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This section illustrates
a previous point about IGRP behavior when a route to 0.0.0.0 is
available. As previously mentioned, the 0.0.0.0 route is
automatically installed as the local gateway of last resort for the
router by the Cisco IOS software. No
ip default- network 0.0.0.0
command
is required. Also, as previously mentioned, IGRP does not advertise
0.0.0.0 to other routers, even if
redistribute static
and a
default metric are configured. The main figure shows the network
topology used in this scenario.
In the
routing table from router A, you can see that the static route to
null0 for 0.0.0.0 is entered as the gateway of last resort. (see RouterA#show ip route
(1) command output).
The partial
configuration from router A shows how this is done. (see RouterA#show
running-config
(1) command output).
In the
output of the debug ip igrp transactions command from
router A, you can see that router A is not advertising 0.0.0.0. (see
RouterA#debug ip igrp
transactions (1)
command output).
Remember that RIP
advertised 0.0.0.0 automatically. The partial
configuration from router A shows that a default metric has been
configured, as well as redistribute static. Router A still does not
advertise 0.0.0.0. (see RouterA#show
running-config
(2) command output).
In the
output of the debug ip igrp transactions command from
router A, you can see that router A is still not advertising
0.0.0.0. (see RouterA#debug ip igrp
transactions (2)
command output).
The partial
configuration from router A shows how to get router A to use 0.0.0.0
as the local default route. It also shows how to advertise a
different candidate default route --- 10.0.0.0 --- that other routers
can consider as a possible gateway of last resort. The ip default
network command is used to tag a route as a candidate
default. (see RouterA#show
running-config
(3) command output).
A router can have
multiple default networks configured; however, 0.0.0.0 is the
preferred route when it is in the router routing table with an equal
or better metric than any other candidate default route. Because
both routes in router A are static, they have identical metrics.
The routing
table from router A shows both static routes. Note that the asterisk
(*) indicates a candidate default route. (see RouterA#show ip route
(2) command output).
The routing
table from router C shows that router C has received the router A
advertisement for 10.0.0.0 and installed it as a gateway of last
resort. (see RouterC#show ip route
command output).
The 0.0.0.0 route is
not as useful for IGRP as it is for RIP. However, it is safer to use
with IGRP for the same reasons that it is not as useful.
The simplest way to
achieve the same effect for IGRP is to create and redistribute a
static route for a fictitious network and flag it as the IP default
network. You don't want to use a real network because you may not be
able to control where the route for the real network is learned. It
may not always be learned via the path on which you want your
traffic to travel.
If you have multiple
routers with exit points to other networks that you want to use as
redundant links to the outside world, you can configure the same
redistributed static route in each of them. If you want one to
always be preferred over the others, give it a much better metric.
Make certain that, for any router in the network, the metric for the
preferred route is better than the metric it receives from the
candidate default of another router.
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