H3C Technologies H3C SR8800 - Manual (Page 40)

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• Garbage-collect timer—Defines the interval from when the metric of a route becomes 16 to when it
is deleted from the routing table. During the garbage-collect timer length, RIP advertises the route
with the routing metric set to 16. If no update is announced for that route after the garbage-collect
timer expires, the route will be deleted from the routing table.
Routing loops prevention
RIP is a distance vector (D-V) routing protocol. Since a RIP router advertises its own routing table to
neighbors, routing loops may occur.
RIP uses the following mechanisms to prevent routing loops:
• Counting to infinity—The metric value of 16 is defined as unreachable. When a routing loop occurs,
the metric value of the route will increment to 16.
• Split horizon—A router does not send the routing information learned from a neighbor to the
neighbor to prevent routing loops and save bandwidth.
• 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 such 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 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. By default, a RIP router sends its routing table to neighbors every 30 seconds.
4. RIP ages out routes by adopting an aging mechanism to keep only valid routes.
RIP versions
RIP has the following versions: RIPv1 and RIPv2.
RIPv1 is a classful routing protocol and supports message advertisement via broadcast only. RIPv1
protocol messages do not carry mask information, which means it can only recognize routing
information of natural networks such as Class A, B, and C. That is why RIPv1 does not support
discontiguous subnets.
RIPv2 is a classless routing protocol, and has the following advantages over RIPv1:
• Supports route tags. Route tags 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 ones on broadcast networks.
• Supports multicasting routing updates to reduce resource consumption. Only RIPv2 routers can
receive these update messages.
• Supports plain text authentication and MD5 authentication to enhance security.