![]() ![]() |
2.3 | ![]() |
VLSM | |
2.3.2 | ![]() |
Classless and classful routing protocols |
For routers in a variably subnetted
network to properly update each other, they must send masks in their
routing updates. Without subnet information in the routing updates,
routers will have nothing but the address class and their own subnet
mask to go on. Only routing protocols that ignore the rules of address
class and use classless prefixes will work properly with VLSM. See the
figure. Routing Information Protocol version 1 (RIP v1) and Interior Gatway Routing Protocol (IGRP), common interior gateway protocols, cannot support VLSM because they do not send subnet information in their updates. Upon receiving an update packet, these classful routing protocols will use one of the following methods to determine the network prefix of an address:
Despite its limitations, RIP is a very popular routing protocol and is supported by virtually all IP routers. The popularity of RIP stems from its simplicity and universal compatibility. However, the first version of RIP, RIP v1, suffers from several critical deficiencies:
In 1988, RFC 1058 prescribed the new and improved RIP version 2 (RIP v2) to address these deficiencies:
Because of these key features, RIP v2 should always be preferred over RIP v1 unless some legacy device on the network does not support it. When RIP is first enabled on a Cisco router, the router listens for version 1 and 2 updates, but sends only version 1. To take advantage of the RIP v2 features, turn off version 1 support and enable version 2 updates with the following command:
The straightforward RIP design ensures that it will continue to survive. A new version has already been designed to support future IPv6 networks.
|