BGP: BGP Community

Technology: Routing

Area: EGP

Vendor: Cisco

Title: BGP aggregation

Software: 12.X , 15.X, IP Services

Platform: Catalyst 3560, 3750, 3850, 4500, 6500, ISR/ASR Routers

BGP community stands for the attributes which can easily be added to the group of prefixes to marked them with the same meaning. With that possibility we can manipulate the whole bunch of prefixes base on single tag which is transitive across BGP autonomous systems.

There are four well-known communities values (https://tools.ietf.org/html/draft-ietf-idr-wide-bgp-communities-04):

1. NO_EXPORT (0xFFFFFF01) – instruct the BGP neighbor not to advertise prefix to any EBGP peers,
2. NO_ADVERTISE (0xFFFFFF02) – obligate the BGP neighbor not to advertise prefix to any BGP peers,
3. NO_EXPORT_SUBCONFED (0xFFFFFF03) – inform the BGP neighbor not to advertise prefix to any BGP peers outside a local sub-confederation,
4. NOPEER (0xFFFFFF04) – advisory to BGP neighbor not to readvertise the route prefix to all external bilateral peers.

To configure router R1 to send prefix marked with community:

R1# configure terminal
R1 (config)# ip bgp- community new-format
R1 (config)# route-map COMMUNITY permit 10
R1 (config-route-map)# set community 100:999
R1 (config)# router bgp 100
R1 (config-router)# network 94.70.40.0 mask 255.255.255.0 route-map COMMUNITY
R1 (config-router)# neighbor 80.50.0.2 remote-as 200
R1 (config-router)# neighbor 80.50.0.2 send-community
R1 (config-router)# exit

R2# configure terminal
R2 (config)# ip bgp-community new-format
R2 (config)# router bgp 200
R2 (config-router)# neighbor 80.50.0.1 remote-as 100
R2 (config-router)# exit

Verification commands:

R1# show bgp ipv4 unicast neighbors 80.50.0.2 | i Community

BGP Community attribute sent to this neighbor

R2# show bgp ipv4 unicast 94.70.40.0/24
BGP routing table entry for 94.70.40.0/24, version 6
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Flag: 0x820
Not advertised to any peer
100
80.50.0.1 from 80.50.0.1 (80.50.0.1)
Origin IGP, metric 0, localpref 100, valid, external, best
Community: 100:999

To configure router R2 to match on specific community and change the prefixes preference:

R2# configure terminal
R2 (config)# ip community-list 10 permit 100:999
R2 (config)# route-map R1_IN permit 10
R2 (config-route-map)# match community 10
R2 (config-route-map)# set local-preference 150
R2 (config)# router bgp 200
R2 (config-router)# neighbor 80.50.0.1 route-map R1_IN in
R2 (config-router)# exit

R2# show bgp ipv4 unicast 94.70.40.0/24
BGP routing table entry for 94.70.40.0/24, version 7
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Flag: 0x820
Not advertised to any peer
100
80.50.0.1 from 80.50.0.1 (80.50.0.1)
Origin IGP, metric 0, localpref 150, valid, external, best
Community: 100:999