3Com Switch 8800 Advanced Software V5 Configuration Guide

RIP Overview 271
■ Poison reverse. A router sets the metric of routes received from a neighbor to
16 and sends back these routes to the neighbor to help delete useless
information from the neighbor’s routing table.
■ Triggered updates. A router advertises updates once the metric of a route is
changed rather than after the update period expires to speed up the network
convergence.
RIP Version RIP has two versions, RIP-1 and RIP-2.
RIP-1, a Classful Routing Protocol, supports message advertisement via broadcast
only. RIP-1 protocol messages do not carry mask information, which means it can
only recognize routing information of natural networks such as Class A, B, C. That
is why RIP-1 does not support discontiguous subnet.
RIP-2 is a Classless Routing Protocol. Compared with RIP-1, RIP-2 has the following
advantages.
■ Supporting route tags. The route tag is used in routing policies to flexibly
control routes.
■ Supporting masks, route summarization and classless inter-domain routing
(CIDR).
■ Supporting designated next hop to select the best next hop on broadcast
networks.
■ Supporting multicast routing update to reduce resource consumption.
■ Supporting Plain text authentication and MD5 authentication to enhance
security.
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RIP-2 has two types of message transmission: broadcast and multicast. Multicast is
the default type using 224.0.0.9 as the multicast address. The interface working in
the RIP-2 broadcast mode can also receive RIP-1 messages.
RIP Message Format RIP-1 message format
A RIP message consists of the Header and up to 25 route entries.
Figure 74 shows the format of RIP-1 message.
Figure 74 RIP-1 Message Format
■ Command: The type of message. 1 indicates Request, 2 indicates Response.
■ Version: The version of RIP, 0x01 for RIP-1.
■ AFI: Address Family Identifier, 2 for IP.
Command
AFI
IP address
Version Must be zero
Must be zero
Must be zero
Must be zero
Metric
0 7 15 31
Route
Entries
Header