January 2002
Beginner
238 pages
6h 20m
English
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Route Filtering
The filtering of routes between ASs is key to implementing routing policies. The following section shows several route-filtering techniques.
Filtering by Prefix (Address/Mask) Information
The following BGP sub-command may be used to filter updates from a neighbor based on the IP prefix in the update packet:
neighborip-address distribute-list{access-list-number|name/prefix-listprefixlistname} {in | out}
ip-address is the address of the BGP peer. The IP prefixes to be filtered may be specified in an access list or a prefix list.
Consider TraderMary’s network again. An access list would be appropriate to block TrdrMary-1 from learning its own internal numbers from ISP-A:
hostname TrdrMary-1 ! interface Serial1 description * to ISP-A * ip address 192.100.100.253 255.255.255.252 ... router bgp 100 neighbor 192.100.100.254 remote-as 192 neighbor 192.100.100.254 distribute-list 1 in ! access-list 1 deny 160.160.0.0 access-list 1 deny 192.200.200.0 access-list 1 permit 0.0.0.0 255.255.255.255
Simple access lists do not allow control over the subnet mask field.
So, ISP-X may advertise 192.156.0.0/16 to peer
a.b.c.d as follows:
hostname ISP-X-1
!
router bgp 222
neighbor a.b.c.d
neighbor a.b.c.d distribute-list 10 out
access-list 10 permit 192.156.0.0However, this access list will permit
192.156.0.0/16, 192.156.0.0/17,
192.156.0.0/18, and so on. To ensure that ISP-X
advertises only 192.156.0.0/16, we need to configure an extended access list that has room to specify the mask ...
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