Dynamic Routing Protocols

A Brief Overview

March 22, 2001Professor Tom Mavroidis

Why Dynamic Routing

• For small networks static routing is fine

• As the network grows it becomes impossible to keep the routing tables consistent

• Dynamic Routing lets the network keep itself up to date

Routing Server or Routing Daemon

• On the internet gateway systems maintain up to date information about routes.

• Each router propagates its routing information to other routers in the network

• They also update their routing tables from information sent by other routers

• This process is known as the routing server or routing daemon

Dynamic Routing...

• Determines the information placed placed in the routing table

• If multiple routes are found the best route will be inserted into the routing table (choice is made according to a routing policy in the router)

Routing Protocols

• Two Classifications– link-state protocols– vector-distance protocols

ICMP Redirect Messages

• Simplest form of dynamic routing

• Host systems are initialized with minimal routing information

• If the router discovers better route to a particular host it send an ICMP redirect message to the router closer to the source who updates its routing table.

Vector-Distance Protocols

• Many vector-distance Protocols have been superseded by link-state Protocols (such as RIP and Hello)

• Vector-distance Protocols forward routes across the network form router to router

• Routes are sent as a table (Destination/Distance)

• If the destination does not exist in the routing table a new route is added

Vector-Distance continued

• If a route already exists it is updated– both longer and shorter routes are updated since

routes may have changed by a router coming off line or going on line

• Rip is the most popular vector-distance protocol– was made available through the 4.2 BSD version of

Unix– Limits hops to 16 (therefore limits size of network)

Link-State Protocols

• OSPF - Open Shortest Path First– Uses a graph searching algorithm– Updates don’t contain routes only status of

directly connected links– When status changes the router rebuilds its

graph and re-computes the routes

Advantages to Link-state Routing

• Each router re-computes routes independently

• Routers are guaranteed to converge - (agree between each other)

• Link Status messages pass across the network unchanged (easier to debug)

• Scaleable - message size doesn’t depend on the number of networks

• Generates less traffic on the network

Other Interior Gateway Protocols

• ICMP Router Discovery Protocol– host selects the most appropriate default route

– work on an advertise/solicit mechanism

– host broadcasts a router solicitation message on startup

– Receiving routers send back an advertisement packet containing its IP address and a preference value

– host will select and update is routing table based on the best preference value

Other Interior Gateway Protocols

• Hello – uses a time delay measurement to choose the

best route

• IDRP - InterDomain Routing Protocol

• IS-IS - Intermediate system to Intermediate system

Exterior Gateways

• A collection of networks and routers under a single administrative control is known as an Autonomous System

• Central core network provides a method of connecting constituent networks

• Admin of the subsections is delegated to the core network

• Packets destined out of the interior network are forwarded to the exterior gateway

Exterior Gateways

• Greatly increase the scalability of the Internet backbone

• Alleviate bandwidth problems

• Keeps the size of of the routing tables to a minimum

Linux

• The routed daemon implemented dynamic routing based on RIP

• A modern replacement for routed is gated– supports more protocols