Read the Label, the VPN l ...
30 August 10

Read the Label, the VPN label.

Posted byINE

One of the frequent questions I hear regarding L3VPNs, is regarding the bottom VPN label.  In this article, we will focus on the control plane that provides both the VPN and transit labels, and then look at the data plane that results because of those labels.

In the topology, there are 2 customer sites (bottom right, and bottom left).  The BGP, VRFs, Redistribution, etc are all configured to allow us to focus on the control and data plane.   Lets begin by verifying that R1 is sourcing the network,

MPLS-class blog3 simple larger canvas

A debug verifies that R1 is sending the updates for to R2.

R1 sources net

R2 has learned the route from R1, has assigned a VPN label for it, and has exported it from the VRF into BGP.  This lucky route was assigned the local label of 16 by R2.

R2 has route in bgp and has label for it

We can also look at the MPLS forwarding table on R2 to see the same tag information.

verfy mpls forwarding table on R2

This prefix, as a VPNV4 route, is sent as an update to the iBGP peer R4.   We can force an update with refresh.

r2 clear ip bgp

The update can be seen on the wire between R2 and R3, (on its way to R4) using a protocol analyzer.  You may also notice that R2 uses outgoing label 19 for forwarding this update to   The label can be seen in the MPLS forwarding output above.

wireshard update from r2 to R4

The VPN label being advertised in the update is Label 16, which is R2's local label for the network.

On R4, which will be the ingress PE for transit traffic destined for, we can see that the VPN label of 16 is associated with destination network of  The next hop of to reach the network, is due to R2 assigning next-hop-self for updates it sends to R4.

R4 has vpn label now learned from R2

We can also see the outgoing MPLS label that R4 will use to reach the next hop of  The label of 18 below, was advertised by R3, as the label to use to reach

R4 next hop for

We can also verify that the route ( has been imported by R4 into the customer vrf.

r4 vrf has

So when a transit packet is sent from R5 to, R4 should impose 2 labels.   The bottom label will be 16 (the VPN label) for the network (R2 told us about that via the iBGP update), and the top label should be 18 (advertised via LDP from R3), to reach the next hop of

On R4 a quick check of the CEF table for the vrf can verify both labels.

cef table on R4

A simple trace from R5, to the destination network of should prove all the labels in the path.

Trace from R5 to

The top label of 18 is used to reach the next hop of, and the bottom label of 16, which is meaningful to R2, because he sourced it, will be used by R2 in forwarding the transit traffic destined to to the next hop, which is R1.

R3 will pop the transit label off, due to R2 advertising implicit-null for the network (itself).

For more information and step by step training on MPLS, take a look at our newest MPLS self paced course!

If you like, an 8 minute video, that reviews the same steps, may be viewed here.

Thanks for reading!


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