Sunday, February 22, 2009

IE Vol 2 Lab 1 Repeat Continued

Section 7 - QoS

Wow, as if QoS wasn't hard enough to begin with, matters sure get even more complex when MPLS is involved.

I actually had to go back and watch the CoD on this to get an idea on what's going on.

The bottom line is there are two key points to keep in mind:
1. There are multiple labels on the stack and the router can only look at the topmost layer on ingress.
2. Labels are swapped and/or popped so extra steps may need to be taken to ensure the packet is classified correctly on egress.

In a full lab scenario, the label stack is likely to consist of:
Bottom: VPN label imposed by the PE router
Middle: TE label imposed in the P network
Top: Link label imposed on a hop-by-hop basis

I still need to work out all the intricacies involved here. But the bottom line is there are two special cases. First, sometimes an explicit null needs to be included, instead of an implicit null. This is called Ultimate Hop Popping, and ensures QoS information is carried throughout the last hop. Second, sometimes a qos-group is needed. I still need a better understanding of when and why this is required.

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