Routing Information Protocol (RIP)

---

The Routing Information Protocol (RIP) is a distance vector routing protocol that uses a hop count to determine the least-cost path to a destination network.

• only considers a single piece of information about the network topology:
  • the next hop router to reach a given network or subnet (vector)
• considers only one metric to select the optimal path to a given destination network:
  • lowest hop count (distance)
• no longer widely deployed
  • useful to understand the function of more advanced protocols
• sends regular updates (every 30s) of its entire routing database to neighboring routers
  • can also send triggered updates whenever changes occur
  • when a router receives an update from a neighbor,
    • it adds unknown routes to its own routing table
    • increases the hop count by one
    • identifies the originator of the updates as the next hop to the specified networks

Example 1

RIP has been used to propagate route information between three routers connected in a chain.

• Router A learns about networks 10.0.3.0/24 and 10.0.4.0/24 from Router B
  • adds 1 to the hop count metric of these routes
• Router B learns about 10.0.1.0/24 from Router A and about 10.0.4.0/24 from Router C
• Router A and Router C do not exchange any information directly
• the distance vector process by which Router A learns about Router C’s networks is often referred to as “routing by rumor”

Example 2

This example illustrates a mesh topology where there are multiple paths between networks.

• Router A has two possible paths to network 10.0.3.0/24
  • learned from Router B and Router C
  • it can forward a packet out of its G1 interface over network 10.0.2.0/24
    • will take 1 hop to reach the destination
  • could also forward the packet out of G2 and reach the destination via Router C and then Router B
    • this takes 2 hops, so is not used as the preferred route

If Router A’s G1 link goes down, those entries will be removed from the routing table and the alternative routes via 10.0.4.0/24 will be selected:

• to help prevent overlooping
  • the maximum hop count allowed is 15
  • consequently, this limits the maximum size of a RIP network
    • since networks that have a hop count of 16 or higher are unreachable