11.9.5 Using BGP Communities Attribute

Following configuration is the most typical usage of BGP communities attribute. AS 7675 provides upstream Internet connection to AS 100. When following configuration exists in AS 7675, AS 100 networks operator can set local preference in AS 7675 network by setting BGP communities attribute to the updates.

router bgp 7675
 neighbor 192.168.0.1 remote-as 100
 neighbor 192.168.0.1 route-map RMAP in
!
ip community-list 70 permit 7675:70
ip community-list 70 deny
ip community-list 80 permit 7675:80
ip community-list 80 deny
ip community-list 90 permit 7675:90
ip community-list 90 deny
!
route-map RMAP permit 10
 match community 70
 set local-preference 70
!
route-map RMAP permit 20
 match community 80
 set local-preference 80
!
route-map RMAP permit 30
 match community 90
 set local-preference 90

Following configuration announce 10.0.0.0/8 from AS 100 to AS 7675. The route has communities value 7675:80 so when above configuration exists in AS 7675, announced route’s local preference will be set to value 80.

router bgp 100
 network 10.0.0.0/8
 neighbor 192.168.0.2 remote-as 7675
 neighbor 192.168.0.2 route-map RMAP out
!
ip prefix-list PLIST permit 10.0.0.0/8
!
route-map RMAP permit 10
 match ip address prefix-list PLIST
 set community 7675:80

Following configuration is an example of BGP route filtering using communities attribute. This configuration only permit BGP routes which has BGP communities value 0:80 and 0:90. Network operator can put special internal communities value at BGP border router, then limit the BGP routes announcement into the internal network.

router bgp 7675
 neighbor 192.168.0.1 remote-as 100
 neighbor 192.168.0.1 route-map RMAP in
!
ip community-list 1 permit 0:80 0:90
!
route-map RMAP permit in
 match community 1

Following exmaple filter BGP routes which has communities value 1:1. When there is no match community-list returns deny. To avoid filtering all of routes, we need to define permit any at last.

router bgp 7675
 neighbor 192.168.0.1 remote-as 100
 neighbor 192.168.0.1 route-map RMAP in
!
ip community-list standard FILTER deny 1:1
ip community-list standard FILTER permit
!
route-map RMAP permit 10
 match community FILTER

Communities value keyword internet has special meanings in standard community lists. In below example internet act as match any. It matches all of BGP routes even if the route does not have communities attribute at all. So community list INTERNET is same as above example’s FILTER.

ip community-list standard INTERNET deny 1:1
ip community-list standard INTERNET permit internet

Following configuration is an example of communities value deletion. With this configuration communities value 100:1 and 100:2 is removed from BGP updates. For communities value deletion, only permit community-list is used. deny community-list is ignored.

router bgp 7675
 neighbor 192.168.0.1 remote-as 100
 neighbor 192.168.0.1 route-map RMAP in
!
ip community-list standard DEL permit 100:1 100:2
!
route-map RMAP permit 10
 set comm-list DEL delete