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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