perrine modified by brierley page 1 chapter 7 ccna2 chapter 7 distance vector protocols

Perrine modified by Brierley

Chapter 7

CCNA2 Chapter 7

Distance Vector Protocols


Chapter 7Dynamic Routing

• Avoids configuration of static routes

• Routers react to changes in the network

• Routers adjust their routing tables accordingly, without the intervention of the network administrator

• There are problems associated with dynamic distance vector routing


Chapter 7

Distance Vector Routing Protocols

• RIP is a distance vector routing protocol that is used in thousands of networks throughout the world

• RIP is based on open standards and is easy to implement makes it attractive to some network administrators

• RIP is a good basic protocol for networking students

• IGRP is another distance vector routing protocol. • Unlike RIP, IGRP is a Cisco-proprietary protocol rather than a

standards-based protocol. • IGRP is simple to implement• IGRP is a more complex routing protocol than RIP and can use

many factors to determine the best route to a destination network. NOTE: for our PacketTracer labs, we’ll use EIGRP)


Chapter 7

Distance vector routing protocols• Require routers to forward their entire routing table when passing along

updates• Routing table information is forwarded to neighbor routers, which continue to

forward the information to their neighbors..• These routing tables include information about the total cost of a route and

the logical address of the first router on the path to each network contained in the table.

• Routers need to update the information in their routing tables to make good path determination decisions.

• Updates may be initiated when topology changes occur • Changes in a network affect the decisions made by a router.

– A router may be taken off line for upgrades or repairs or an interface on a router may go down.

– If not aware of the changes that have occurred in a network, routers may switch packets to interfaces that are no longer connected to the best route.

• Distance vector routing protocols typically send out updates at certain time intervals

– Every 30 seconds for RIP.. – Every 90 seconds for IGRP


Chapter 7

A Network converges when all the routers in the network have the same routing information.

If a link goes down, it is possible that invalid updates will continue to loop through out the network. This is called the count to infinity.

RIP routing protocol counts the count to infinity by hop count. RIPs maximum hop count is 15.

After 15 hops the packet is discarded by RIP.

Routing Loops


Chapter 7

A packet arrives at Router 1 at time t1. Router 1 has already been updated and knows that the optimal route to the destination calls for Router 2 to be the next stop

Router 1 therefore forwards the packet to Router 2. Router 2 has not yet been updated and believes that the optimal next hop is Router 1. Router 2 therefore forwards the packet back to Router 1

The packet will continue to bounce back and forth between the two routers until Router 2 receives its routing update or until the packet has been switched the maximum number of times allowed

This process illustrates the count to infinity problem - there are several solutions to this problem:


Chapter 7

Split Horizon –

Disables the router from sending information about a ‘failed’ route in the routing table. This is done by not sending the information through the same interface that it learned about the failed route

That is, it would prevent Router A from sending the updated information if received from Router B back to Router B

A B

171.10.0.0

Network 171.10.0.0 is down

Get to network 171.10.0.0 via BIs Down!

Split Horizon


Chapter 7

Poison Reverse –

A route that is not ‘good’ is sent a poison reverse which removes the route

Network 4 Network 5

C E

When Network 5 goes down, Router E initiates route poisoning by entering a table entry for Network 5 as 16, for RIP, unreachable.

By this poisoning of the route to Network 5, Router C is not susceptible to incorrect updates about the route to Network 5.

When Router C receives a router poisoning from Router E, it sends an update, called a poison reverse, back to Router E.

This makes sure all routes on the segment have received the poisoned route information.

Poison Reverse


Chapter 7

Holddown –

Is used to prevent regular update messages from reinstating a route that may have gone bad

When a router receives an update from a neighbor indicating that a previously accessible network is not working - is inaccessible, the holddown timer will start

If a new update arrives from a different neighbor with a better metric than the original network entry, the holddown is removed and data is passed

However, if an update is received from the same neighbor router before the holddown timer expires, and it has a lower metric than the previous route, the update is ignored and the holddown timer keeps ticking

One Solution to Count to Infinitive


Chapter 7

RouterA# config t

RouterA(config)# router rip

RouterA(config-router)# network 192.10.10.0



RouterA(config)#int s0

RouterA(config-if)# ip rip triggered

s0s1

e0

192.10.10.0 net

172.120.0.0 net

210.45.20.0 net

If topology changes, this command will ‘triggered’ those updates to the next router. Only applied to a serial interface.

Configure RIP


Chapter 7

RIP uses the following techniques to reduce routing loops and count to infinity. In some cases, configuration is required:

• count-to-infinity

• split horizon

• poison reverse

• holddown counters

• triggered updates

To disable split horizon do:

RouterA(config-if)# no ip split-horizon

RIP Configuration Issues


Chapter 7

To change RIP’s update interval do:

RouterA(config-router)# update-timer <seconds>

To disable sending RIP updates do:

RouterA(config-router)# passive-interface <interface>

Command to receive either version of RIP, do

RouterA(config-if)# ip rip receive version 1

RouterA(config-if)# ip rip receive version 2

RouterA(config-if)# ip rip receive version 1 2



Chapter 7

Router# config term

Router(config)# router rip

Router(config-router)# timers basic update

Intervals between updates

invalid

route is invalid after receiving no updates in secs

holddown

holddown time

flush

when route is flushed from table

update – 30 seconds

holddown - 180 seconds

Administrative Distance - 120



Chapter 7

NOTE for RIP:

Its metric to determine a route to a destination is the hop count.

As a packet goes from router to router, RIP increments a counter called hop count.



Chapter 7

Use the following commands to make RIP verifications:

show ip route

The routing table will have “R” by the routes determined by the RIP routing protocol

show ip protocols

This will verify:

• RIP routing is configured (which protocol is configured)

• Which interfaces are sending & receiving RIP updates

• Which network it is sending information to

RIP Configuration Verification


Chapter 7

Some RIP debugging commands are:

debug ip rip

show ip rip database

show ip interface brief

Debugging Commands for RIP


Chapter 7

NOTE:

Router Rip cannot handle Classless Routing, but Rip ver2 can.

A supernet route (classless route) is a route that covers a greater range of subnets with a single entry. An example a supernet of 172.16.0.0/16 could be 172.16.0.0/13.

However, a router by default assumes that all subnets of a directly connected network should be present in the routing table.

If a packet is received with an unknown destination address within an unknown subnet of a directly attached network, the router assumes that the subnet does not exist, and will drop this packet.

To get around this problem, use a global command: ip classless.

Classless Routing


Chapter 7

Load-balancing describes the ability of a router to transmit packets to a destination IP address over more than one path

When a router learns multiple routes to a specific network, the route with the lowest administrative distance is entered into the routing table

To set maximum number of parallel paths:

RouterA(config-router)#maximum-paths [number]

RIP – Load Balancing


Chapter 7

Route Source Default Distance

Connected interface 0

Static route 1

EIGRP summary route 5

External BGP 20

Internal EIGRP 90

External EIGRP 170

IGRP 100

OSPF 110

IS-IS 115

RIP 120

EGP 140

Internal BGP 200

Unknown 255

Administrative Distance


Chapter 7

Floating static routes are static routes configured with an administrative distance value that is greater than that of the primary route (or routes).

Essentially, floating static routes are fallback routes, or backup routes, that do not appear in the routing table until another route fails.

Example:

RouterA(config)#ip route 200.10.10.0 255.255.255.0 192.16.10.1 30

Floating Static Routes


Chapter 7

RTA(config)# ip route 0.0.0.0 0.0.0.0 s0

RTA(config)# router rip

RTA(config-router)# default-information originate

For RIP, if a static route is assigned to an interface that is not one of the networks defined in a network command, no dynamic routing protocols advertise the route. Use redistribute static command.

To redistribute static default route, must use the default-information originate command.

Example:

RIP – Redistribute Static Routes


Chapter 7

IGRP:

• must be assigned an “AS” (autonomous system # - 16 bit number)

• Cisco proprietary

• distance-vector

• metrics

• delay

• bandwidth (1200 bps - 10 Gbps)

• reliability (1-224) (higher the number, more reliable)

• load (1-244) (higher the number, more it is under load)

• sends updates every 90 seconds• maximum hop count is 255 (default 100)

IGRP


Chapter 7

IGRP has number of features that are designed to enhance its stability:

• holddowns

• split horizons

• poison reverse updates

IGRP


Chapter 7

Router# config term

Router(config)# router igrp 100

Router(config-router)# timers basic 90 270 280 630 [Default settings]

Intervals between updates

route is invalid after receiving no updates in secs

holddown time

when route is flushed from table

update invalid holddown flushRouter(config-router)# timers basic

Setting IGRP Basic Timers


Chapter 7

RouterA# config t

s0s1

e0

192.10.10.0 network address



RouterA(config)# router igrp 101




Configure IGRP


Chapter 7

Helpful commands for troubleshooting IGRP:

• show ip protocols

• show ip route

• debug ip igrp events

• debug ip igrp transactions

• ping

• traceroute

Troubleshooting IGRP

perrine modified by brierley page 1 chapter 7 ccna2 chapter 7 distance vector protocols

Documents

routing information

rip routing protocol

routing tables

dynamic routing

routing update

routing loops

network routers

complex routing protocol