Inter-AS MPLS VPN - Option C (BGP+Label)

In this post, we will look into the Inter-AS MPLS VPN - Option C which is also known as "BGP + Label". 

Option C uses eBGP IPv4 session between ASBRs to exchange reachability to PE loopbacks. There will be a VPNv4 neighbourship between service providers VPNv4 route-reflectors (RRs).

Option C takes away the heavy reliance on ASBRs. In this case, ASBRs are only used to exchange the loopback prefixes using eBGP IPv4 sessions.

To understand how it works, let's look at our topology below.

We have two service providers. Both are connected through ASBR routers. In this specific instance, we have local VPNv4 route-reflector for each SP. The customer site CE1 is connected to SP1 and CE2 is connected to SP2.

Inter-AS MPLS VPN - Option B (VPNv4 EBGP between ASBRs)

So far we have seen the Inter-AS MPLS VPN using "back to back VRF" method. Even though it's a relatively easy method to implement, it has few drawbacks.

This procedure doesn't require MPLS enabled on the link between ASBRs however it does not scale very well. 

There is another method in which ASBR's establish VPNv4 neighbourship, exchange MPLS labels and can maintain end-to-end LSP. It is know as "Option B".

Let's see how it works. We are going to use the same topology as we used in last post.

The IP addressing and the core routing remains the same. The only difference is the configuration on ASBR where I have removed the EBGP neighbourship statement and removed the local VRF configuration.

Inter-AS MPLS VPN - Option A (Back to Back VRF)

We have seen how standard L3 MPLS works in the previous post http://ciskonetwork.blogspot.co.uk/2015/03/layer-3-mpls-vpn.html. 

In normal implementation, customer sites are served by a single service provider. The PE routers maintain an IBGP session and the routing information gets exchanged through VPNv4 neighbourship.

What if two customer sites are connected through different service providers? The PE routers of each service provider will not be able to establish IBGP neighbourship with each other hence won't be able to exchange VPNv4 routes.

There are mainly three ways to handle this type of a situation. One of three methods is called "Option A - Back to Back VRF".