Subject: [Isis-wg] Overloaded DIS problems

From: "Jonnala, Nagi" <[email-masked]>
Date: Thu, 29 May 2003 08:16:14 -0400

All,

We have a LAN having 10 routers. After sometime for some reason,
the DIS gets *overloaded*  and cannot forward the PDUs.

Since the DIS is overloaded, as per the 7.2.8.1 section of ISO-10589
document,
the decision process(es) of a router(s) has no alternative but to drop the
pseudo node LSP. Hence no route is calculated to any of the neighbors in the

LAN (Obviously no route to other segments of the network). To be specific,
technically you can ONLY have End system neighbors(Connected IP routes) 
reachabilities of the DIS even though all other routers are connected
directly.

The major concern is if you have one router gone bad, should you allow a
whole bunch 
of others non-functional? I'm aware that sys-ad can change the DIS priority
on
another router and make things work. But shouldn't protocol take care of
that case?

The second problem is -  overloaded DIS anyway cannot do its flooding job
properly
and there is always a possibility of all sorts of issues. If we know that
DIS 
can't do a proper job, can't we make him resign from the job,  provided
there is
backward compatibility and a simpler solution is available.

I think it is a considerable issue to be addressed by the group. I can think

of a simpler solution but before digging into the solution space I would
like to receive
group's response on the problem.

Let me know if we are not in sync.

Thanks
Nagi.

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From: mike shand <[email-masked]>
Date: Thu, 29 May 2003 14:15:16 +0100

>the DIS gets *overloaded*  and cannot forward the PDUs.

I think you have a misunderstanding of what is meant by "overloaded". 
"Overloaded" in this context means that a router is unable to store the 
entire LSP database (and therefore any routes which it computes are likely 
to be wrong). It does NOT mean that it has failed in some way. If it has 
failed it should really be taking itself out of the network (perhaps by 
setting a zero holding time on its hellos). Merely setting the "LSP 
database overload" bit is not necessarily enough. If it has enough smarts 
to set the overload bit, then it has enough smarts to take itself out.

Are you saying that the DIS is setting the overload bit for the wrong 
reason? (See 7.3.19)

I'm a little concerned that you seem to imply that the overload bit is 
being set in the pseudonode LSP. There is no reason to do this, since 
routes computed "through" the pseudonode, do not (in general) require 
forwarding by the DIS. I don't believe you will find anywhere in ISO/IEC 
10589 which requires the setting of the overload bit in the pseudonode LSP.

In general the setting of the overload bit is a strategy of last resort 
designed to allow the network to recover when the temporary condition which 
caused it goes away (for example accidental area merge). If it is the case 
that one router on a LAN has a much lower LSP storage capacity, then it 
might be an idea to set that with a lower priority of being DIS.

Overload is something which should never occur in a well designed network. 
(Of course the overload bit may be set for other reasons such as BGP 
synchronization, but that is a different issue).

         Mike