| When using multiple IP routing
protocols on a router, the default distances usually suffice.
However, some circumstances call for changing the administrative
distance values on a router.
For example, if a router is running
both IGRP and OSPF, it may receive routes to the same network from
both protocols. The default administrative distances favor IGRP
routes over OSPF routes, as shown in the figure. However, because IGRP
does not support CIDR, you may want the router to use the OSPF route
instead. In this case, you can configure the local router to apply a
custom administrative distance to all OSPF routes, as shown in the
Figure.
With the distance
95 OSPF configuration
command, RTZ compares the IGRP and OSPF routes and comes up with a
different result.
In its broadest application, the distance
command can be used to modify the administrative distance value
applied to all routes learned via a specific routing process.
The commands in the figure will assign the value of 95 to all routes
learned by the OSPF 1 process. Note that these values are local to
the router. Although RTZ assigns the 10.0.0.0 network an
administrative distance of 95, all other Cisco OSPF routers will
apply a value of 110, unless otherwise configured.
You can also apply the distance
command with optional arguments to make changes to selected routes
based on where they originate. The expanded syntax of the distance
command is as follows:
Router(config-router)#distance
weight [source-ip-address
source-mask (access-list-number | name)]
After running multiple protocols on a
boundary router, you may discover that one or two suboptimal paths
have been installed because of their lower administrative distance.
Rather than assign a new distance value to all routes learned by a
process, specific routes can be identified based on their source IP.
Using the optional arguments, you can configure a router to apply an
administrative distance of 105 to all RIP routes received from
10.4.0.2:
RTZ(config)#router rip
RTZ(config-router)#distance 105 10.4.0.2 255.255.255.255
Alternatively, you can apply an
administrative distance value to only certain routes from that same
source by specifying an access list.
Remember that the administrative
distance defaults exist for a reason and will serve a network well
in most circumstances. Use the distance
command only when you are certain that it is necessary to guarantee
optimal routing.
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