H3C Technologies H3C S10500 Series Switches - Manual (Page 37)

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• Suppress timer—Defines how long a RIP route stays in the suppressed state. When the metric of a
route is 16, the route enters the suppressed state. In the suppressed state, only routes coming from
the same neighbor and whose metric is less than 16 will be received by the router to replace
unreachable routes.
• 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, then the route is deleted from the routing table.
Routing loops prevention
RIP is a distance vector (D-V) routing protocol. Because 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 routing information back to the neighboring router the
information came from, preventing routing loops and saving 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 instead of
after the update period expires to speed up network convergence.
Operation of RIP
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 receiving this information, the router 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 version
RIP has the following versions: RIPv1 and RIPv2.
RIPv1, a classful routing protocol, 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 ones on broadcast networks.
• Supports multicasting routing updates to reduce resource consumption. Only RIPv2 routers can
receive these update messages.