Re: [GROW] Potential route flap with bgp-diverse-path

[Jakob Heitz <[e-mail]>]

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:[e-mail]] 
> Sent: Sunday, March 27, 2011 1:46 PM
> To: Jakob Heitz
> Cc: [e-mail]; 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.
> 
>