1 1-Dec-15 S Ward Abingdon and Witney College Distance Vector Routing CCNA Exploration Semester 2...

18 Apr 2023 S Ward Abingdon and Witney College 1

Distance Vector Routing

CCNA Exploration Semester 2

Chapter 4


Topics

Characteristics of distance vector routing protocols

Distance vector routing protocols in use today How they discover routes How they maintain routing tables Routing loops


RIP v1RIP v2IGRPEIGRP

Routing protocols

Interior Exterior

Distance vector Link state

OSPFIS-IS

EGPBGP


Distance vector knowledge

A distance vector protocol learns: The distance to a network, measured in hops or in

some other way The direction of the network: which port should be

used to reach it It puts the routes in the routing table It does not know any more details of the route

or the other routers along the way


Distance vector

Network 192.168.48.0 is 3 hops away using

port fa0/0

Network 192.168.22.0 is 2 hops away using

port fa0/1


Link state knowledge

A link state routing protocol finds out about all the routers in the system and the networks they link to.

It builds up a complete picture of the topology It can then work out the best path to any

network It puts these best paths in the routing table


Link stateI know all the routers and paths in

this system of networks.


Metrics

RIP v1 and 2 hop count, maximum 15

IGRP and EIGRP bandwidth, delay,load, reliability


Distance vector

Exchange complete routing tables with immediate neighbours

Do this at regular intervals Adjust the metric, e.g. add 1 to the hop count,

or add number based on bandwidth and delay of link.


Adjust the metric

192.168.13.0 is 2 hops

away

192.168.13.0 is 3 hops

away

Send update


Sending updates

RIP v1 Whole routing tableBroadcast every 30 sec

RIP v2 Whole routing tableMulticast every 30 sec

IGRP Whole routing tableBroadcast every 90 sec

EIGRP Initial learning process then small updates when topology changes


RIP v1RIP v2IGRP

Routing protocols

Distance vector Link state

OSPFIS-IS

EIGRP

Slow to convergeEasy to configureSmall networksLittle use of resources

Fast to convergeHarder to configureLarge networksMuch use of resources


Distance vector updates

Routers start up.

R1 adds directly connected networks to table.

Network Interface Hop

10.1.0.0 Fa0/0 0

10.2.0.0 S0/0/0 0

10.1.0.0 10.2.0.0 10.3.0.0 10.4.0.0



Exchange of routing table information.

10.1.0.0 10.2.0.0 10.3.0.0 10.4.0.0



R1 has learned about 10.3.0.0 from R2.

It does not know about 10.4.0.0


10.1.0.0 Fa0/0 0

10.2.0.0 S0/0/0 0

10.3.0.0 S0/0/0 1

10.1.0.0 10.2.0.0 10.3.0.0 10.4.0.0



Exchange of routing table information.

10.1.0.0 10.2.0.0 10.3.0.0 10.4.0.0



R1 has learned about 10.4.0.0 from R2.

R2 previously learned about it from R3.


10.1.0.0 Fa0/0 0

10.2.0.0 S0/0/0 0

10.3.0.0 S0/0/0 1

10.4.0.0 S0/0/0 2

10.1.0.0 10.2.0.0 10.3.0.0 10.4.0.0


Update timer

R 10.3.0.0 [120/1] via 10.2.0.2, 00:00:04, Serial0/0

Show ip route gives number of seconds since last update.

Routing Protocol is “rip”Sending updates every 30 seconds, next due in 3 seconds

Show ip protocols says when next update is due.

Update timer default is 30 seconds


RIP timers

Routing Protocol is “rip”Sending updates every 30 seconds, next due in 26 secondsInvalid after 180 seconds, hold down 180, flushed after 240

R 10.3.0.0 [120/1] via 10.2.0.2, 00:00:04, Serial0/0 R 10.4.0.0 [120/2] via 10.2.0.2, 00:00:04, Serial0/0

Routing table contains two RIP routes


RIP timers



30 seconds – updateRoute to 10.3.0.0 refreshedRoute to 10.4.0.0 not included


RIP timers





RIP timers





RIP timers





RIP timers





RIP timers





RIP timers





RIP timers


R 10.3.0.0 [120/1] via 10.2.0.2, 00:00:00, Serial0/0


Route has been removed.


RIP_JITTER

RIP updates can become synchronised This is a problem if routers are linked by hubs

because the updates will collide RIP_JITTER is a random variable that makes

updates vary a little from the default 30 seconds


Triggered updates

These are to speed up convergence Interface goes up/down, route

added/removed Router detects change, sends update to

neighbour at once without waiting for timer Neighbour passes on update at once.


EIGRP

Does not send regular updates Does not send its whole routing table Sends only information about changes Sends only to routers that need the

information Non-periodic, partial, bounded.


Routing loop

A packet is sent from router to router in a loop until it is eventually dropped when its TTL field drops to 0

Caused by incorrect or out of date information in routing tables

Very bad for network – uses up bandwidth and processing power in routers


Avoiding routing loops

Defining a maximum metric to prevent count to infinity

Holddown timers Split horizon Route poisoning or poison reverse Triggered updates


Maximum metric

Routers exchanging wrong information can report higher and higher values of the metric.

RIP sets a maximum metric. The hop count can go up to 15. If it reaches 16 then the route is regarded as

unreachable.


Holddown timers

Router receives update saying that a network is down.

Router marks the network as possibly down and starts holddown timer.

Update with a better metric for that network arrives: network is reinstated and holddown timer removed.

Update with the same or worse metric for that network arrives: update is ignored.

Timer runs out : network removed from table. Packets still forwarded to network while timer runs.


Split horizon

Router receives information about a route through an interface.

It will not send out information about the same route through that interface.

Route to 10.1.1.0 in

3 hops

Route to 10.1.1.0 in

4 hops


Route poisoning

A router detects that a route has gone down. It marks that route as unreachable in its

routing table. (16 hops for RIP) It sends out updates that show the route as

unreachable. Neighbour routers pass on these “poison”

updates.


Poison reverse

This is an exception to split horizon. If a router receives an update marking a route

as unreachable then it will send this information back to the router that sent it.


RIP v1 and RIP v2

RIP v1 Classful, does not send

subnet mask in updates so does not support VLSM

Sends updates as broadcasts

No authentication No manual route

summarisation

RIP v2 Classless, includes the

subnet mask in routing updates, so supports VLSM.

Sends updates as multicasts

Authentication for security

Supports manual route summarization.


RIP v2 or EIGRP?

RIP runs on any make of router, EIGRP only on Cisco routers.

EIGRP is suitable for large networks EIGRP uses a more efficient metric and may

choose faster routes. EIGRP converges faster than RIP EIGRP uses less bandwidth but it needs

more processing power and RAM RIP is simpler to configure


The End

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