Hello Costa
When implementing MPLS L3VPNs, there are cases where you may have some remote customer sites in locations where the same service provider cannot serve both sites. The result is that the backbone network that interconnects the sites, has more than one AS, thus some sort of mechanism must be established to allow inter-AS communication to take place without disrupting the VPN.
There are three ways to do this, and they are called Options A, B and C:
- Option A: Back-to-Back VRFs
- Option B: MP-eBGP for VPNv4
- Option C: Multihop MP-eBGP between RR + Labels
Because these topics are outside of the scope of current Cisco certifications at all levels, so a lesson on these is not found on the site. However, Cisco has some good content that may help you out. I’m adding a few links below:
https://learningnetwork.cisco.com/s/article/mpls-l3vpn-inter-as-option-a-part-1
You can also see this Youtube video from INE describing configurations in detail.
I hope this has been helpful!
Laz