There are four cases when summarization shouldn’t be used in MPLS Backbone:
- MPLS VPNs
- 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 10.1.1.1/32 and P2 router summarize it to 10.1.0.0/16. In this case, P2 will have two routes in it’s routing table – 10.1.1.1/32 from the origin and 10.1.0.0/16 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 10.1.0.0/16 network.
Now, P1 thinks it have to pop label for whole 10.1.0.0/16 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.