BGP: Route Filtering Using Communities

BGP: Route Filtering Using Communities

·         In BGP, communities are basically used to tag the routes so that we can filter them where ever we want.

·         Let me take a small topology to configure and verify route filtering based on communities.

R1-------------------R2--------------------R3

·         R1: AS 100

·         R2: AS 200

·         R3: AS 300

·         R1 is advertising a prefix 1.1.1.1/32 with community value 1

·         R1 is advertising a prefix 11.11.11.11/32 with community value 11

·         Configuration in R2:

·         Strip the community value 1 while forwarding it to R3 for 1.1.1.1/32

·         Change the community value to 123 for 11.11.11.11/32

·         Let me configure it and see…

R1#show run

router bgp 100

 no synchronization

 bgp router-id 1.1.1.1

 bgp log-neighbor-changes

 network 1.1.1.1 mask 255.255.255.255

 network 11.11.11.11 mask 255.255.255.255

 neighbor 100.1.12.2 remote-as 200

 neighbor 100.1.12.2 send-community

 neighbor 100.1.12.2 route-map COMM out

 no auto-summary

!

!

ip prefix-list 1 seq 5 permit 1.1.1.1/32

!

ip prefix-list 11 seq 5 permit 11.11.11.11/32

!

route-map COMM permit 10

 match ip address prefix-list 1

 set community 1

!

route-map COMM permit 20

 match ip address prefix-list 11

 set community 11

!

route-map COMM permit 100

!

R1#

R2(config)#do show run | begin router bgp 200

router bgp 200

 no synchronization

 bgp log-neighbor-changes

 neighbor 100.1.12.1 remote-as 100

 neighbor 100.1.23.3 remote-as 300

 neighbor 100.1.23.3 send-community

 no auto-summary

!

R2#show ip bgp

BGP table version is 10, local router ID is 2.2.2.2

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,

              r RIB-failure, S Stale

Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path

*> 1.1.1.1/32       100.1.12.1               0             0 100 i

*> 11.11.11.11/32   100.1.12.1               0             0 100 i

R2#

R2#show ip bgp 1.1.1.1

BGP routing table entry for 1.1.1.1/32, version 10

Paths: (1 available, best #1, table Default-IP-Routing-Table)

Flag: 0x820

  Advertised to update-groups:

     2

  100

    100.1.12.1 from 100.1.12.1 (1.1.1.1)

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

      Community: 1

R2#

R2#show ip bgp 11.11.11.11

BGP routing table entry for 11.11.11.11/32, version 9

Paths: (1 available, best #1, table Default-IP-Routing-Table)

  Advertised to update-groups:

     2

  100

    100.1.12.1 from 100.1.12.1 (1.1.1.1)

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

      Community: 11

R2#

Observations:

·         R2 has received 2 prefixes from R1, 1.1.1.1/32 and 11.11.11.11/32

·         1.1.1.1/32 has community value of 1

·         11.11.11.11 has community value of 11

Let me configure to change these community values on R2:

R2(config-router)#do show run | begin router bgp 200

router bgp 200

 no synchronization

 bgp log-neighbor-changes

 neighbor 100.1.12.1 remote-as 100

 neighbor 100.1.23.3 remote-as 300

 neighbor 100.1.23.3 send-community

 neighbor 100.1.23.3 route-map COMM1 out

 no auto-summary

!

ip forward-protocol nd

!

no ip http server

!

!

!

ip prefix-list 1 seq 5 permit 1.1.1.1/32

!

ip prefix-list 11 seq 5 permit 11.11.11.11/32

!

route-map COMM1 permit 10

 match ip address prefix-list 1

 set community none

!

route-map COMM1 permit 20

 match ip address prefix-list 11

 set community 123

!

route-map COMM1 permit 100

!

!

R2(config-router)#

R3#show ip bgp

BGP table version is 13, local router ID is 3.3.3.3

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,

              r RIB-failure, S Stale

Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path

*> 1.1.1.1/32       100.1.23.2                             0 200 100 i

*> 11.11.11.11/32   100.1.23.2                             0 200 100 i

R3#

R3#

R3#show ip bgp 1.1.1.1

BGP routing table entry for 1.1.1.1/32, version 12

Paths: (1 available, best #1, table Default-IP-Routing-Table)

Flag: 0x820

  Not advertised to any peer

  200 100

    100.1.23.2 from 100.1.23.2 (2.2.2.2)

      Origin IGP, localpref 100, valid, external, best

R3#

R3#show ip bgp 11.11.11.11

BGP routing table entry for 11.11.11.11/32, version 13

Paths: (1 available, best #1, table Default-IP-Routing-Table)

Flag: 0x820

  Not advertised to any peer

  200 100

    100.1.23.2 from 100.1.23.2 (2.2.2.2)

      Origin IGP, localpref 100, valid, external, best

      Community: 123

R3#

Observations:

·         R2 is stripping the community value for 1.1.1.1/32 while forwarding it to R3, that’s why I didn’t see any community value for 1.1.1.1/32 in R3

·         R2 is changing the community value to 123 for 11.11.11.11/32 while forwarding it to R3, that’s why I have community value as 123 for 11.11.11.11/32 in R3.