3Com WX3000 operation manual RIP Startup and Operation, RIP timers, Routing loops prevention

zInterface: Outbound interface on this router, through which IP packets should be forwarded to reach the destination.

zMetric: Cost from the local router to the destination.

zRoute time: Time elapsed since the routing entry was last updated. The time is reset to 0 every time the routing entry is updated.

RIP timers

As defined in RFC 1058, RIP is controlled by three timers: Period update, Timeout, and Garbage-collection.

zPeriod update timer: The period update timer defines the interval between routing updates.

zTimeout timer: The timeout timer defines the route aging time. If no update for a route is received after the aging time elapses, the metric of the route is set to 16 in the routing table.

zGarbage-collection timer: The 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) based 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.

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z

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 back to the neighbor to prevent routing loops and save the bandwidth.

RIP Startup and Operation

The whole process of RIP startup and operation is as follows:

zOnce RIP is enabled on a router, the router broadcasts or multicasts a request packet to its neighbors. Upon receiving the packet, each neighbor running RIP answers a response packet containing its routing table information.

zWhen this router receives a response packet, it updates its local routing table and sends a triggered update packet to the neighbors. Upon receiving the triggered update packet, the neighbor sends the packet to all its neighbors. After a series of update triggering processes, each router can get and keep the updated routing information.

zBy default, RIP sends its routing table to its neighbors every 30 seconds. Upon receiving the packets, the neighbors maintain their own routing tables and select optimal routes, and then advertise update information to their respective neighbors so as to make the updated routes known globally. Furthermore, RIP uses the aging mechanism to handle the timeout routes to ensure real-time and valid routes.

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