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Chapter 11
Access Policies
and so on. For a block size of 32, the ranges are 031, 3263, 6495, and
so on.
Using the command any is the same thing as writing out the wildcard
0.0.0.0 255.255.255.255.
Route Filtering
The distribute route command can be used to limit the number of networks
advertised by permitting advertisements only to those specified. Distribution
lists are applied from within a routing protocol to manipulate which route
updates are sent and accepted on the specified interface. All that's necessary
for networks in which the edge routers rely only on the default route is to
advertise a few choice networks to them. Reducing the advertised routes
conserves bandwidth and reduces the load on the edge router.
Controlling the routing table of the core block reduces the routing table
size on the core and can also stop users from accessing networks that you
don't advertise unless you specifically provide them with a static route.
You can also use access lists to manipulate route advertisements. As an
example of this, look at the route table in the router output below before any
changes are made in the EIGRP session:
Router_C#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - [output cut]
Gateway of last resort is not set
172.16.0.0/16 is variably subnetted, 4 subnets, 3 masks
C 172.16.40.4/30 is directly connected, Serial0
D 172.16.30.0/24 [90/2195456] via 172.16.40.5 , 00:42:51,
Serial0
D 172.16.20.4/30 [90/2681856] via 172.16.40.5 , 02:33:25,
Serial0
D 172.16.0.0/16 is a summary, 03:03:56, Null0
Router_C#
This information shows that there are three routes learned via EIGRP and
one route that's directly connected. Next, we'll filter out the route for the
172.16.30.0 network:
Router_C#conf t
Enter configuration commands, one per line. End with
CNTL/Z.
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