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Wednesday, May 1, 2013

MPLS Route Distinquisher l2vpn in comparison to l3vpn connections.

With l3vpn we have 2 mpls label the top label for communicating between the PE and the PE (most likely loopback ip’s of these routers) and we have the mpls vpn (inner) label with consists of the ip prefix + the route distinquisher so mpls know how to differentiate same routes from multiple customers.

Unfortunately now our confusion starts, with l2vpn connections you also have route distinquishers, but why do we have them there? For instance with Juniper you have a remote site ID 1 which is communicating with remote site ID 2 and we do nothing with prefixes at all. So if i say this RD is used for making every l2vpn connection in the cloud unique, is this a correct way of saying it?


The inner label in L3 VPN is not related to the route distinguisher

The VPNv4 prefix is formed by prepending the RD to the original 32 bit IPv4 prefix.

the route distinguisher makes the prefix unique in the signalling plane allowing to discriminate between overlapping prefixes in different VRFs /VPNs.

The inner label is an attribute of the VPNv4 NLRI and is part of the forwarding plane, the sending PE node tells to all the potential peers what inner label it expects to receive when traffic is sent to this specific NLRI.

In Juniper L2VPN signalling is made with MP BGP using a different address family the l2vpn address family.
This is called Kompella L2VPN from the name of its inventor.

As you have guessed also in this case the RD assumes the role of identifying the site. If you can look at the l2vpn MP BGP route you will see the site-id at the end of the composite prefix.

We could say that in L2VPN the prefix is indeed the site id prepended by some other information including the RD.



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