|
IGRP treats 0.0.0.0/0 routes differently. In
fact, IGRP does not recognize the network 0.0.0.0/0 and will not include it
in updates. To configure a dynamic exchange of default information in an
IGRP network, you must use the ip
default-network command. The ip
default-network command can be used
to flag a route to any IP network, not just 0.0.0.0/0, as a candidate
default route, using the following command syntax:
Router(config)#ip default-network ip-network-address
A candidate default route is denoted by an
asterisk in the routing table and will be considered (along with any other
candidates) for the role as gateway of last resort. As an example of how
this command works, consider the internetwork shown in the figure.
As the "boundary router," RTB
must be manually configured to send default traffic to its link partner,
the ISP router. You could configure a 0.0.0.0/0 route on RTB, but this
route will not be propagated by IGRP to the other routers. To avoid manually
configuring 0.0.0.0/0 routes on all routers, you can configure RTB to flag
its route to 207.21.20.0/24 as a candidate default route, as shown:
RTB(config)#ip default-network
207.21.20.0
The network 207.21.20.0/24 now has special
properties as an exterior network (the outside network that serves as a
gateway of last resort). RTB will send this information in IGRP routing
updates to RTA and RTC. These routers can now dynamically learn that
network 207.21.20.0/24 is an exterior network, making RTB the gateway of
last resort for both of these routers. Both RTA and RTC will propagate
this route, flagged as a candidate default, to other IGRP neighbors, if
present.
Unlike a static 0.0.0.0/0 route
configuration, the ip
default-network command provides an
administrator with a great deal of flexibility. In complex topologies,
several networks can be flagged as candidate defaults. Routers can then
choose from among the available candidates to pick the lowest-cost route.
If you are running IGRP, you must use the ip
default-network command to enable
the exchange of default information. If you are using RIP, a 0.0.0.0/0
route will usually suffice. However, you can use the ip
default-network command on a RIP
router, but RIP routers propagate IP default networks as 0.0.0.0/0 routes.
In other words, a RIP router configured with the ip
default-network
192.168.1.0 command will
send neighbors a route to 0.0.0.0/0, not a flagged route to 192.168.1.0.
Note that the ip
default-network command is classful,
which means that if the router has a route to the subnet indicated by this
command, it installs a static route to the major net and then flags that
route as a candidate default route.
|