H3C Technologies H3C S12500 Series Switches - Manual (Page 37)

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• Counting to infinity—A destination with a metric value of 16 is considered unreachable. When a
routing loop occurs, the metric value of the route will increment to 16 to avoid endless looping.
• Split horizon—Disables RIP from sending routing information on the interface from which the
information was learned to prevent routing loops and save bandwidth.
• Poison reverse—Enables RIP to set the metric of routes received from a neighbor to 16 and sends
back these routes to the neighbor so the neighbor can delete such information from its routing table
to prevent routing loops.
• Triggered updates—RIP immediately advertises triggered updates for topology changes to reduce
the possibility of routing loops and to speed up network convergence.
RIP operation
The following procedure describes how RIP works:
1. After RIP is enabled, the router sends request messages to neighboring routers. Neighboring
routers return response messages, including information about their routing tables.
2. After the router receives this information, it updates its local routing table, and sends triggered
update messages to its neighbors. All routers on the network do this to keep the latest routing
information.
3. RIP ages out routes to keep only valid routes.
RIP versions
RIP has RIPv1 and RIPv2.
RIPv1 is a classful routing protocol, and supports message advertisement through broadcast only. RIPv1
protocol messages do not carry mask information, so it can only recognize routing information of natural
networks such as Class A, B, and C. For this reason, RIPv1 does not support discontiguous subnets.
RIPv2 is a classless routing protocol, and has the following advantages over RIPv1:
• Supports route tags, which are used in routing policies to flexibly control routes.
• Supports masks, route summarization, and Classless Inter-Domain Routing (CIDR).
• Supports designated next hops to select the best next hops on broadcast networks.
• Supports multicast routing update to reduce resource consumption. Only RIPv2 routers can receive
these update messages.
• Supports plain text authentication and MD5 authentication to enhance security.
NOTE:
RIPv2 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 operating in RIPv2 broadcast mode can also receive
RIPv1 messages.
RIP message format
A RIP message consists of a header and up to 25 route entries. (A RIPv2 authentication message uses the
first route entry as the authentication entry, leaving 24 available.)