Introduction to BGP Confederations

Earlier we discussed on BGP Route Reflector for solving the BGP full mesh scenario and we have another way of solving BGP full mesh scenario. 

What is Full mesh in iBGP ?
With the full meshed network you will have to maintain all the links from all the routers to each other in the internal BGP network. So let us suppose you have 10 routers in your network and you want to configure iBGP network between them, so how many links you required to maintain the fully meshed network here. Well you need to have the 45 links between these 10 routers to establish the iBGP links between all of them.

You ever think that this may be of higher cost and ever router needs to have the information of the other routers in the network. Well to sort out iBGP full meshed network scenario we have two methods and these methods are Confederations and Route Reflectors.

Let's go through the other method of Confederations resolving BGP full Mesh scenario. 

Fig 1.1- BGP Confederations

With the help of the Confederations, we can reduce the number of the peers within the AS. We use confederations to divide the AS into multiple mini-ASs and assign the mini-ASs to a confederation. Each mini-AS is fully meshed, and IBGP is run among its members. Each mini-AS has a connection to the other mini-ASs within the confederation. 

Even though the mini-ASs have EBGP peers to ASs within the confederation, they exchange routing updates as if they were using IBGP and that is, the next hop, MED, and local preference information is preserved. To the outside world, the confederation looks like a single AS. 

    Configurations on NDNA_R1

If you look on the configurations showing above, the bgp confederation identifier router configuration command specifies that Router NDNA_R1 belongs to confederation 100. The first two neighbor remote-as router configuration commands establish IBGP connections to the other two routers within AS 65050. 

The second two neighbor remote-as commands establish eBGP connections with confederation peers 65060 . The last neighbor remote-as command establishes an EBGP connection with external AS 200. 

    Configurations on NDNA_R2

Now look on the configuration of the router NDNA_R2. The router bgp global configuration command specifies that Router D belongs to AS 65060.The bgp confederation identifier router configuration command specifies that Router NDNA_R2 belongs to confederation 100.

Here the same we have the the first neighbor remote-as router configuration command establishes an IBGP connection to the other router within AS 65060. The second two neighbor remote-as commands establish eBGP connections with confederation peers 65050. The last neighbor remote-as command establishes an EBGP connection with AS 300 and AS 400. The following commands configure Router NDNA_R3.

Configurations on Router NDNA_R3
!
router bgp 200
neighbor 50.50.50.2 remote-as 100
!

The neighbor remote-as command establishes an eBGP connection with Router NDNA_R1. Router NDNA_R3 is unaware of AS 65050 and AS 65060. Router NDNA_R3 only has knowledge of AS 100.

Configurations on Router NDNA_R4    
!
router bgp 300
neighbor 60.60.60.2 remote-as 100
!

The neighbor remote-as command establishes an eBGP connection with Router NDNA_R2. Router NDNA_R4 is unaware of AS 65050 and AS 65060. Router NDNA_R4 only has knowledge of AS 100.

Configurations on Router NDNA_R5    
!
router bgp 400
neighbor 70.70.70.2 remote-as 100
!

The neighbor remote-as command establishes an eBGP connection with Router NDNA_R2. Router NDNA_R5 is unaware of AS 65050 and AS 65060. Router NDNA_R5 only has knowledge of AS 100.