Hi Jakob,
The "corner" case happens 1/2 the time.
If you insist let's focus on it then ...
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.
There is no "flap" in the above. Let's see what client does in this case ...
Initially you have net X via P1 and P2.
P1 goes down so P2 becomes active in prefix independent convergence way
no packet drops other then IGP notification about P1 down bad event.
Now RR2 sends implicit withdraw and advertises P3 over the session so
client does RIB update for net X and in place modification for it's next
hop (assuming that P2 and P3 have even different next hops).
In decent implementation this is not removal of the path, drop then it's
new installation. In decent bgp implementation switching one active and
valid next hop with a given net with another valid next hop is not
traffic impacting (if that is what you mean by route flap).
So with the light of this what problem are you trying to address in the
first place ?
Originally, I proposed a delay on Plane2 advertisements,
but you didn't like it. How "small" should the delay be?
In non decent implementation the delay of advertising 2nd best may cover
for it. I would not make any recommendation for the value at this point
and leave it as an configuration option.
I think longer than MRAI, since Plane1 may be running
that timer for its P2 advertisement.
Well this now depends how MRAI is implemented ... does it allow always
first transition to go or not. Besides default MRAI for IBGP and this is
IBGP we are discussing has been for a long time recommended to be of zero.
What is the "extra session"?
Is it a 3rd RR plane?
Additional session allows you to receive from the same RR both best and
second best paths one on each session. Hence non of the issues you are
describing do apply then.
Cheers,
R.
--
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.
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