MPLS VPN packet forwarding

[auosdavid2000]

I read in book (MPLS fundamentals) in section (Packet Forwarding in an MPLS VPN Network):-

How does the egress PE router know which VRF the packet belongs to? This information is not in the IP header, and it cannot be derived from the IGP label, because this is used solely to forward the packet through the service provider network. The solution is to add another label in the MPLS label stack. This label indicates which VRF the packet belongs to. Therefore, all customer packets are forwarded with two labels: the IGP label as the top label and the VPN label as the bottom label.

Ok, I so confuse, why I put other label to now which VRF, I believe this should do by RT.

[Giuseppe Larosa]

Hello Auosdavid2000,

with MPLS there is a clear distinction between forwarding plane ( = user traffic ) and the signalling plane ( routing protocols and other protocols like LDP, RSVP TE).

The route target is a signalling plane key concept ( a BGP extended community attribute) that allows a PE to decide if a VPNv4 route has to be imported in a VRF table or not ( a sort of color of the route).

The VPN label is associated to the VPNv4 prefix and it is a unique MPLS label picked by MP BGP from the originator PE label address space to be used in the forwarding plane when sending packets from remote PE to the specific VPNv4 destination.

It is the way that packets are sent from remote PE nodes to the egress PE node.

Another important aspect is the penultimate hop popping PHP the penultimate MPLS router removes the external label ( the IGP label in your book) and traffic is sent to the egress PE with only an MPLS label that is unique in the MPLS label space of the node and allows to direct the IPv4 packet to the correct VRF.

MPLS label stack is used for MPLS  L3 VPN forwarding. The VPN label acts as a service discriminator (demultiplexer) and allows for efficient packet forwarding.

Hope to help

Giuseppe

[auosdavid2000]

Dear Giuseppe,

You are great, thx a lot.