[c-nsp] Questions regarding 6PE and route aggregation

Spyros Kakaroukas skakaroukas at rolaware.com
Tue Feb 25 14:55:16 EST 2014


I’ve been trying to evaluate 6PE as a transition mechanism lately and I’ve stumbled upon something I didn’t initially expert. My understanding of 6PE is as follows ( and feel free to correct me if I’m wrong ☺ ) : PE-A and PE-B peer over IPv4, they exchange routes and labels. Packets transit the core over ipv4 and have 2 labels; outer label describes the LSP towards the remote PE while the inner label describes the actual IPv6 next hop ( “CE” or whatever you want to call it ). Label allocation is per-prefix ( unless you switch to 6VPE, where you can actually change it, with all the consequences that follow ). So, I’ve been thinking, what if I don’t want to send the full table to some PEs, but only internal routes and a default route ? Internal routes obviously shouldn’t pose an issue, but aggregating everything to the default is somewhat trickier. So, I fired up my lab, got an ebgp session with an upstream and…here’s what happened:

Scenario A, get a default route from an upstream and advertise it downstream while suppressing all other external routes: Everything works fine. Would it work fine with >1 upstream though, or would I get suboptimal routing, as the PE with the full table would just perform a lookup on the inner label ( which would correlate to whatever single default route was preferred from upstreams ) ?

Scenario B, no default route from upstreams. Just generating one towards downstreams with default-information originate: Control plane looks ok , data plane just fails!

Scenario C, same as scenario B apart from the addition of a static default to Null0 on the PE originating the default route: Surprisingly, it works. Is it supposed to work though ? If so, should I assume it’s doing 2 lookups ( one of the label and one for the actual prefix ) with all the performance implication this carries ?

Also, any advise on how you tackled similar issues would be greatly appreciated. Assuming scenario C is a valid design, it feels kinda wasteful. I could use 6PE only on nodes that can handle a full table and dual-stack the rest, but that’s not really far from foregoing 6PE altogether and dual-stacking everything ( which may or may not be a “good thing”, depending on whether/when we see LDP/TE/etc over IPv6 ).

My thoughts and words are my own.

Kind Regards,


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