Originating a Default Route with IGRP

As with RIP, you can configure a default route on an IGRP router like this:

router igrp network 172.18.0.0

The default route 0.0.0.0 is used locally by the router; however, it cannot be advertised to any other router with IGRP. IGRP does not propagate 0.0.0.0 even if you redistribute it into IGRP with the redistribute static command.

To make IGRP originate a default route and advertise it to others, configure IGRP as follows (this also works for EIGRP):

router igrp ICC redistribute static network 172.18.0.0

ip route 30.0.0.0 255.0.0.0 192.168.202.1 ip default-network 30.0.0.0

where ip route 30.0.0.0 255.0.0.0 192.168.202.1 creates a static route to a fictitious network 30.0.0.0 (a major net). This network does not actually exist in the organization. It's just a placeholder and a substitute for ip route 0.0.0.0 0.0.0.0 192.168.202.1.

The command ip default-network 30.0.0.0 declares that 30.0.0.0, the fictitious network, is to be treated as a gateway of last resort. Traffic with an unknown destination is sent to this network. Ilie router knows the route to this network is through 192.168.202,1 because of the previous command, ip mute 30.0.0.0 255.0.0.0 192.168.202.1.

The command redistribute static is necessary to redistribute the static route, 30.0.0.0, into IGRP so that it can be advertised to other IGRP routers. Upon receiving 30.0.0.0. other routers will use it as a default route. This is equivalent to a RIP router receiving 0.0.0.0.

To verity the configuration of the originator of the default, issue show ip route: IGRP-Router#sh ip ro

Codes: C • connected, S static, I • IGRP, R RIP, M - mobile, B BGP D • EIGRP, EX - EIGRP external, 0 OSPF, IA - OSPF inter area N1 OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E EGP i • IS IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * candidate default U per-user static route, o - 0DR T • traffic engineered route

Gateway of last resort is 192.168.202.1 tQ network 3«.0.0.0

172.16.0.0/24 is subnetted, 2 subnets C 172.16.13.0 is directly connected, Serial0

C 172.16.2.0 is directly connected, Seriall

<lines deleted for brevity>

The preceding output agrees with the configuration: Gateway of last resort is correct, and network 30.0.0.0 is codcd with S*, indicating it is a default route. If you forget to configure the ip default-network command, the asterisk (*) will not appear and the static will not be a default route.

Was this article helpful?

0 0

Post a comment