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Chapter 5
IP Routing
The information included in the show ip protocols command includes
the AS, routing timers, networks being advertised, gateways, and AD (100).
The invalid timer is set at 270 seconds (three times the update timer). If a
router update is not received in three update periods, the route is considered
invalid. The holddown timer is 280, which is around three times the update
timer. This is the number of seconds a route is suppressed while waiting for
a new update to be received. If a new update is not received before the hold-
down timer expires, the flush timer will start. When the flush timer expires,
the route is removed from the routing table.
The Debug IP RIP Command
The debug ip rip command sends routing updates as they are sent and
received on the router to the console session. If you are telnetted into the
router, you'll need to use the terminal monitor command to be able to
receive the output from the debug commands.
Notice in the following router output that RIP is both sent and received
on serial 1, serial 0, and Ethernet 0 interfaces. This is a great troubleshooting
tool. The metric is the hop count.
2501B#debug ip rip
RIP protocol debugging is on
2501B#
07:12:56: RIP: received v1 update from 172.16.40.2 on
Serial1
07:12:56: 172.16.50.0 in 1 hops
07:12:56: RIP: received v1 update from 172.16.20.1 on
Serial0
07:12:56: 172.16.10.0 in 1 hops
In the above debug output, notice the route updates received on the
2501B serial 0 and serial 1 interfaces. These are from Routers 2501A and
2501C, respectively. What you want to notice is that spit horizon rules stop
the 2501A and C routers from advertising back routes that they learned from
2501B, which means only network 172.16.50.0 is being advertised from
2501C and 172.16.10.0 is being advertised to 2501B from 2501A.
07:12:58: RIP: sending v1 update to 255.255.255.255 via
Ethernet0 (172.16.30.1)
07:12:58: subnet 172.16.40.0, metric 1
07:12:58: subnet 172.16.20.0, metric 1
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