Re: Persistent loops when mixing rtgwg-enterprise-pa-multihoming and rtgwg-dst-src-routing

[Fred Baker <[e-mail]>]

> On Jul 27, 2017, at 2:06 AM, Matthieu Boutier <[e-mail]> wrote:
> 
> Did you agree that:
> 
>  1. destination first give the correct behaviour as-is.
> 
>  2. source first needs extra mechanism and route duplication.

Actually, I don't. I can produce cases in which source first gives the wrong route, and in which destination first gives the wrong route. The only way I see to make doing either one first *always* gives the right result is if a small set of routes is duplicated.

The issue is when prefixes overlap. If you have sources S1 and S2, destinations D1 and D2, D1 is a more specific of D2, and D1 is advertised by S1 but not S2, and D2 is advertised by S2. If you are looking from S1, you should find S1->D1, and if you are looking from S2, you should find S2->D2. If you look destination first, and happen to be looking from S2, I think you wind up trying to find S2->D1, which doesn't exist.

Every time I get my head into this space, I have to rethink it, and the emails I wrote a few years back are unavailable to me now as I am no longer at Cisco. I need to think the source version through again. But you get the idea. I have pretty much convinced myself that you need to duplicate S2->D2 as S2->D1 but with the next hop associated with S2->D2 in order to make destination first work. There is a similar case regarding source-first lookup.

This is the reason I have suggested a PATRICIA algorithm or something like it that looks up both addresses at the same time.