The "corner" case happens 1/2 the time. Initial: Plane1 advertises P1. Plane2 advertises P2.
P1 goes down: Plane1 advertises P2. Plane2 advertises P3. The case that the client receives Plane2 message first is 50% likely. During the flap, it has: Plane1: P1 Plane2: P3 P1 is unreachable, so it chooses P3. Originally, I proposed a delay on Plane2 advertisements, but you didn't like it. How "small" should the delay be? I think longer than MRAI, since Plane1 may be running that timer for its P2 advertisement. What is the "extra session"? Is it a 3rd RR plane? -- Jakob Heitz. > -----Original Message----- > From: Robert Raszuk [mailto:[email protected]] > Sent: Sunday, March 27, 2011 1:46 PM > To: Jakob Heitz > Cc: [email protected]; IETF IDR > Subject: Re: [GROW] Potential route flap with bgp-diverse-path > > Hello Jakob, > > > If there are 3 paths: P1, P2 and P3 > > and 2 RR planes. > > Plane1 advertises best, Plan2 advertizes 2nd best. > > Ok. > > > When the bestpath P1 goes away, the client can > > flap to the 3rd best path, P3, before coming > > back to the 2nd bset path P2. > > Nope it will not .. see below on comments to your description. > > > Here is the initial condition: > > P1 is best, P2 2nd best, P3 3rd best. > > Plane1 advertises P1, Plane2 advertises P2 > > Perfect ! > > > Because the advertisements from the RR planes are > > not synchronized, the order of events could be: > > > > 1. P1 goes down: P2 becomes best, P3 becomes 2nd best. > > Ok. > > > 2. Client receives IGP message of P1 down: Client chooses P2. > > Ok. > > > 3. Client receives P3 from Plane2: Client chooses P3. > > This is only when client would not have P2 at all. IMHO > corner case (see > below why) > > > 4. Client receives P2 from Plane1: Client chooses P2. > > P2 should not go away from client. > > > Possible fix: Recommend at least as many RR planes > > as possible paths for any destination. > > Nope. > > There are two simple solution here: > > - Presence of an extra session from client to either RR to make sure > that your second best is not removed before overall best is advertised > > - Small delay on the RR when advertising 2nd best to clients to make > sure overall best is there first in some implementations just > configuring per session mrai may help here. > > Cheers, > R. > > _______________________________________________ GROW mailing list [email protected] https://www.ietf.org/mailman/listinfo/grow
