Influence routing decision with BGP Community

You can use community as a flag in order to mark a route. Upstream routers will use these flags to define routing policy. In the schema below, you can image that AS 100 is a customer site and AS 200 is a service provider. We have negotiated with the provider that:

• Traffic destined to routes with a community 100 have to pass by R3
• Traffic destined to routes with a community 200 have to pass by R2

In order to realize this, R1 marks routes with a community field, R2 and R3 modify the local_pref to follow the rules aboves.

In our case:

•  Lo1 will be 'marked' with a community 200:100 by R1
•  Lo2 will be 'marked' with a community 200:200 by R1
• Route with a community 200:100 will have a local_pref of 1000 (R3). 
• Route with a community 200:200 will have a local_pref of 500 (R2).

Configuration

R1:

interface Loopback1

 ip address 172.16.1.1 255.255.255.0

!

interface Loopback2

 ip address 172.16.2.1 255.255.255.0

!

router bgp 100

 bgp log-neighbor-changes

 network 172.16.1.0 mask 255.255.255.0

 network 172.16.2.0 mask 255.255.255.0

 neighbor 10.1.1.2 remote-as 200

 neighbor 10.1.1.2 send-community

 neighbor 10.1.1.2 route-map Peer-R2 out

 neighbor 10.1.2.3 remote-as 200

 neighbor 10.1.2.3 send-community

 neighbor 10.1.2.3 route-map Peer-R3 out

!

ip access-list standard LO1

 permit 172.16.1.0 0.0.0.255

ip access-list standard LO2

 permit 172.16.2.0 0.0.0.255

!

route-map Peer-R3 permit 10

 match ip address LO2

 set community 200:200

route-map Peer-R3 permit 15

 match ip address LO1

 set community 200:100

!

route-map Peer-R3 permit 20

!

route-map Peer-R2 permit 10

 match ip address LO1

 set community 200:100

!

route-map Peer-R2 permit 15

 match ip address LO2

 set community 200:200

!

route-map Peer-R2 permit 20

R2:

router bgp 200

 bgp log-neighbor-changes

 neighbor 10.1.1.1 remote-as 100

 neighbor 10.1.1.1 next-hop-self

 neighbor 10.1.1.1 route-map Peer-R1 in

 neighbor 20.1.1.3 remote-as 200

 neighbor 20.1.1.3 next-hop-self

 neighbor 20.1.2.4 remote-as 200

 neighbor 20.1.2.4 next-hop-self

!

ip bgp-community new-format

ip community-list 1 permit 200:200

!

route-map Peer-R1 permit 10

 match community 1

 set local-preference 500

!

route-map Peer-R1 permit 20

R3:

router bgp 200

 bgp log-neighbor-changes

 neighbor 10.1.2.1 remote-as 100

 neighbor 10.1.2.1 route-map Peer-R1 in

 neighbor 20.1.1.2 remote-as 200

 neighbor 20.1.1.2 next-hop-self

 neighbor 20.1.3.4 remote-as 200

 neighbor 20.1.3.4 next-hop-self

!

ip forward-protocol nd

!

ip bgp-community new-format

ip community-list 1 permit 200:100

!

route-map Peer-R1 permit 10

 match community 1

 set local-preference 1000

!

route-map Peer-R1 permit 20

Validation

Check that routes are flags on R2:

R2#sho ip bgp 172.16.1.0

BGP routing table entry for 172.16.1.0/24, version 6

Paths: (2 available, best #1, table default)

  Advertised to update-groups:

     2

  Refresh Epoch 1

  100

    20.1.1.3 from 20.1.1.3 (20.1.3.3)

      Origin IGP, metric 0, localpref 1000, valid, internal, best

  Refresh Epoch 1

  100

    10.1.1.1 from 10.1.1.1 (172.16.2.1)

      Origin IGP, metric 0, localpref 100, valid, external

      Community: 200:100

R2#sho ip bgp 172.16.2.0

BGP routing table entry for 172.16.2.0/24, version 3

Paths: (1 available, best #1, table default)

  Advertised to update-groups:

     3

  Refresh Epoch 1

  100

    10.1.1.1 from 10.1.1.1 (172.16.2.1)

      Origin IGP, metric 0, localpref 500, valid, external, best

      Community: 200:200

Check policies applied by R2 and R3 on R4:

R4#sho ip bgp

BGP table version is 9, local router ID is 20.1.3.4

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

 *>i 172.16.1.0/24    20.1.3.3                 0   1000      0 100 i

 *>i 172.16.2.0/24    20.1.2.2                 0    500      0 100 i