What is basic BGP community concept?

What is basic BGP community concept?

 

This instruction is good reference. In fact, I will analysis this instruction. BGP community is used for filtering and tagging information over routing information. Therefore, routing table information will be controlled by these condistions. Thus I can select the routing information to advertise or update in the table.

 

1. Pre-requiste (Basic BGP configuration)

This is my environment for this post. 

S1	S2	S3	S4
interface Ethernet1/1    no switchport    ip address 100.12.1.11/16    no shutdown  !  interface loopback0    ip address 1.1.1.1/32  !  interface loopback1    ip address 11.11.11.11/32  !   router bgp 1    router-id 1.1.1.1    log-neighbor-changes    address-family ipv4 unicast      network 1.1.1.1/32      network 11.11.11.11/32    neighbor 100.12.2.11      remote-as 2      update-source Ethernet1/1      address-family ipv4 unicast	interface Ethernet1/1   no switchport   ip address 100.12.2.11/16   no shutdown interface Ethernet1/2   no switchport   ip address 100.23.2.12/16   no shutdown ! interface loopback0   ip address 2.2.2.2/32 ! interface loopback1   ip address 22.22.22.22/32 !   router bgp 2   router-id 2.2.2.2   log-neighbor-changes   address-family ipv4 unicast     network 2.2.2.2/32     network 22.22.22.22/32   neighbor 100.12.1.11     remote-as 1     update-source Ethernet1/1     address-family ipv4 unicast   neighbor 100.23.3.12     remote-as 3     update-source Ethernet1/2     address-family ipv4 unicast	interface Ethernet1/2   no switchport   ip address 100.23.3.12/16   no shutdown interface Ethernet1/3   no switchport   ip address 100.34.3.13/16   no shutdown ! interface loopback0   ip address 3.3.3.3/32 ! interface loopback1   ip address 33.33.33.33/32 ! router bgp 3   router-id 3.3.3.3   log-neighbor-changes   address-family ipv4 unicast     network 3.3.3.3/32     network 33.33.33.33/32   neighbor 100.23.2.12     remote-as 2     update-source Ethernet1/2     address-family ipv4 unicast   neighbor 100.34.4.13     remote-as 4     update-source Ethernet1/3     address-family ipv4 unicast	interface Ethernet1/3   no switchport   ip address 100.34.4.13/16   no shutdown ! interface loopback0   ip address 4.4.4.4/32 ! interface loopback1   ip address 44.44.44.44/32 ! router bgp 4   router-id 4.4.4.4   log-neighbor-changes   address-family ipv4 unicast     network 4.4.4.4/32     network 44.44.44.44/32   neighbor 100.34.3.13     remote-as 3     update-source Ethernet1/3     address-family ipv4 unicast

For nexus, "address-family ipv4 unicast" in neighbor parameter and "address-familiy ipv4 unicast" in global parameter are necessary to advertise to the peer. After configure these, I can verify the routing table and BGP information. From S2, 

From S3,

From S4, 

I can confirm that the information from S1 such as "1.1.1.1" and "11.11.11.11" are advertised to S2, S3 and S4.

 

2. Configure send community

 

To use the BGP community, I have to configure "send community". There are 2 types, standard and extended. In this struction, the extended community has more information such as VPN services.

I will added the configuration like below.

S1	S2	S3	S4
router bgp 1   neighbor 100.12.2.11     address-family ipv4 unicast       send-community both	router bgp 2   neighbor 100.12.1.11     address-family ipv4 unicast send-community both   neighbor 100.23.3.12     address-family ipv4 unicast send-community both	router bgp 3   neighbor 100.23.2.12     address-family ipv4 unicast send-community both   neighbor 100.34.4.13     address-family ipv4 unicast send-community both	router bgp 4   neighbor 100.34.3.13     address-family ipv4 unicast send-community both

Please note that there are any differences of routing table after these configuration above.

 

3. Configure Route-map in and out

 

To use the BGP community, the route-map is necessary. This route-map affect the routing table. In S3, I will add "route-map" configuration.

! router bgp 1 neighbor 100.12.2.11 address-family ipv4 unicast route-map to-remote-as2 out end !

"route-map to-remote-as2 out" mean that routing information to transfer out will be controlled with this route-map. Therefore, after this configuration, the routing table will be changed. "1.1.1.1" and  "11.11.11.11" are removed. 

Now, I will add the policy to set community.

ip prefix-list ip-prefix-1 seq 10 permit 1.1.1.1/32 ip prefix-list ip-prefix-2 seq 10 permit 11.11.11.11/32 route-map to-remote-as2 permit 10   match ip address prefix-list ip-prefix-1   set community 12:130 route-map to-remote-as2 permit 20   match ip address prefix-list ip-prefix-2   set community 12:150

I check the routing table from S4. "1.1.1.1" and "11.11.11.11" will be updated. 

And I can see more detail. I can see the community values are changed on each.

So far, I learn how to set the community with BGP. From now, I will handle how to get the community and use this. This is so similar with above. I will add configure into the S2.

! router bgp 2 neighbor 100.12.1.11 address-family ipv4 unicast route-map from-remote-as1 in end !

After configure this, the routing table will be changed. "1.1.1.1" and "11.11.11.11" are removed again.

Now, I will add some configuration to get community and use it in S2. 

ip community-list expanded community-1 seq 1 permit "12:130" ip community-list expanded community-2 seq 1 permit "12:150" route-map from-remote-as1 permit 10   match community community-1   set local-preference 130 route-map from-remote-as1 permit 20   match community community-2   set local-preference 150

These configuration effect S2 routing table like below.

However, it does not effect S3 and S4, even if the community value is left.

 

Reference

 

[ 1 ] https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/28784-bgp-community.html

[ 2 ] http://www.ciscopress.com/articles/article.asp?p=2756480&seqNum=12

[ 3 ] https://blog.naver.com/happy_jhyo/221291791846