3Com Switch 8800 Advanced Software V5 Configuration Guide

270 CHAPTER 28: RIP CONFIGURATION
■ Egress interface: Packet outgoing interface.
■ Metric: Cost from the local router to the destination.
■ Route time: Time elapsed since the routing entry was last updated. The time is
reset to 0 every time the routing entry is updated.
■ Route tag: Identifies a route, used in routing policy to flexibly control routes.
For information about routing policy, refer to
“Routing Protocol Overview” on
page 189.
RIP initialization and running procedure
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 all information about
their routing tables.
2 The router updates its local routing table, and broadcasts the triggered update
messages to its neighbors. All routers on the network do the same to keep the
latest routing information.
3 RIP ages out timed out routes by adopting an aging mechanism to keep only valid
routes.
RIP timers
RIP employs four timers, Update, Timeout, Suppress, and Garbage-Collect.
■ The update timer defines the interval between routing updates.
■ 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.
■ The 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 which come from the same neighbor and whose
metric is less than 16 will be received by the router to replace unreachable
routes.
■ 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.
■ 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 the bandwidth.