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This example
demonstrates how you can redistribute in one direction and use a
default route in the other direction, instead of redistributing in
both directions.
Assume an internetwork that uses three
ASs with OSPF
is the core protocol and RIP is the "edge" protocol. The
following illustrates how to configure a system:
- Allow the OSPF
backbone to know all the routes in each AS. This is done by
configuring redistribution on the ASBRs so that RIP routes are
redistributed into OSPF.
- Allow the RIP ASs
to know only about their internal routes, and use a default
route to networks that are not in the AS. This is done by
configuring a default route on the ASBRs. The default route is
advertised by the ASBRs into the RIP ASs.
Note that many other
ways to configure redistribution exist, so you must understand your
network topology and requirements in order to choose the best
solution.
The network shown in
Figure
is the basis for the output shown on subsequent pages. The
routers focused on are P1R3 (ASBR) and P1R1 (internal router).
Figures -
show the configurations for one of the ASBRs and a router in one of
the RIP networks. Points about each configuration follow:
- Internal RIP
router
- No redistribution
configuration is necessary because the intent is not to have
this router learn about external routes.
- The ip
classless command is required on all RIP/IGRP routes that
must use a default route. This command allows for classless
forwarding, meaning that the default paths can be used for
unconnected subnets of the same major classful network rather
than perform traditional classful behavior. Traditional classful
behavior means the router drops packets when the target subnet
is not present for the directly connected classful network.
- ASBR
- When
redistributing into OSPF, you need the subnets keyword for
subnetted networks to be redistributed.
- Define the default
network to be advertised to the edge protocols.
The output illustrates the ASBR route tables of P1R3 after redistribution was
enabled on both ASBRs. (see
P1R3#show ip route
(1) command
output).
For comparison, an
example of the route table prior to redistribution is shown in the
figure to the left. (see
P1R3#show ip route
(2) command
output).
Notice that in the
"before" output, the 10.2.0.0/24 networks do not appear
until redistribution is configured.
The output
shows one of the internal route tables of P1R1 after the default
route was configured on the P1R3 ASBR. (see
P1R1#show ip route
command output).
Using this route
table, the P1R1 can successfully ping, for example, any network in
the other RIP AS. (see
P1R1#ping 10.2.2.1
command output).
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