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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).
RIPv1 and 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:
- If the router receives information about
a network, and if the receiving interface belongs to that same network (but on a different subnet), the router
applies the subnet mask that is configured on the receiving interface.
- If the router receives information about
a network address that is not the same as the one configured on the receiving interface, it applies the default
(by class) subnet mask.
Despite its limitations, RIP is a very popular routing protocol and is
supported by virtually all IP routers. RIP's popularity stems from its simplicity
and universal compatibility. However, the first version of RIP (RIPv1) suffers from
several critical deficiencies:
- RIPv1 does not send subnet mask information in its
updates. Without subnet information, VLSM and CIDR cannot be
supported
- Its updates are broadcast increasing
network traffic.
- It does not support authentication.
In 1988, RFC 1058 prescribed the new (and
improved) RIP version 2 to address these deficiencies:
- RIPv2 does send subnet information and
therefore supports VLSM and CIDR.
- It multicasts routing updates using the
Class D address 224.0.0.9, providing better efficiency.
- It provides for authentication in its
updates.
Because of these key features, RIPv2 should always be preferred over RIPv1, unless some
legacy device on the network can 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 version 2's features,
you can turn off version 1 support and enable version 2 updates with the following
command:
Router(config)#router rip
Router(router-config)#version 2
RIP's straightforward design ensures that
it will continue to survive. A new version has already been designed to support future IPv6 networks.
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