Hi John, The way I understood this is intending to work in practice is simply to create IBGP session between GW1 & GW2,
If we have this IBGP session then there are two cases: * we receive route to X from peer GW so we know peer GW can reach X hence it is safe to advertise X with both GWs as NHs * we send route to X to peer GW so we know that peer GW can reach X (at least via sender's GW) hence it is again ok to advertise X with both GWs as NHs Seems like this logic can solve your question ... But good catch :) Cheers, Robert On Fri, May 14, 2021 at 10:12 PM John Scudder <jgs= [email protected]> wrote: > Hi Adrian, > > Thanks for your reply. Pressed for time at the moment but one partial > response: > > On May 14, 2021, at 1:04 PM, Adrian Farrel <[email protected]> wrote: > > Agree with you that "stuff happens." I think that what you have described > is a window not a permanent situation. > When GW2 knows it can't reach X any more, it will stop advertising X, and > GW1 will receive that and will update what it advertises on behalf of GW2. > > > Ah, perhaps I have badly misunderstood the way this works. I had thought > it went something like this: > > - GW1 knows it can reach GW2 because of GW2’s auto discovery route > - GW1 knows the set S of internal prefixes it can reach > - GW1 advertises each prefix from S with both GW1 and GW2 in the tunnel > attribute > > In the description above, there’s no notion of GW2 telling GW1 what > internal prefixes GW2 can reach, or GW1 caring. Now I suppose you are > telling me that it goes: > > - GW1 knows it can reach GW2 because of GW2’s auto discovery route > - GW1 knows the full set of prefixes GW2 can reach. _How does it know > this?_ > - GW1 constructs each advertisement listing only the correct set of > gateways in the tunnel attribute > > The key question is the one I’ve highlighted: how does GW1 come to know > GW2’s internally-reachable prefixes? I didn’t notice any of this in the > spec. Maybe it was just my sloppy reading, I’ll look again. > > Further, if GW1 can no longer receive advertisements from GW2 then it will > stop advertising on behalf of GW2. > > > Yes, that’s understood, but I was positing a case where just because GW1 > can reach GW2 stably, and just because GW1 can reach X stably, it does not > imply GW2 can reach X. > > —John > _______________________________________________ > BESS mailing list > [email protected] > https://www.ietf.org/mailman/listinfo/bess >
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