IEEE 802.3 use the CSMA/CD protocol to ensure proper data transfer. The CSMA/CD standard gives devices the ability to detect a data collision, which occurs when two network devices transmit at the same time and the data become garbled. After a collision is detected, a device will retransmit the message after a random delay time.

Topic 2.1.1: Ethernet Types

*Ethernet Bus Topology
Ethernet and IEEE 802.3 have a bus topology and can have a signaling rate of 10, 100, or even 1000 Mbps. LAN nodes are connected to the bus to form the network. Terminators are placed at both ends of the bus to absorb signals, which prevents signal reflection. Standard names for Ethernets include the signal speed. For example, an Ethernet name beginning with 10Base would operate at 10 Mbps and one beginning with 100Base would operate at 100 Mbps.

*Thin Ethernet
10Base-2, or Thin Ethernet, uses 0.25-inch diameter coaxial cable. Each 10Base-2 cable segment may be up to 185 meters in length. Thin Ethernet cables are connected to individual nodes with the use of BNCs (British Naval Connectors) and BNC tees.

*Thick Ethernet
10Base-5, or Thick Ethernet, uses 0.4-inch diameter coaxial cable. Each segment on Thick Ethernet may be up to 500 meters in length. Thick Ethernet cables are connected to individual nodes with the use of transceivers and AUIs (attachment unit interfaces). Transceivers transmit and receive data to and from the network, and AUIs provide the connection for the cable to the node.

*10BaseT
10Base-T uses UTP cable up to 100 meters in length. UTP cable can only connect at the cable end points, and therefore connects only two nodes. For this reason, 10Base-T almost always uses a hub, which is a central connection point for all the nodes. The hub is also considered a node, with cable segments attached to all the other nodes.
10Base-FL is similar to 10Base-T, but uses fiber optic cable.

*Fast Ethernet
Fast Ethernet operates at 100 Mbps. Examples of Fast Ethernet are 100Base-TX, with UTP cables up to 100 meters in length, and 100Base-FX, with fiber optic cables up to 400 meters in length. There is also a Gigabit Ethernet, which operates at 1000 Mbps. Fast Ethernet and Gigabit Ethernet are covered in the next unit of this course.

Question 3

Question 4

Topic 2.1.2: Ethernet Interface

*Network Interface Cards
Ethernet network interface cards (NICs) allow each network device, or node, to communicate across the cable media. Each node is identified by its MAC address.

*NIC Connections
10Base-5 is connected to the bus with AUIs and transceivers. The AUI is a 15-pin socket located on the NIC.
A cable connects the AUI to the transceiver, which is attached to the bus. Some NICs have a built-in transceiver, in which case the AUI can be directly attached to the bus.

*Connecting 10Base-T
The interface for 10Base-T is different from those of 10Base-2 and 10Base-5. Since 10Base-T uses UTP cables with RJ-45 connectors, the network cable is plugged directly into the NIC. The other end of the cable is attached to the network hub.

*Ethernet Hubs
Ethernet hubs provide a central location for linking the nodes of the network. They also serve as an attachment site to link the local network to a larger network. This centralized wiring makes it much easier for administrators to troubleshoot cable problems. It is important to remember that even when an Ethernet uses a star topology, it still behaves like a bus in that all nodes receive all transmissions at about the same time.

Topic 2.2: Ethernet/802.3 Operation

*Packets to Frames
Ethernet nodes encapsulate data that needs to be sent over the network into a packet. When the packet is ready to be sent over the local network, the node adds source and destination MAC addresses to the packet. This process converts the packet into a frame.

*Transmitting Frames
When a node transmits a frame, the signal travels the entire network. Every node in the network receives and examines the frame. When the data reaches the endpoints of the bus, terminators absorb the signal to prevent reflection.

*Processing a Frame
When a node receives a frame, it checks for the destination MAC address in the frame. If that MAC address matches its own MAC address, the node will process the frame. If the MAC addresses do not match, the frame is discarded.

*Frame Errors
Frames are also discarded if they contain errors. An example of a frame error would be when a frame does not conform to size specifications. Frames must be between 64 and 1518 bytes in length. Frames less than 64 bytes in length are called runts. Frames greater than 1518 bytes are called giants. Both runts and giants are considered errors and are discarded by network nodes.

*Frames and the OSI Model
Frames operate on the Data Link layer of the OSI model. They must be moved from the Data Link layer to the Physical layer to be transmitted over the network. Ethernet and IEEE 802.3 frames have differences because IEEE 802.3 frames provide fields for LLC services and Ethernet does not.

*Transmission Time
The time required to move a frame between the Data Link layer to the Physical layer is called the transmission time. Each Ethernet bit has a window of 100 nanoseconds. A packet size of 64 bytes transmits in 51.2 microseconds. A packet of:
512 bytes transmits in 410 microseconds
1000 bytes transmits in 800 microseconds
1518 bytes transmits in 1214 microseconds.

Question 5

Topic 2.2.1: Ethernet Frames

*The Preamble
Ethernet frames begin with a preamble. The preamble is an eight-byte segment that provides clock synchronization and lets receiving stations know that a frame is coming. The preamble ends with a 10101011 bit pattern, called the Starting Delimiter, to indicate the arrival of data.

*Destination Address
The preamble is followed by the destination address (DA). This is the six-byte MAC address of the intended receiver of the frame. This address may include a single node (unicast), a group of nodes (multicast), or all nodes on the network (broadcast). The unicast address is the destination node's MAC address, the multicast address sets the first transmitted bit of the DA to a value of 1, and the broadcast address is all 1s (hexadecimal FFFF.FFFF.FFFF).

*Source Address
The DA is followed by the source address (SA). This is the six-byte MAC address of the originator of the frame. Since the originator of a frame is always a single node, the source address will always be unicast.

*Type Field
The SA is followed by a type field. This field is two bytes long and identifies the upper-layer protocol that will receive the data in the frame after the frame is received and processed by the lower-layer protocols.

*Data
The type field is followed by the actual data being transmitted. This data can be anywhere from 46-1500 bytes in length.

*Frame Check Sequence Field
Following the data is a frame check sequence (FCS) field. This field is four bytes long and contains the cyclical redundancy check (CRC) value and a remainder number. These values are used to check for errors in transmission. When a destination node receives a frame, it divides the length of the frame by the CRC value. The remainder should equal the remainder number in the FCS. If the remainders do not match, the frame is considered damaged, probably due to a collision.

Question 6

Question 7

Topic 2.2.2: IEEE 802.3 Frames

*IEEE 802.3 Frames
The first three sections of an IEEE 802.3 frame are identical to an Ethernet frame. An eight-byte preamble is followed by a six-byte DA, followed by a six-byte SA. The last section is the same as well: a four-byte FCS field.

*Length Field
After the source address, however, IEEE 802.3 frames contain a two-byte length field instead of a type field. The length field is used to indicate the length of the data (in bytes) that lie between the length field and the FCS field. The length field is then followed by either an 802.2 LLC (Logical Link Control) header or a SNAP (SubNetwork Access Protocol) header.

*802.2 LLC Header
An 802.2 LLC header consists of three segments of one byte each. They are the DSAP (Destination Service Access Point), SSAP (Source Service Access Point), and a control field. The DSAP and the SSAP are both pointers to memory buffers that tell the NIC where to put the information. This is very handy when multiple protocol stacks are running.

*SNAP Header
The SNAP header is designed to allow vendors to use their own protocols (called proprietary protocols) in the 802.2 LLC frame. In a SNAP header, the DSAP and SSAP are both set to the hexadecimal value of AA and are followed by a control field. This is then followed by the vendor OUI (3 bytes) and the Ethernet type for the frame (2 bytes).

Question 8

Question 9

Question 10

Question 11

Topic 2.2.3: CSMA/CD

*Collision Detection
CSMA/CD (Carrier Sense Multiple Access with Collision Detection) is an important aspect of Ethernet operation. It is a very reliable method to provide an orderly transmission of data. In CSMA/CD, nodes constantly monitor the network for transmissions. When a node is ready to transmit, CSMA/CD allows the node to determine whether other nodes are currently transmitting. If there are no transmissions on the network, the node will then proceed with the transmission.

*Jam Signals
Sometimes, however, two different nodes may begin transmitting at approximately the same time, causing a collision. When the transmitting node realizes a collision has occurred, it will send out a jam signal to alert the other nodes. All nodes then stop transmitting for a random time period, called the backoff time, before trying to retransmit.

*Retransmitting
If the backoff times of two nodes are too close to the same value, then subsequent collisions will occur and the process will repeat. The backoff time doubles after each collision until the 10th retry. After the 16th retry, a node will stop trying to transmit the data.

Topic 2.3: Half-Duplex versus Full-Duplex Ethernet

*Half-Duplex Ethernet
Half-duplex Ethernet is the most popular of all LAN topologies. It uses a single channel to both transmit and receive data. Transmitting and receiving on the same channel can cause problems like collisions and loopbacks. Loopbacks occur when the sending node operates at a faster speed than the receiving node, and the signal loops back. Half-duplex Ethernet uses CSMA/CD and loopback detection to deal with collisions and loopbacks.

*Full-Duplex Ethernet
Full-duplex Ethernet, on the other hand, can transmit and receive simultaneously. This is accomplished by using two separate channels in a point-to-point connection from the transmitter of one device to the receiver of the receiving device. Full-duplex Ethernets do not share bandwidth, and are thus collision-free systems.

*Collision-Free
Since a full-duplex Ethernet is collision free, the CSMA/CD protocol does not have a function in this mode. The CSMA/CD protocol needs to be disabled in any full-duplex Ethernet implementation.

*Implementing Full-Duplex Ethernet
Full-duplex Ethernet can be used in 10Base-T, 100Base-T, 10Base-FL, and 100Base-FX. To implement a full-duplex Ethernet, requirements below must be met.

*Avoiding Problems
Problems will occur if half- and full-duplex modes are mixed. If one side of a connection is configured for full-duplex and the other side of the connection is configured for half-duplex, a bad FCS will occur. If the bad FCS has an integral number of octets, the error is called an FCS error. If the bad FCS has a non-integral number of octets, the error is called an alignment error.

Question 12


* Exercise 1
Try searching the World Wide Web for more information on Ethernets.

Examine the following table
Step Action
1 Use your browser to navigate to your favorite search engine (i.e., Yahoo!, LookSmart, Excite, etc.)
2 Search for terms you found in this unit.
3 Follow the links to find out more information on the terms you selected.


Topic 2.4: Unit 2 Summary

In this unit you learned how to identify the different Ethernet types and the various cables used in Ethernet connections. You also learned how to distinguish between Ethernet and IEEE 802.3 frames and how CSMA/CD works to control media access and errors. In addition, you learned how to distinguish between half-duplex and full-duplex Ethernet.
In the next unit you will learn about Fast Ethernets.

Unit 3. Fast Ethernet



In this unit you will learn about the different types of Fast Ethernet. You will learn about the cables and distance limitations in Fast Ethernet. You will also learn how to calculate Fast Ethernet cable lengths when repeaters are used and when fiber optic cable is used to replace a copper cable segment.
Finally, you will learn about Gigabit Ethernet.

After completing this unit, you should be able to:
  • List the different types of Fast Ethernet

  • Identify the cables used for different types of Fast Ethernet

  • List the guidelines and distance limitations of Fast Ethernet

  • Calculate Fast Ethernet cable lengths when repeaters are used

  • List the differences between Fast Ethernet and Gigabit Ethernet


This unit provides information that is relevant to the following CCNA exam objectives:
  • Describe the features and benefits of Fast Ethernet
  • Describe the guidelines and distance limitations of Fast Ethernet


Topic 3.1: What Is Fast Ethernet?

*Fast Ethernet
Fast Ethernet is a form of Ethernet with data transfer rates much higher than the 10 Mbps rate of classic Ethernet. The rate at which Fast Ethernet transfers data is 100 Mbps, and is known as 100Base-T. 100Base-T was approved as the Fast Ethernet standard in 1995 by the IEEE and designated IEEE 802.3u.

*Fast Ethernet Collision Window
Most implementations of Fast Ethernet use the CSMA/CD LAN media access method, just like classic Ethernet. However, since the bandwidth is increased by a factor of ten, the collision window, which is the amount of time in which a collision may occur, is reduced to one tenth.

*Implementing Fast Ethernet
Implementations of Fast Ethernet include 100Base-T4, 100Base-TX, 100Base-FX, and 100VG-AnyLAN. 100Base-T4, 100Base-TX, and 100Base-FX all have MAC layers that are compatible with the IEEE 802.3 MAC layer. The MAC layer for 100VG-AnyLAN, however, is not compatible with the IEEE 802.3 standard because it uses a different method to control media access.

*100Base-T4
100Base-T4 varies from 10Base-T at the Physical layer. In 100Base-T4, the 100-Mbps data stream is divided into three 33-Mbps streams. The three streams created are then sent over three pairs of UTP wire. A fourth pair of UTP wire is used by the hub to send a collision signal to a workstation when a collision occurs.

*100Base-TX Protocols
The 100Base-TX is a true derivative of 10Base-T and both use the same protocols. Thus, 100Base-TX supports both half- and full-duplex modes. The 100-Mbps speed is achieved by sending the signal 10 times faster than 10Base-T. Signal strength is retained by allowing only two repeaters between nodes, as opposed to four repeaters between nodes in 10Base-T.

*100Base-FX
100Base-FX is similar to 100Base-TX, but uses fiber optic cable instead of copper wires. Segments are usually limited to 400 meters, but 62.5/125-micron fiber optic cables in full-duplex mode have no distance restrictions. Since 100Base-FX and 100Base-TX are similar in all but cable type, they are sometimes grouped together and designated as 100Base-X.

*100VB-AnyLAN
100VG-AnyLAN uses Category 3 or 5 UTP and utilizes aspects of both Ethernet and Token Ring. When a node is ready to transmit, it sends a request to the hub, which then grants access to the media depending on priority and network load. The hub for 100VG-AnyLAN must have at least one uplink port, and all other ports must be capable of being used as downlink ports. Token Rings are discussed in the next unit of this course.

Topic 3.1.1: Cabling for Fast Ethernet

*Cabling 100Base-TX
100Base-TX uses two pairs of Category 5 UTP with RJ-45 connectors. This type of cable cancels electromagnetic interference by increasing the twist ratio of the wires. However, it is more expensive than Category 3 UTP.
The maximum length for a cable segment is 100 meters.

*Cabling 100Base-T4
100Base-T4 requires four pairs of Category 3, 4, or 5 UTP cables with RJ-45 connectors. Since the data stream is split over three of the four pairs, the full-duplex mode is not supported.
The maximum segment length is 100 meters.

*Cabling 100Base-FX
100Base-FX uses a two-strand multimode fiber optic cable. This cable may have a diameter of 50/125 or 62.5/125 microns and requires an SC, ST, or MIC fiber optic connector.
The maximum segment length is 400 meters.

*Cabling 100VG-AnyLAN
100VG-AnyLAN uses four pairs of Category 3 or 5 UTP wires with RJ-45 connectors. The maximum segment length is 600 meters for Category 3 UTP, and 900 meters for Category 5 UTP. If the hubs are in the same closet, the maximum lengths drop to 200 meters for Category 3 UTP and 300 meters for Category 5 UTP. Cascaded hubs (for a cascaded star topology) have maximum distances between hubs of 100 meters (Category 3 UTP) or 150 meters (Category 5 UTP).

*Avoiding Late Collisions
Sometimes a network exceeds the IEEE 802.3 limitations on cable lengths. Exceeding these specifications may result in late collisions. A late collision is a collision that is detected late in the data transmission. Late collisions are usually not a big problem, but they can lead to a slowdown in network traffic.

Question 13

Question 14

Topic 3.1.2: Guidelines

*Upgrading Classic Ethernet
One of the advantages of Fast Ethernet is that it allows a gradual upgrade from classic Ethernet. There are a few issues that need to be addressed during this process. For example, classic Ethernet can have up to 4 hubs or repeaters between nodes. In Fast Ethernet, only one hub is allowed per collision domain, which is a group of nodes that share the same bandwidth. This limitation may be overcome by using switching hubs, which break up the collision domain.

*Identifying Potential Bottlenecks
When upgrading an Ethernet segment from 10Base-T to 100Base-T, potential bottlenecks should be identified and switches placed to alleviate the bottlenecks. All cables should be inspected to make sure they are adequate for the implementation. Also, the NICs should be checked to see if they are capable of 100 Mbps speed.

*Autonegotiation
Networks that combine 10Base-T and 100Base-T usually have an assortment of NICs with varying capabilities. This can be very confusing for network administrators, especially since both 10Base-T and 100Base-T use RJ-45 connectors. IEEE 802.3 provides for a method to help with this problem, called autonegotiation.

*Transmitting a Pulse
In autonegotiation, nodes and hubs transmit a pulse along their connection. The 10Base-T pulse is called an NLP (Normal Link Pulse). If a node or hub detects a NLP from its connection, it will only transmit at 10 Mbps along that connection. The 100Base-T pulse is called an FLP (Fast Link Pulse), and contains a code word describing its capabilities. The station and hub then compare capabilities and automatically exchange data at the highest common level.

*Advantages of Fast Ethernet
The advantages of upgrading from classic Ethernet to Fast Ethernet include improvements in bandwidth, throughput, and overall performance. There is also an easy migration from classic Ethernet to Fast Ethernet using existing cables and network equipment — and both networks use the reliable CSMA/CD.

*Disadvantages of Fast Ethernet
There are few disadvantages in upgrading to Fast Ethernet, but there is an increase in costs and the use of the same type of cables and connectors can be confusing. An upgrade to a 100VG-AnyLAN might also produce problems associated with the different media access methods of 100VG-AnyLAN and classic Ethernet.

Question 15

Topic 3.2: Repeaters in 100BaseT

*Repeaters
There are two different types of repeaters that can be used in Fast Ethernet networks, Class I and Class II. Class I repeaters limit networks to only one repeater between nodes, but have more functionality than Class II repeaters. Class II repeaters are faster than Class I repeaters and allow networks to use multiple repeaters between nodes.

*Class I Repeaters
Class I Fast Ethernet repeaters perform the normal function of repeating signals, but they are also capable of identifying corrupt frames, translating data for different Ethernet implementations, and performing error handling. Class I repeaters also partition off malfunctioning nodes, such as nodes that stop using CSMA/CD, send endless streams of bits (jabbers), or transmit frames with invalid preambles.

*Stacking Repeaters
A Fast Ethernet network with a repeater connecting two nodes, and each node at the maximum 100 meter cable limit, has a total cable distance of 200 meters. Stacking two Class II repeaters allows a total distance of 205 meters, including the cable connecting the two repeaters.

*Cable Distance
The cable distance is the longest possible path between nodes. For example, Node 1 is attached with 40 meters of cable to Repeater 1, Repeater 1 is attached to Repeater 2 with 16 meters of cable, and Repeater 2 is attached to Node 2 with 75 meters of cable. The total cable distance would be 40 + 16 + 75 = 131 meters.

*Maximum UTP Cable Distances
Some Class II repeaters, such as Cisco FastHub 300, exceed the IEEE 802.3u specifications. These repeaters allow longer cable distances, provided the 100 meter UTP segment limit is not exceeded. The following distances are maximum UTP cable distances using repeaters: 1 repeater any type — 200 m, 2 Class II repeaters — 205 m, 1 FastHub + 1 other Class II repeater — 214 m, 2 FastHubs — 223 m.

*Fiber Optic Cables
Even longer cable distances may be achieved if fiber optic cables are used in place of one of the UTP cable segments. When determining maximum cable distances, fiber optic segments must be converted to their UTP equivalents by multiplying the fiber optic length by 0.9. When converting a UTP length to a fiber optic equivalent, multiply by 1.11.

*Single Repeaters and Fiber Optic Cable
If fiber optic cable is used for one of the cable segments, the maximum cable distances (in UTP equivalents) using a single repeater in 100Base-T networks then become: 1 Class I repeater — 245 m, 1 Class II repeater — 287 m, 1 FastHub — 296 m

*Calculating Cable Distances
Here is an example that demonstrates how to calculate cable distances when both UTP and fiber optic cable are used:
A FastHub, connected to node A with 50 meters of UTP and to node B with 85 meters of UTP, can be connected to node C with fiber optic cable as far away as 234 meters. This is calculated by using 296 (longest FastHub distance using UTP and fiber optic), subtracting 85 (the longest length of UTP from FastHub to node A or B), and multiplying by 1.11 (to convert UTP to fiber optic).

Question 16

Question 17

Question 18

Question 19

Question 20

Question 21

Topic 3.3: Gigabit Ethernet

*Gigabit Ethernet
Gigabit Ethernet, or 1000Base-X, is a relatively new Ethernet implementation with data transfer rates of 1000 Mbps (1 Gbps). The standards for 1000Base-X are governed by IEEE 802.3z and combines aspects of the IEEE 802.3 Ethernet standards with elements of the ANSI X3T11 Fibre Channel technology. The result is a fast network that is identical to Ethernet from the Data Link layer upward.

*Gigabit Ethernet Cabling
Gigabit Ethernet can transmit over different media. 1000Base-LX uses long wave lasers over single mode and multimode fiber optic cables, 1000Base-SX uses short wave lasers over multimode fiber optic cables, and 1000Base-CX uses up to 25 meters of STP. Still in development is 1000Base-T, which will use Category 5 or higher UTP up to 100 meters in length.

*Operating Modes
Gigabit Ethernet can operate in either half-duplex or full-duplex modes. In half-duplex mode the IEEE 802.3 CSMA/CD is used, and problems occur when frames are smaller than the slot time. The slot time is the unit of time in bits for Ethernet MAC to handle collisions. When a frame is smaller than the slot time, Gigabit Ethernet adds bits called carrier extensions to a frame to make it bigger than the slot time.

*Frame Bursting
Gigabit Ethernet also uses a technique called frame bursting in the half-duplex CSMA/CD environment. Frame bursting allows a transmitting node to prevent other nodes from transmitting until it has sent all of its frames. This is done by filling the idle time between frames with a stream of bits so that other nodes will see that the network is busy.

*Gigabit Ethernet Advantages
Gigabit Ethernet is expected to become a popular LAN alternative due to its large bandwidth, the use of the IEEE 802.3 Ethernet frame format, and backward compatibility with slower forms of Ethernet. Initial deployment of 1000Base-X will probably be as a fast backbone for 100 Mbps subnetworks.

Question 22


* Exercise 1
Try searching the World Wide Web for more information on terms discussed in this unit.

Examine the following table
Step Action
1 Use your browser to navigate to your favorite search engine (i.e., Yahoo!, LookSmart, Excite, etc.)
2 Perform a search for terms like Fast Ethernet, repeaters, Gigabit Ethernet, or other terms you are interested in.
3 Follow the links to find out more about the terms you selected.


Topic 3.4: Unit 3 Summary

In this unit you learned about the different types of Fast Ethernet, such as 100Base-T4, 100Base-TX, 100Base-FX, and 100VG-AnyLAN. You learned about the cabling requirements for these implementations as well as their guidelines and distance limitations of Fast Ethernet. In addition, you learned how to calculate Fast Ethernet cable lengths when repeaters are used and when fiber optic cable is used in place of a copper segment. You also learned about Gigabit Ethernet.
In the next unit, you will learn about Token Rings and FDDI.

Unit 4. Token Ring and FDDI



In this unit you will learn about Token Rings. You will learn about Token Ring operation and the active monitor. You will also learn the components of Token Ring frames.
In addition, you will learn about FDDI networks. You will learn about FDDI operation and the reliability of the dual ring. You will also learn the components of FDDI frames.

After completing this unit, you should be able to:
  • Identify a Token Ring network and its components

  • Assemble a Token Ring frame

  • Identify an FDDI network and its components

  • Assemble a FDDI frame


This unit does not address any specific Cisco objectives. However, it does provide background information that is essential for the CCNA exam.
In the course index, questions about background information are indicated with the abbreviation BCK and a short description of the question subject matter.

Topic 4.1: Token Ring

*Token Ring Networks
The Token Ring network was developed by IBM. IEEE 802.5 and IBM's Token Ring are nearly identical specifications, and now the terms are virtually synonymous. Token Rings are characterized by their ring topology and the use of an electronic token-passing method to determine media access.

*Token Ring Topology
The Token Ring topology is a logical ring but a physical star. Each node is connected to a central hub called an MSAU (multistation access unit) by STP or UTP cables.

*Multistation Access Unit
Each MSAU can connect up to eight nodes. More nodes may be connected by using stackable Token Ring switches instead of shared MSAUs. This would also allow increased performance and port density.

*Lobe Length
The length of the cable from a node to the MSAU is known as the lobe length. The maximum lobe length for UTP cables is 100 meters. For STP cables this length is 300 meters.

*NICs
Token Ring NICs support transfer rates of either 4 or 16 Mbps, and some Token Ring NICs can be configured for either speed. NICs operating at 4 Mbps cannot receive data from NICs operating at 16 Mbps. If these speeds are mixed on a single Token Ring network, the integrity of the ring is destroyed and the ring will not operate properly.

*Protocol
IEEE 802.5 protocol follows IEEE 802.3 in instances like MAC sublayer and physical layer services and it relies on the 802.2 LLC sublayer and upper-layer protocols for point-to-point services. However, IEEE 802.5 uses a token to determine LAN medium access, while IEEE 802.3 uses CSMA/CD.

Question 23

Question 24

Topic 4.1.1: Token Ring Operation

*Taking Turns Transmitting
Token Rings provide a collision-free network where nodes take turns transmitting. Nodes receive signals from their NAUN (nearest active upstream neighbor) and repeat those signals to their downstream neighbors.

*Token Passing
A node can transmit only when it is in possession of a token, and then it can transmit only a single frame. The node appends the frame to be transmitted to the token and sends it to the next node. If the node has no data to send, it passes the token to the next node where the process is repeated.
Token passing is always the first step in data transmission in Token Ring networks.

*Transmitting a Frame
When a node transmits a frame, the frame travels the ring and is examined by each node in turn. When the destination node receives the frame and recognizes the destination address as its own MAC address, it copies the frame and then tags the original frame in the Frame Status field.
The original frame then continues along the ring until it reaches the originating node, which reads the Frame Status field to see if the frame was received by the destination node and then removes the frame from the ring.

*Early Token Release Feature
There is only one frame on the Token Ring at any given time unless the ETR (early token release) feature is used. The ETR allows a node to send the token onto the ring immediately after transmitting a frame.
The next node would then receive the token while the frame is traversing the ring, in which case it could be possible for that node to transmit a frame while the first frame was still on the ring.

*Ring Access Priority
Certain nodes may require greater access to the ring than other nodes. Ring access priority is accomplished by placing priority and reservation bits on the token.
Nodes must have a priority greater than or equal to that on the token in order to use the token. Nodes can reserve a token only if they have a priority greater than that of the transmitting node.

*Beaconing and Fault Management
Token Rings contain a fault management algorithm called beaconing. Beaconing begins when a node detects a fault, such as not receiving anything from its NAUN. The node then sends out a beacon frame to define the failure domain, which includes the node, its NAUN, and everything in between.
All nodes in the failure domain then undergo autoreconfiguration in an attempt to reconfigure the network around the failed areas. This process is not always successful, and manual reconfiguration is sometimes necessary.

Topic 4.1.2: Token Ring Frames

*Token Ring Frames
Token Ring frames consist of a 1-byte Starting Delimiter, a 1-byte Access Control field, a 1-byte Frame Control field, two 6-byte MAC addresses for the destination and source, a variable-length RIF (Routing Information Field), the encapsulated data, a 4-byte FCS (Frame Check Sequence), a 1-byte Ending Delimiter, and a 1-byte Frame Status field.

*Delimiters
The Starting and Ending Delimiters indicate the beginning and end of the frame or token. The Frame Control field indicates the frame type, and the RIF is used to indicate a route through a network.

*Access Control Field
The Access Control field contains Priority bits (P) to indicate the priority of the frame or token, Reservation bits (R) to indicate the priority required for the next token to access the ring, a Token bit (T) to differentiate a token from a frame, and a Monitor bit (M) to determine if a frame has been on the ring for too long. The pattern for the Access Control field is PPPTMRRR.

*Frame Status Field
The Frame Status field contains an address recognition bit (A-bit) and a copied bit (C-bit). The originating node sets these bits to 0 before sending the frame onto the ring. The receiving node changes the A-bit to 1 if it recognizes its own address and the C-bit to 1 if it successfully copies the frame. When the frame returns to the originating node, the node examines the A-bit and C-bit to see if the frame was successfully delivered.

*Frame Status Field Combinations
There are four possible combinations in the Frame Status field. 00 means the destination was not found. 01 means data was copied but not acknowledged. 10 means data was acknowledged but not copied. 11 means the data was acknowledged and copied, or sent to another ring by a bridge. The Frame Status field is in the form ACrrACrr, where A equals the A-bit and C equals the C-bit.

Question 25

Topic 4.1.3: The Active Monitor

*Active Monitor
There is always one node within the ring that acts as the active monitor. One of the jobs of the active monitor is to check the monitor bit (M-bit) of each frame as it travels around the ring. If this bit is a 0, the active monitor will change it to 1 and then forward the frame. When the active monitor detects an M-bit of 1, it will not forward the frame, thus removing the frame from the ring. This process is how a frame is removed if the node that originated the frame shuts down before the frame returns to it.

*Generating a New Token
Another job of the active monitor is to generate a new token if the old token is lost. The active monitor checks the TRT (token rotation time) and times out if the TRT exceeds a threshold value set by the network administrator — which would mean that a token has been lost.
If a token is lost, the active monitor generates a new token and sends it along the ring.

*Claiming Active Monitor Status
There can be only one active monitor at any given time, but any node on the ring can become the active monitor. For example, if the node that is the current active monitor is shut down, the first node to detect the loss of the active monitor and claim active monitor status will become the new active monitor.

Question 26

Topic 4.2: FDDI

*Dual Ring Configuration
FDDI, or Fiber Distributed Data Interface, is another popular network type. FDDI uses single mode or multimode fiber optic cables configured as a dual ring.
In many respects, FDDI is similar to the Token Ring network, but the dual ring configuration allows greater flexibility and backup capabilities and the fiber optic medium allows faster data transfer rates of 100 Mbps.

*Nodes and Distance
A FDDI LAN may have a maximum of 500 nodes with a maximum distance between nodes (lobe length) of 2 kilometers using multimode fiber optic cables. The maximum circumference of a FDDI ring is 100 kilometers. If single mode cables are used, the maximum distance between two nodes is increased to 60 kilometers.

*Fiber Optic Cable
The use of fiber optic cable makes FDDI a good network choice in instances where there is a large distance between nodes, or where a large bandwidth is needed. The immunity of fiber optic cable to electromagnetic interference also makes FDDI a good choice in hostile environments.

*Other Network Media
Twisted pair cables (UTP and STP) may also be used as network media. If these cables are used, then the network is designated as a CDDI (Copper Distributed Data Interface). FDDI and CDDI are very similar in most respects, but twisted pair cables limit the lobe length to about 100 meters.

Question 27

Topic 4.2.1: FDDI Connections

*Modes
A mode is a ray of light entering a fiber optic cable at a certain angle. Multimode cables use LEDs (light emitting diodes) to generate their signals and allow multiple modes.
This causes modal dispersion, a phenomenon where the different modes enter the cable at different angles and arrive at the destination at different times. Modal dispersion limits the bandwidth and length of multimode cables.

*Single Mode Fiber Optic Cable
Single mode fiber optic cables use lasers to generate the signal and allow only a single mode. There is no problem with modal dispersion in single mode fiber optic cable, so this cable does not limit the bandwidth like multimode cable does. Single mode fiber optic cable provides better connectivity over larger distances than multimode cable.

*FDDI Topology
The topology of a FDDI network is a physical star but a logical ring. Fiber optic cables connect each node to a dual ring network hub, called a DAC (Dual Attached Concentrator).
The two rings are the primary ring (shown in red) and the secondary ring (shown in yellow), and data travels along these rings in opposite directions. Normally, the primary ring is used for data transmission while the secondary ring is idle. This allows for a backup in case the primary ring fails.

*Single Attachment Station
Each node is attached via the DAC to the primary ring of the network. A node with only one connection to the DAC is known as an SAS (Single Attachment Station). An SAS will not affect the operation of the FDDI ring if it is powered down or disconnected.

*Dual Attachment Stations
Some nodes have a double connection to the DAC and are attached to both the primary ring and the secondary ring. These nodes are known as DASs (Dual Attachment Stations).
A powered down or disconnected DAS will affect operation of the FDDI ring, but this can be overcome with the optical bypass switch located in the NIC. This switch uses mirrors to route the light signals through itself to maintain ring integrity when the DAS is disconnected or powered down.

*Dual Homing
A technique called dual homing allows for additional network reliability. In this technique, a dual-homed router or mainframe host has a single attachment to two different DACs on the FDDI dual ring. This provides for an active primary link as well as a backup connection to the FDDI network.

Question 28

Question 29

Topic 4.2.2: Operation

*FDDI Operation
FDDI, like a Token Ring, uses tokens to determine which node may transmit data. Unlike Token Ring, however, FDDI nodes can attach new tokens to the end of a transmission. A downstream node can then attach its own frame to an existing frame and several frames may be on the ring at any given time.

*FDDI Frames
FDDI frames are very similar to Token Ring frames. However, they begin with a 8-byte Preamble, like Ethernet, for clock synchronization. The Preamble is followed by a 1-byte Start Delimiter, a 1-byte Frame Control field, the 6-byte Destination and Source addresses, the transmitted data, a 4-byte Frame Check Sequence, a 1-byte End Delimiter, and a 1-byte Frame Status field.

*Frame Transportation
A frame travels from the source node through the ring, being forwarded by each node in turn, until it gets back to the source node, which removes the frame from the ring by not forwarding it. A FDDI node must see its own MAC address in the frame before it removes the frame.
There are other fields in a frame before the Source Address, and these fields are forwarded before the source node recognizes its own MAC address and stops the forwarding process. This causes extra data to float around the ring. This extra data is periodically scrubbed to clear the network.

*FDDI Nodes and Active Monitors
All FDDI nodes are active monitors, and check for things like lost tokens, persistent frames (frames not removed from the ring by the source node), or faults in the ring. FDDI, like Token Ring, uses beacon frames to find faults. When a node does not receive tokens from its NAUN, the node will send out a beacon frames to find the fault and notify the rest of the ring. When the fault is found, the dual ring loops (or wraps) to form a single ring and thus bypass the problem.

Question 30


* Exercise 1
Try making a diagram of a FDDI network.

Examine the following table
Step Action
1 Create a diagram of a FDDI network with two DACs and four SASs.
2 Now add two DASs.
3 Now add a dual-homed router.


Topic 4.3: Unit 4 Summary

In this unit you learned about Token Ring and FDDI networks. You discovered how these networks operate and how they maintain integrity when nodes fail. You also learned the components of Token Ring and FDDI frames.
You were presented with the basic principles of LAN operation in this course. You learned about LAN implementations such as Ethernets, Fast Ethernets, Token Rings, and FDDIs. You saw how each of these implementations packaged frames, granted access to the media, and controlled errors.
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