CCNA 2 Week 7 Distance Vector Routing. Copyright © 2005 University of Bolton Topics Distance Vector Protocol Issues RIP IGRP

CCNA 2 Week 7

Distance Vector Routing

Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton

Topics

Distance Vector Protocol Issues

RIP

IGRP


Routing Updates

Routing tables are learned dynamically from neighbours

Updates occur periodically and when topology changes

Each router sends its entire routing table to its neighbours


Routing Loops

Net1 Direct FA0/0Net1 3hop via Rtr B Net1 1Hop via Rtr E

Net1 2Hop FA0/0

Net1 2Hop via Rtr A

Net1 unreachable

X

Net1 unreachable

Net1 unreachable

Net1 unreachable

Net1 4hop via Rtr C

Net1 5hop via Rtr D


Maximum Hop Count

In a routing loop, traffic continuously circulates despite target network being down

Routers count to infinity while packets loop

DV protocols define infinity as specific maximum number to prevent endless looping

If max is 15, networks become unreachable once count is exceeded.


Split Horizon Rule

Split horizon is used to avoid loop situation.

If a routing update about Network 1 arrives from Router A, Router B or Router D cannot send information about Network 1 back to Router A.

Split horizon reduces incorrect routing information and routing overhead.


Route Poisoning

If a network goes down, router can set a distance vector one greater than maximum to poison route

Once poisoned update propagates other routers are unaffected by incorrect updates

Router C sends poison reverse back to E so that it can't learn an incorrect route either


Triggered Updates

Protocols like RIP send updates periodically every 30 seconds

Triggered updates are sent immediately when the routing table changes, with neighbours also forwarding the changes immediately

When used with route poisoning, triggered updates help to ensure unreachable networks are correctly propagated


Holddown Timers

When a router receives an update marking a route as down the router marks the route as inaccessible and starts a holddown timer

While timer running:– An update from the same neighbour saying route is back then

route restored, timer cancelled– An update from a different neighbour with a better metric then the

router marks the network as accessible and removes the timer. – If an update is received from a different router with a higher

metric before the holddown timer expires, the update is ignored

RIP Protocol


RIP

Widely implemented standard

Uses a maximum hop count (15) to prevent routing loops

Implements features such as holddown timers and split horizon to prevent loops

RIP v2 supports extra features


Basic Setup

Updates sent every 30 seconds unless triggered

Received hop count increased by 1

Routers use best path (lowest hop count)


Classless routing

Sometimes a router receives packets destined for an unknown subnet of a network that has directly connected subnets – won't use default

Rtr(glaboal)# ip classless forwards packets to best supernet route.

A supernet route covers a range of subnets in one entry.

e.g. 10.10.0.0 /16, is a supernet route for 10.10.10.0 /24

The ip classless command is enabled by default since IOS 11.3. To disable use the no ip classless


Further configuration

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

Router(config-router)#timers basicupdate invalid holddown flush [sleeptime ]

RIP timer defaults 30s, 180s, 180s, 240s [N/A]

Holddown should be just greater than max time to propagate updates around network


Further configuration

Turn off broadcasts on unwanted interfaces– BHM(config-router)# passive-interface FA0/0

On networks that don't support broadcast– BHM(config-router)# neighbor ip address


Verify RIP

show ip protocols– shows how RIP is configured– details of interfaces– networks being advertised

show ip route– contents of routing table– direct and learned routes

Additional information:– show interfaceinterface – show ip interfaceinterface – show running-config


Troubleshooting

debug ip rip– displays routing updates as received

Other debug commands– show ip rip database – show ip protocols {summary} – show ip route – debug ip rip {events} – show ip interface brief


Preventing Updates

Route filtering regulates information advertised or received into routing table

Only apply to Distance Vector protocols

Router(config-router)# passive interface s0/0– Prevents route table being sent out via interface– Will still accept incoming routing tables


Load Balancing

RIP will load balance up to 6 equal cost routes

Cost refers to the routing metric– Hop count for RIP– Takes no account of link speed

Round-robin balancing 1-2-3-4-1-2-3-4


Load Balancing in detail

Where several protocols in use, router adds route with lowest administrative distance in table

– Effectively a priority list for routing protocols

Two routes with the same AD – lowest metric used

Same AD and metric – load balancing occurs

Limits vary by protocol, can set using:Router(config-router)# maximum-paths [number ]


Static routes

Administrator can configure routes manually if none learned – often used for gateway route

Allow static routes to be overridden by learned routes by setting a high AD

Static routes pointing out an interface are redistributed via RIP

Static routes not associated with a network command only sent if redistribute static command used

IGRP

Interior Gateway Routing Protocol


Routing metric

Uses composite metric– Bandwidth – The lowest bandwidth value in the path – Delay – The cumulative interface delay along the path – Reliability – The reliability on the link toward the destination as

determined by the exchange of keepalives – Load – The load on a link toward the destination based on bps

show ip route displays five weightings K1-K5

By default bandwidth and delay are used


IGRP routes

InteriorBetween subnets of a route attached to an interface

If not subnetted, IGRP does not advertise

SystemOther networks within Autonomous System

Derived from direct connections and other routers

ExteriorBetween Autonomous Systems

Gateway chosen from list of exterior routes in IGRP routing table


Avoiding Loops

IGRP supports– Split horizon– Holddown timers– Poison Reverse Updates

Increasing metric by factor of 1.1 triggers poison reverse update to place route in holddown

Timers as before include Update, Holddown, Invalid and Flush – default update time is 90s


Configure IGRP

BHM(config)# router igrp 101– 101 is the AS number and does vary!

BHM(config-router)# network 192.233.99.0

Cancel either command using the no prefix


Debug IGRP

show ip route– IGRP routes prefixed with 'I'

Useful commands– show interfaceinterface – show running-config – show running-config interfaceinterface – show running-config | begin interfaceinterface – show running-config | begin igrp – show ip protocols


Debug commands

show ip protocols

show ip route

debug ip igrp events

debug ip igrp transactions

ping

traceroute


Documents

CCNA 2 Week 7 Distance Vector Routing. Copyright © 2005 University of Bolton Topics Distance Vector Protocol Issues RIP IGRP