CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 75 doc

Upon completion of this chapter, you will be able to ■ Describe initial router conﬁguration ■ Describe RIP characteristics ■ Describe IGRP characteristics ■ Describe and test load balanc

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Upon completion of this chapter, you will be able to

■ Describe initial router conﬁguration

■ Describe RIP characteristics

■ Describe IGRP characteristics

■ Describe and test load balancing over multiple paths

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Chapter 16

Distance Vector Routing Protocols

Now that you have learned about routing protocols, you are ready to conﬁgure IP routing protocols As you know, routers can be conﬁgured to use one or more IP routing protocols

In this chapter, you learn the initial conﬁguration of the router to enable the Routing Information Protocol (RIP) and the Interior Gateway Routing Protocol (IGRP) In addition, you learn how to monitor IP routing protocols

Please be sure to look at this chapter’s associated e-Labs, Videos, and PhotoZooms that you will ﬁnd on the CD-ROM accompanying this book These CD elements are designed

to supplement the material and reinforce the concepts introduced in this chapter

Initial Router Conﬁguration

After testing the hardware and loading the Cisco IOS Software image, the router finds and applies the configuration statements These entries provide the router with details about router-specific attributes, protocol functions, and interface addresses Remember that if the router cannot locate a valid startup-config file, it enters an initial router config-uration mode called setup mode or system configconfig-uration dialog

With the setup mode command facility, you can answer questions in the system configu-ration dialog This facility prompts you for basic configuconfigu-ration information The answers that you enter enable the router to build a sufficient but minimal router configuration

The setup facility provides the following:

■ An inventory of interfaces

■ An opportunity to enter global parameters

■ An opportunity to enter interface parameters

■ A setup script review

■ An opportunity to indicate whether or not you want the router to use this conﬁguration

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712 Chapter 16: Distance Vector Routing Protocols

After you confirm setup mode entries, the router uses the entries as a running configu-ration The router also stores the configuration in nonvolatile random-access memory (NVRAM) as a new startup-config, and you can start using the router For additional protocol and interface changes, you can then use the enable mode and enter the

com-mand conﬁgure.

Distance Vector Routing

This section discusses distance vector routing protocols and their shortcomings, as well

as identiﬁes solutions to the problems presented by distance vector routing Distance vector–based routing logarithms pass periodic copies of a routing table from router to router These regular updates between routers communicate topology changes

Maintaining Routing Information Through Distance Vector Protocols

Routing tableupdates occur periodically when the topology in a distance vector proto-col network changes It is important for a routing protoproto-col to be efﬁcient in updating the routing tables As with the network discovery process, topology change updates proceed systematically from router to router Figure 16-1 illustrates how distance vector protocols handle topology changes

Figure 16-1 Distance Vector Topology Changes

Distance vector algorithms instruct each router to send its entire routing table to each

of its adjacent neighbors The routing tables include information about the total path cost, as deﬁned by the metrics, and the logical address of the advertising router on the path to each network

B

Process to Update This Routing Table

Router A Sends Out This Updated Routing Table

A

Process to Update This Routing Table

Topology Change Causes Routing Table Update

chpt_16.fm Page 712 Tuesday, May 27, 2003 2:16 PM

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Distance Vector Routing 713

Load Balancing Across Multiple Paths

Load balancing describes the capability of a router to transmit packets to a destination

IP address over multiple paths Load balancing is a concept that allows a router to take

advantage of multiple paths to a given destination The paths are derived either

stati-cally or with dynamic protocols, such as RIP, Enhanced IGRP (EIGRP), Open Shortest

Path First (OSPF), and IGRP Figure 16-2 shows an example of load balancing

When a router learns of multiple routes to a speciﬁc network through multiple routing

processes or routing protocols, it installs the route with the lowest administrative

dis-tance into the routing table Sometimes, the router must choose from many routes

pro-vided by the same routing process with the same administrative distance In this case,

the router chooses the path with the lowest cost or metric to the destination Each

routing process calculates its cost differently, and the costs might need to be manually

conﬁgured to achieve load balancing

If the router receives and installs multiple paths with the same administrative distance

and cost to a destination, load balancing can occur Cisco IOS Software imposes a six

equal-cost routes limit on the routing table, but some Interior Gateway Protocols (IGPs)

set their own limitations For example, EIGRP allows up to four equal-cost routes

By default, most IP routing protocols install a maximum of four parallel routes in a

routing table Static routes always install six routes The exception is the exterior

rout-ing protocol Border Gateway Protocol (BGP), which by default allows only one path

to a destination

The number of maximum paths ranges from one to six paths To change the maximum

number of parallel paths allowed in a routing table, use the following command while

in router conﬁguration mode:

IGRP can load balance up to six unequal links RIP networks must have the same hop

count to load balance, whereas IGRP uses bandwidth to determine how to load balance

In Figure 16-2, there are three ways to access Network X:

■ E to B to A with a metric of 30

■ E to C to A with a metric of 20

■ E to D to A with a metric of 45

Router E chooses the second path, E to C to A with a metric of 20, which is a lower

cost than 30 and 45 If two or more of the paths had the same metric, load balancing

could occur

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Figure 16-2 Load Balancing

When routing IP, Cisco IOS Software offers two methods of load balancing:

■ Per-packet load balancing

■ Per-destination load balancing

If process switching is enabled, the router alternates paths on a per-packet basis If fast switching is enabled, only one of the alternate routes is cached for the destination address, so all packets in the packet stream bound for a speciﬁc host take the same path Packets bound for a different host on the same network might use an alternate route, in which case trafﬁc is load balanced on a per-destination basis

How Routing Loops Occur in Distance Vector

Routing loops can occur if a network experiences slow convergenceas the result changes

in the network or routing topology causing inconsistent routing entries Figure 16-3 demonstrates routing loops

Figure 16-3 Routing Loops

E

B

C

D

A

20

10

20

10

25

Net X

C

D

B

1

X

Network 1 Unreachable

Alternate Route:

Network 1, Distance 3

Alternate Route: Use

Network 1 Down

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The process of how a routing loop occurs (based on Figure 16-3) is as follows:

1. Just before the failure of Network 1, all routers have consistent knowledge and

correct routing tables The network is said to have converged Assume for the remainder of this example that for Router C, the preferred path to Network 1 is

by way of Router B, and the distance from Router C to Network 1 is three

2. When Network 1 fails, Router E sends an update to Router A Router A stops

routing packets to Network 1, but Routers B, C, and D continue to do so because they have not yet been informed of the failure When Router A sends out its update, Routers B and D stop routing to Network 1 However, Router C has not received an update To Router C, Network 1 is still reachable through Router B

3. Now Router C sends a periodic update to Router D, indicating a path to

Net-work 1 by way of Router B Router D changes its routing table to reﬂect this incorrect information, and sends the information to Router A Router A sends the information to Routers B and E, and so on Any packet destined for Network

1 now loops from Router C to B to A to D and back again to C

Deﬁning a Maximum to Prevent Count to Inﬁnity

The invalid updates of Network 1 continue to loop until some other process stops the

looping This condition, called count to inﬁnity, loops packets continuously around

the network in spite of the fundamental fact that the destination network, Network 1,

is down While the routers are counting to inﬁnity, the invalid information allows a

routing loop to exist, as illustrated in Figure 16-4

Figure 16-4 Counting to Inﬁnity

Without countermeasures to stop the process, the distance vector or the metric of hop

count increases each time the packet passes through another router Metrics are

cov-ered in Chapter 15, “Routing and Routing Protocols.” These packets loop through the

network because of incorrect information in the routing tables

C

D

B

1

X

≠

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Distance vector routing algorithms are self-correcting, but a routing loop problem can require a count to infinity to resolve To avoid this prolonged problem, distance vector protocols define infinity as a specific maximum number This number refers to a routing metric, which might simply be the hop count Figure 16-5 demonstrates this concept

Figure 16-5 Deﬁning a Maximum Metric

By assigning a maximum number to inﬁnity, the routing protocol permits the routing loop to continue until the metric exceeds its maximum allowed value Figure 16-5 shows the metric value as 16 hops, which exceeds the distance vector default maximum

of 15 hops, and the router discards the packet In any case, when the metric value exceeds the maximum value, Network 1 is considered unreachable

Eliminating Routing Loops Through Split Horizon

Another possible source for a routing loop occurs when incorrect information that has been sent back to a router contradicts the correct information that the router originally distributed The following process, as shown in Figure 16-6, explains how this problem occurs:

1. Router A passes an update to Router B and Router D indicating that Network 1

is down However, Router C transmits an update to Router B indicating that Network 1 is available at a distance of four by way of Router D This action does not violate split-horizon rules

2. Router B concludes, incorrectly, that Router C still has a valid path to Network 1, although at a much less favorable metric Router B sends an update to Router A, advising Router A of the new route to Network 1

C

D

B

1

X

Routing Table Maximum metric is 16.

Network 1 is unreachable.

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3. Router A now determines that it can send information to Network 1 by way of

Router B Router B determines that it can send information to Network 1 by way

of Router C, and Router C determines that it can send information to Network 1

by way of Router D Any packet introduced into this environment loops between the routers

4. Split horizon attempts to avoid this situation If a routing update for Network 1

arrives from Router A, then Router B and Router D cannot send information about Network 1 back to Router A, as in Figure 16-6 Split horizonthus reduces incorrect routing information and reduces routing overhead

Figure 16-6 Split Horizon

Route Poisoning

Route poisoning is used by various distance vector protocols to overcome large

rout-ing loops and offer explicit information when a subnet or network is not accessible

Route poisoning is usually accomplished by setting the hop count to one more than the

maximum

Poison reverse is another way of avoiding routing loops Its rule states:

Once you learn of a route through an interface, advertise it as unreachable back through that same interface.

Assume that the routers in Figure 16-7 have poison reverse enabled When Router One

learns about Network A from Router Two, it advertises Network A as unreachable

C

D

B

X

B: Do Not Update Router A About Routes to Network 1

D: Do Not Update Router A About Routes to Network 1

Network 1 Unreachable

Network 1 Down

1

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through its link to Routers Two and Three Router Three, if it shows any path to Net-work A through Router One, removes that path because of the unreachable advertise-ment EIGRP combines these two rules to help prevent routing loops

Figure 16-7 Route Poisoning

EIGRP uses split horizon or advertises a route as unreachable when

■ Two routers are in startup mode (exchanging topology tables for the ﬁrst time)

■ Advertising a topology table change

■ Sending a query When route poisoning is used with triggered updates, it will speed up convergence time because neighboring routers do not have to wait 30 seconds before advertising the poisoned route Route poisoning causes a routing protocol to advertise infinite-metric routes for a failed route Route poisoning does not break split-horizon rules Split horizon with poison reverse is essentially route poisoning, but specifically placed on links that split horizon would not normally allow routing information to flow across

In either case, the result is that failed routes are advertised with inﬁnite metrics

Avoiding Routing Loops with Triggered Updates

New routing tables are usually sent to neighboring routers on a regular basis RIP updates occur every 30 seconds However, a triggered update is sent immediately in response to some change in the routing table The router that detects a topology change immediately sends an update message to adjacent routers Those routers then generate triggered updates, notifying their adjacent neighbors of the change When a route fails, an update is sent, rather than waiting on the update timer to expire The use of triggered updates, in conjunction with route poisoning, ensures that all routers know of failed routes before any hold-down timers can expire

One

Two

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Four

b: 56 d: 2000

b: 128 d: 1000

b: 10000 d: 100

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