Nov 20 2008
Inter-AS MPLS VPN Option C
Option C is the most complex of the three inter-AS MPLS VPN options, however its complexity has significant benefits in that it scales well.
However there are some trust issues which need to be overcome. Usually this Option is not implemented unless you have a merger for example and two ASs need to provide an MPLS VPN.
A Sample topology is given below.
Option C requires two bordering ASBRs to share an eBGP session to transport both IPv4 prefixes as well as IPv4 labels.
The IPv4 prefixes are usually the PE and Route Reflector loopback addresses.
BGP is then redistributed into the IGP on the ASBR.
Once the neighboring ASs loopback addresses are in the IGP database, LDP can assign a label for them and an LSP can be built from a source PE in one AS to a destination PE in another AS.
There is also an MP-eBGP peering between the route reflectors in each AS. The MP-eBGP session is configured such that the next hop is unchanged using the command below.
neighbor rr-ip-address next-hop-unchanged
The link between the ASBRs is an IP link, however when you configure Option C you will notice the command below appears on the ASBR interface.
mpls bgp forwarding
The VPN label is originated at the source PE and does not change at any point in the LSP.
