05 Jun

Why summarization is not always good in MPLS VPN networks

There are four cases when summarization shouldn’t be used in MPLS Backbone:

  • MPLS TEs
  • MPLS-enabled ATM network
  • Transit BGP where core routers are not running BGP

So those are the cases where end-to-end LSPs are required. Let’s look why.

Let’s consider following topology:

Summarization is done on P2 router. Let’s say egress PE router is advertising it’s loopback address of and P2 router summarize it to In this case, P2 will have two routes in it’s routing table – from the origin and as a summary. P2 sends only summary to P1. Router P2 also informs P1 router he’s the PHP (penultimate hop popping) router on the path and should remove label from packet to network.

Now, P1 thinks it have to pop label for whole network before sending packet to R2. The R2 router will receive packet with no label or in case of multiple labels on stack with label that it don’t understand. This will result in packet dropping.

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