Distribute list

FIGURE 9 . 7 Distribute list
AS 100
AS 200
192.168.24.0 /24 R1
R3 R2
302 Chapter 9  Advanced Border Gateway Protocol
1. The route to R1’s Ethernet segment needs to be blocked from R3.
2. In order to prevent unnecessary UPDATE messages for the route, it would be a better
choice to put an outbound filter on R2.
3. Create the access list on R2:
R2#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R2(config)#access-list 1 deny 192.168.24.0 0.0.0.255
R2(config)#access-list 1 permit any
R2(config)#^Z
R2#
When creating an access list, there is an implicit deny all at the end of the
access list. If a permit any had not been added to the access list, all routes
would have been denied.
4. Add the distribute list to the BGP session on R2 for R3. The address used by R3 for the BGP
session is 3.3.3.3:
R2#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R2(config)#router bgp 200
R2(config-router)#neighbor 3.3.3.3 distribute-list 1 out
R2(config-router)#^Z
R2#
That’s all there is to configuring distribute lists. The major drawback of distribute lists for
BGP filtering is that they rely on an access list. Access lists are not flexible. If you need to deny
a new route or permit a new route, you need to delete the access list and reconfigure it with the
new information. That is where prefix lists can help.