Subject: Re: Link Flap Damping

From: "Manral, Vishwas" <[email-masked]>
Date: Fri, 26 Jul 2002 01:18:17 -0400

Hi Acee,

>   1. Dampening the delivery/processing of "link up" events is a very good
idea.
>       However,  I believe that this should be done locally at layer 2
rather
>       than in OSPF.
I agree here. Infact check the mail I sent to the list some time back.
http://discuss.microsoft.com/SCRIPTS/WA-MSD.EXE?A2=ind0108&L=OSPF&P=R7962&I=
-3 where I agree to what you have said, link flap is best dealt with the
lower layer, the closer to hardware the better.

>   2. The OSPF specified MinLSInterval is 5 seconds (RFC 2328, appendix B).
This
>        is already quite a long time and I don't think further dampening is
required.
There is a difference here. MinLSInterval does prevent the same LSA from
being originated too soon. However this does not prevent the entire
adjacencies to have to be reformed/broken. Damping the flap can prevent us
from doing this(in case the thing is not done at a lower layer).

>   3. OSPF is an IGP and, as such, an OSPF instance should span a single
routing
>       domain under a single administrative authority. Hence, it should be
possible
>       to address and remedy link flapping issues.
Agreed !!!

>   4. I haven't heard a requirement for such a feature from a network
operator
> (public or enterprise). Has anyone?
Not exactly, but I guess the study by Cengiz and Steve on faster convergence
does talk about advantages of asymmetricaly damping Up/Down events, where
and how it is done I guess is left to the implementor. This was presented at
one of the recent NANOG meetings. The study was done on the Qwest backbone.

In our draft(with the AT&T folks) we did find this feature useful in case of
congestion/scalability conditions too. Infact I guess Nokia also did a study
on this and came with similar results.

Thanks,
Vishwas

---

From: Acee Lindem <[email-masked]>
Date: Fri, 26 Jul 2002 02:15:58 -0400

> Hi Acee,

Hi Vishwas,

> from doing this(in case the thing is not done at a lower layer).

You're right. After I sent the E-mail I thought of at least two scenarios where
MinLSInterval  doesn't prevent multiple originations and flushes in the
same 5 second interval. There are probably more.

    1. You have a  broadcast network with two routers. The non-DR is flapping.
        The DR will originate and purge his network LSA at the rate it takes to
        for the two routers to establish an adjacency (the DR's router LSA will
        be origination rate limited by MinLSInterval) . This could easily be
        handled - does anyone keep a timestamp in the OSPF interface data
       structure to prevent this?

   2. An ABR's intra-area routes are flapping faster than MinLSInterval. This
        could happen if multiple routers are advertising the same unstable network
        but are not in sync as far as originating LSAs. Each of the intra-area routers
        will be limited by MinLSInterval but the ABR could be purging and
        originating the corresponding summary LSAs at a faster rate.

> on this and came with similar results.

I'll take a look at the draft again.

--
Acee

---

From: Amit Srivastava <[email-masked]>
Date: Mon, 29 Jul 2002 02:10:51 -0700

Hi All,
    We know that vitual link are basically for the
back bone connectivety. Now consider a scenirie where
we have R1 connected to area 1 , area 2 ,area 3 and R2
connected to area 2 and area 4 now if i want to
configure a virtual link between R1 and R2 what should
be the LSAs flowing over the virtual link.(As there in
no back bone connectivety)
Regards
Amit

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