1-2
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Egress interface: Packet outgoing interface.
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Metric: Cost from the local router to the destination.
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Route time: Time elapsed since the routing entry was last updated. The time is reset to 0 every time
the routing entry is updated.
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Route tag: Identifies a route, used in a routing policy to flexibly control routes. For information about
routing policy, refer to
Routing Policy Configuration
in the
IP Routing Volume
.
RIP timers
RIP employs four timers, update, timeout, suppress, and garbage-collect.
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The update timer defines the interval between routing updates.
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The timeout timer defines the route aging time. If no update for a route is received within the aging
time, the metric of the route is set to 16 in the routing table.
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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.
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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) 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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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.
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Split horizon. A router does not send the routing information learned from a neighbor to the
neighbor to prevent routing loops and save bandwidth.
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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.
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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.
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 such information, the router updates its local routing table, and sends triggered
update messages to its neighbors. All routers on the network do the same 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 two versions, RIPv1 and RIPv2.
