Using BGP Community Attribute

TOPOLOGY

Objectives:



In the topology, customer AS 30 wishes to achieve this routing policy with the community attributes.

• The traffic inbound from AS 100 destined to network 6.6.6.0/24 comes through the R1-R3 link. In case the R1-R3 link fails, all traffic comes in through R2-R3.
  
• The traffic inbound from AS 100 destined to network 7.7.7.0/24 comes through the R2-R3 link. In case the R2-R3 link fails, all traffic comes in through R1-R3.
  

In order to achieve this routing policy, R3 announces its prefixes as follows:
To R1:

• 6.6.6.0/24 with a community attribute 100:300
  
• 7.7.7.0/24 with a community attribute 100:250
  

To R2:

• 6.6.6.0/24 with a community attribute 100:250
  
• 7.7.7.0/24 with a community attribute 100:300
  
  Path preferred without modifying any BGP Attributes:
  
  
  
  
  Configurations:
  R1
  router bgp 100
   no synchronization
   bgp log-neighbor-changes
   neighbor 10.10.12.2 remote-as 100
   neighbor 10.10.12.2 next-hop-self
   neighbor 10.10.13.3 remote-as 30
   neighbor 10.10.13.3 route-map Peer-R3 in
  !-----Configures the inbound policy route-map "Peer-R3" when peering with R3.
   no auto-summary
   !
  ip bgp-community new-format
  ip community-list 1 permit 100:300
  ip community-list 2 permit 100:250
  
  

  !--- Defines community list 1 and 2.

  route-map Peer-R3 permit 10
   match community 1
   set local-preference 130
  

   !--- Sets local preference 130 for all routes
  matching community list 1.
  

  
  route-map Peer-R3 permit 20
   match community 2
   set local-preference 125

  !--- Sets local preference 125 for all routes
  matching community list 2.
  

  
  route-map Peer-R3 permit 30
  !--- allowing all other traffics
  R2
   router bgp 100
   no synchronization
   bgp log-neighbor-changes
   neighbor 10.10.12.1 remote-as 100
   neighbor 10.10.12.1 next-hop-self
   neighbor 10.10.23.3 remote-as 30
   neighbor 10.10.23.3 route-map Peer-R3 in
   no auto-summary
  !
  ip bgp-community new-format
  ip community-list 1 permit 100:300
  ip community-list 2 permit 100:250
  !
  
  route-map Peer-R3 permit 10
   match community 1
   set local-preference 130
  !
  route-map Peer-R3 permit 20
   match community 2
   set local-preference 125
  !
  route-map Peer-R3 permit 30
  !
  R3
  router bgp 30
   no synchronization
   bgp log-neighbor-changes
   network 6.6.6.0 mask 255.255.255.0
   network 7.7.7.0 mask 255.255.255.0
   neighbor 10.10.13.1 remote-as 100
   neighbor 10.10.13.1 send-community
   neighbor 10.10.13.1 route-map Peer-R1 out
   neighbor 10.10.23.2 remote-as 100
   neighbor 10.10.23.2 send-community
   neighbor 10.10.23.2 route-map Peer-R2 out
   no auto-summary!access-list 101 permit ip host 6.6.6.0 host 255.255.255.0
  access-list 102 permit ip host 7.7.7.0 host 255.255.255.0
  !
  route-map Peer-R1 permit 10
   match ip address 101
   set community 100:300
  

  !--- Sets community 100:300 for routes matching access-list 101.

  
  route-map Peer-R1 permit 20
   match ip address 102
   set community 100:250

  !--- Sets community 100:250 for routes matching access-list 102.

  
  route-map Peer-R2 permit 10
   match ip address 101 
•  set community 100:250

  !--- Sets community 100:250 for routes matching access-list 101.

  
  route-map Peer-R2 permit 20
   match ip address 102
   set community 100:300

  !--- Sets community 100:300 for routes matching access-list 102.

  Verification:

  From R1: 

  R1 receives prefixes 6.6.6.0/24 and 7.7.7.0/24 with communities 100:300 and

  100:250, as shown in bold in the show ip bgp output of this section.

  

  
  !--- Prefix 6.6.6.0/24 with community 100:300 received from
  10.10.13.3 (R3) is assigned local preference 130.

  
  !--- Received prefix 7.7.7.0/24 over iBGP from 10.10.12.2
  
  (R2) with local preference 130.
  
  

    !--- Prefix 6.6.6.0/24 with community 100:250 received from  10.10.23.3 (R3) is assigned local preference 125.

!--- Prefix 7.7.7.0/24 with community 100:300 received from
10.10.23.3 (R3) is assigned local preference 130.