Default Routes

Default Routes
When redistributing default routes into RIP, IGRP, and EIGRP, you redistribute them the same
way you redistribute static routes. Redistributing default routes into IS-IS and OSPF differs
from the way you do it for their counterpart static routes.
OSPF
OSPF ASBRs do not by default advertise default routes. In order to enable the advertising of a
default route, the default-information originate command needs to be used. Whatever
OSPF router you enter this command on, even if the router is not currently an ASBR, will
become an ASBR. It’s just like issuing the redistribute command. The command makes the
router become an ASBR in software.
The default-information originate command advertises default routes to the other
routers participating in the OSPF process if the ASBR has a default route of its own. So if the
default route were to be removed from the ASBR’s routing table, the ASBR would cease originating
the default information and OSPF would declare that route unreachable. In order to
overcome the issue of the ASBR requiring its own default route before originating the default
route, OSPF allows for the keyword always to be appended to the default-information
originate command. With the use of the always keyword, the ASBR always sends a default
route into OSPF, even if there is not a default route available in its own routing table. If the
OSPF process has only one default, this is a good approach. If there is more than one router originating
a default route, you should not use the always keyword. The following is an example
of a default route being redistributed into OSPF:
R1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#ip route 0.0.0.0 0.0.0.0 10.10.10.1
R1(config)#router ospf 1
R1(config-router)#default-information originate
R1(config-router)#^Z
R1#
IS-IS
Like OSPF, IS-IS uses the default-information originate command to redistribute default
routes into IS-IS. IS-IS, however, does not support the use of the always keyword. Default routes
redistributed into IS-IS are advertised only to the Level 2 area. The default route is not advertised
Configuring Redistribution 349
into a Level 1 area. Level 1 areas have their own way of discovering default gateways. The following
is a sample of a default route being redistributed into IS-IS:
R1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#ip route 0.0.0.0 0.0.0.0 10.10.10.1
R1(config)#router isis
R1(config-router)#default-information originate
R1(config-router)#^Z
R1#
OSPF allows you to set a metric value and a metric type for the default route. IS-IS does not
allow you to set these values for a default route.