BGP Community Attributes: Introduction
BGP community is an optional transitive attribute. We can use route-maps to set communities on the BGP updates. By default the BGP communities aren’t sent with BGP updates hence we have to specifically mention ‘send-community’ command to send the update with community.
Few of the well-known communities are as below:
- No-export: Do not advertise to EBGP peers
- No-advertise: Do not advertise this route to any peer
- Internet: Advertise this route to the internet community, any router belongs to it.
Communities can also be used in BGP for the route-filtering also known as community filtering.
BGP Community Attributes
Let us use the topology below to configure and verify the community filtering:
We are running eBGP between R1 – R2 and R2 – R3. R3 will advertise the route for 3.3.3.3 to R2 with the BGP community attribute applied to it. First, we will apply community no-advertise and we will see once R2 receives the route with community no-advertise it won’t announce the route about 3.3.3.3 to R1. Secondly we will use community internet while advertising 3.3.3.3 from R3 and will be able to see that the route of 3.3.3.3 is announced to R1.
Configuration
R3
bgp log-neighbor-changes neighbor 9.9.23.2 remote-as 200 neighbor 9.9.23.2 send-community neighbor 9.9.23.2 route-map Community out route-map Community permit 10 match ip address ACL set community no-advertise
R3#sh ip access-lists ACL
10 permit ip any any (2 matches)
We are using a route-map on R3 which call matches an access-list named ACL. ACL is configured to allow all traffic. Hence all routing updates from R3 will be matched in ACL and hence will be set with community no-advertise.
Verification –
We see R2 has received route about 3.3.3.3 with community no-advertise as below:
R2#sh ip bgp 3.3.3.3
Paths: (1 available, best #1, table default, not advertised to any peer)
Not advertised to any peer
Refresh Epoch 1
300
9.9.23.3 from 9.9.23.3 (3.3.3.3)
Origin IGP, metric 0, localpref 100, valid, external, best
Community: no-advertise
rx pathid: 0, tx pathid: 0x0
We also see R2 isn’t advertising the route about 3.3.3.3 to R1 as below:
R2#sh ip bgp neighbors 9.9.12.1 advertised-routes
Status codes: s suppressed, d damped, h history, * valid, > best, i – internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i – IGP, e – EGP, ? – incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 1.1.1.1/32 9.9.12.1 0 0 100 i
*> 2.2.2.2/32 0.0.0.0 0 32768 i
Total number of prefixes 2
R1#sh ip bgp
Status codes: s suppressed, d damped, h history, * valid, > best, i – internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i – IGP, e – EGP, ? – incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 1.1.1.1/32 0.0.0.0 0 32768 i
*> 2.2.2.2/32 9.9.12.2 0 0 200 i
Now let us configure the route-map on R3 to set community to internet and we shall be able to see R2 advertising the route of 3.3.3.3 to R1.
Configurations –
R3:
match ip address ACL set community internet
Verification –
Let us check on R2 now we will see the community as internet and 3.3.3.3 is announced to R1 as below:
R2#sh ip bgp 3.3.3.3
Paths: (1 available, best #1, table default)
Advertised to update-groups:
1
Refresh Epoch 2
300
9.9.23.3 from 9.9.23.3 (3.3.3.3)
Origin IGP, metric 0, localpref 100, valid, external, best
Community: internet
rx pathid: 0, tx pathid: 0x0
R2#sh ip bgp neighbors 9.9.12.1 advertised-routes
Status codes: s suppressed, d damped, h history, * valid, > best, i – internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i – IGP, e – EGP, ? – incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 1.1.1.1/32 9.9.12.1 0 0 100 i
*> 2.2.2.2/32 0.0.0.0 0 32768 i
*> 3.3.3.3/32 9.9.23.3 0 0 300 i
Total number of prefixes 3
Route is received on R1:
R1#sh ip bgp
Status codes: s suppressed, d damped, h history, * valid, > best, i – internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i – IGP, e – EGP, ? – incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 1.1.1.1/32 0.0.0.0 0 32768 i
*> 2.2.2.2/32 9.9.12.2 0 0 200 i
*> 3.3.3.3/32 9.9.12.2 0 200 300 i
Related-
ABOUT THE AUTHOR
I am here to share my knowledge and experience in the field of networking with the goal being – “The more you share, the more you learn.”
I am a biotechnologist by qualification and a Network Enthusiast by interest. I developed interest in networking being in the company of a passionate Network Professional, my husband.
I am a strong believer of the fact that “learning is a constant process of discovering yourself.”
– Rashmi Bhardwaj (Author/Editor)
