Routing Information Protocol RIP I and II

Routing Information Protocol (RIP) is a distance vector protocol, which uses hop counts as the metric. This metric determines whether an IP network will be placed in the routing table. There are two versions of RIP—RIP I and RIP II.

Both RIP versions I and II operate over UDP using port 520. As discussed in Chapter 2, RIP uses holddowns, split horizon, and poison reverse to avoid routing loops. RIP version I cannot carry subnet information, and it applies the default network mask to all networks. Hence, all networks within a RIP network must have the same subnet mask throughout. RIP II does carry subnet information, because RIP II can use variable length subnet masking (VLSM).

RIP version I characteristics can be summarized as follows:

► Distance vector protocol

► Broadcasts full routing table every 30 seconds

► RIP can load balance if the hop count is the same

RIP version II characteristics can be summarized as follows:

► Distance vector protocol

► Supports VLSM (carries subnet information in updates)

► Authentication of IP routing updates

The deficiencies encountered in RIP I, such as no support for variable length subnet masks, spurred the release of RIP version II. RIP II's biggest improvement over RIP I is that RIP II supports variable length subnetting and authentication of routing updates. RIP II is also a classless routing protocol, whereas RIP Version I is classful. RIP II still has a hop count limit of 15. Furthermore, RIP II supports multicast updates. While RIP II provides advancements over RIP I, it still contains some of the deficiencies found in RIP I. For example, RIP II continues to send full routing updates every 30 seconds and limits hop counts to 15 hops.

Cisco routers support both RIP I and RIP II. By default a Cisco router runs RIP version I. Therefore, by default, the router will listen to RIP II updates but will only forward RIP I updates unless configured otherwise.

Note: In order to configure RIP version II, you will have to be in the "router rip" configuration mode.

To further clarify the operation of RIP I and RIP, let's look at a sample RIP configuration on a small network consisting of four Cisco routers.

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