Subject: Re: Link Flap Damping

From: "Manral, Vishwas" <[email-masked]>
Date: Tue, 30 Jul 2002 10:06:05 -0400

Hi Yasu,

I will try reply in brief.

> So, hellos may be dropped anyway whether L2 dampening feature is on or
> not (off).
Exactly. The packets would be dropped if the Lower-Layer is down. By
damping, we are keeping it down even when it isn't really, so that we do not
have persistent cycles of up/down. I think we have some slight confusion
here, regarding what you are thinking and what I am.

>Yes, I've read the draft and it'll be very effective where hello/dead
>are 10/40s (or similar). But none of this will solve the problem
>essentially.
I was talking about doing something like section 4.2.2.3.

What I meant was imagine a case where we get a flap from the lower-layer to
OSPF, a Down and then an UP. What we could do in such a case would be as
soon as we get the down event, we do advertize the link as down, however we
do not actually bring the adjacency down for some short period of time. So
incase the link comes up immediately we do not have to reform adjacency all
over again. If the other end however breaks the adjacency we anyway form
adjacency from the beginning again.

>No, I don't think so.
>
>In usual case below:
>
>  [sw1]-[sw2]-[sw3]
>    |           |
>    R1         R2
>
> If L2 switch [sw2] or the link connected to it flaps, R1 and R2 would
> not be able to recognize the flaps even if there's some conversation
> about the link's (L2) information above [sw1], [sw2] and [sw3].
If the link is flapping on sw2, sw2 would damp it wouldn't it. If we did
have flap dampening at layer 2. then the two ends R1 and R2, would not form
an adjacency. By the way L2 damping would help not only OSPF but all other
protocols above L2.

> That's very similar to what I am looking for ;p)
>
> But how did you recognize the flap ? All I want to know is this.
> How did you distinct the flap from the link completely down, when
> dead-interval have elapsed without receiving Hello ? How immediately
> could you recover the link's availability when the link's flaps
> vanished ?
To recognize a link flap the flap history of the link will have to be
stored. If the link went up/down x times in time t, we could damp it. We
could have a link damped for at most a time Tand so on. Methods are left to
the implementor, as well as the value of x ,T and t.  We could however do
things, as I pointed out earlier.

>Really waiting for your reply.
Guess the wait is finally over now(though it did take considerable time).
;-)

Thanks,
Vishwas

---

From: Manav Bhatia <[email-masked]>
Date: Wed, 31 Jul 2002 11:14:30 +0530

Hi Manral,

| >
| >  [sw1]-[sw2]-[sw3]
| >    |           |
| >    R1         R2
| >
| > If L2 switch [sw2] or the link connected to it flaps, R1 and R2 would
| > not be able to recognize the flaps even if there's some conversation
| > about the link's (L2) information above [sw1], [sw2] and [sw3].
| If the link is flapping on sw2, sw2 would damp it wouldn't it. If we did
| have flap dampening at layer 2. then the two ends R1 and R2, would not
form
| an adjacency. By the way L2 damping would help not only OSPF but all
other
| protocols above L2.

not exactly.

link flap damping implemented at L2 will only work if the time period of
the flap is high and the duration is small. What can be done if an
interface fails after every 10 mins and takes 20 seconds to come up. Will
we flap the link at L2 after 30 mins (hypothetical figures) when its has
flapped thrice for some 10 mins? We will thus be blocking all the traffic
which might otherwise flow over this link. If we do it at the L3 then we
can specify which all applications needs to do the damping and rest of the
things work normally following the traditional best delivery model.

I can cite cases when we may like to continue with a flapping link than
having no link/communication channel at all. If you decided to let L2 take
care of link flap damping then it can  adversely affect BGP functioning
under some circumstances. BGP may continue to use this link as the last
resort claiming reachability to some destinations through it and assigning
it a higher MED and announcing this to its external peers. The system
administrator can then suitably set the route flapping parameters
associated with that link and things will work out fine for him!

Thanks and Regards,
Manav