Nov 19 2008
Inter-AS MPLS VPN Option b – Type 2b
Inter AS MPLS VPN Option b has three options as described below.
In the second option ie 2b an MP-eBGP session is configured between ASBRs to exchange VPNv4 prefixes.
The Link between the ASBRs does NOT run LDP/TDP and does not require an IGP.
The link is a straight forward point to point IP link.
A sample topology can be seen below.
In this option, the ASBRs do not change the next hop of the destination prefix when advertising the prefixes using MP-iBGP.
When an ASBR receives a VPNv4 prefix from another ASBR, a /32 host route is created for the sending ASBR.
This host route needs to be redistributed into your IGP to ensure the LSP can be built.
The VPN label for the prefix changes only twice compared to three times of option b, 2a.
For a prefix Generated by CE1, PE1 will generate the first VPN label. ASBR1 will then change the VPN label when it sends the VPNv4 prefix using MP-eBGP to ASBR2. ASBR2 will then send the VPNv4 prefix to PE2 with the VPN label unchanged.
To can see the VPN label generated as an “in” label using the commands
sh ip bgp vpnv4 all label
On PE1 and ASBR1 you will see the locally generated “in” label.
If you run the above command on ASBR2 you will notice their is no “in” label, this shows the difference between the two sub options 2a and 2b.
