Dynamic Routing

Add the RIP(routing information protocol) routing protocol to your configuration.

RIP is a distance vector routing protocol that uses hop count as its metric. The maximum hop count is 15 so 16 hops is deemed unreachable.

RIP updates are broadcast every 30 seconds by default.

RIP is enabled by typing.

Router(config)#router rip

This puts you in router configuration mode. You then have to associate attached networks with the RIP process. You only associate directly attached networks.

Router(config-router)#network 192.168.10.0

This would add the 192.168.10.0 network to the routing process.

Add the IGRP routing protocol to you configuration.

IGRP is a distance vector routing protocol designed by Cisco(You can only use cisco routers in this siutation since this protocol is CISCO propertiary). The maximum hop count is 255 default is 100 and it uses a combination of variables to determine a composite metric.

Bandwidth

Delay

Load

Reliability

Maximum Transmission Unit (MTU)

Only Bandwidth & Delay is used by default

Routing updates are sent at 90 second intervals by default.

IGRP is enabled by typing

Router(config)#router igrp 12

Where 12 is the autonomous system number.

You then have to associate directly connected networks in the same way as you did with RIP

Router(config-router)#network 192.168.20.0

This would add the 192.168.20.0 network to the routing process.

List problems that each routing type encounters when dealing with topology changes and describe techniques to reduce the number of these problems.

Distance Vector Concept

Distance vector based routing algorithms pass periodic copies of a routing table from router to router. Regular updates between routers communicate topology changes.

Each router receives a routing table from its direct neighbour and increments all learned routes by one.

This is the way that the algorithm learns the internetwork topology, via second hand information. Distance Vector algorithms do not allow a router to know the exact topology of an internetwork.

RIP and IGRP are Distance Vector Routing Protocols.

Distance Vector Topology Changes

When the topology in a distance vector network changes, routing table updates must occur. As with the network discovery process topology change notification must occur router to router.

Distance Vector protocols call for each router to send its entire routing table to each of its adjacent neighbours.

When a router receives an update from a neighbouring router, it compares the update to its own routing table. If it learns about a better route (smaller hop count) to a network from its neighbour, the router updates its own routing table.

Problems with Distance Vector

Distance Vector routing protocols are prone to Routing Loops and counting to infinity.

Routing loops can occur if the internetwork’s slow convergence on a new configuration causes inconsistent routing entries.

Counting to infinity continuously loops packets around the network, despite the fundamental fact that the destination network is down.

To over come these you can implement

Defining a maximum number of hops.

Specify a maximum distance vector metric as infinity. 16 with RIP and 256 with IGRP.

Split Horizon

If you learn a protocol’s route on an interface, do not send information about that route back out that interface.

Route Poisoning

Information past out on an interface it was learned from is marked as unreachable by setting the hop count to 16 for RIP

Hold Down Timers

Routers ignore network update information for some period.