Re: Command Preference?

Right, that's the key; the DBD MTU check is done on purpose. What I should have said is that MTU mismatch is a well known design problem, in which the OSPF control plane attempts to alert you of; it's not an OSPF design issue per-se. Disabling the MTU check fixes the symptom, but not the underlying problem.

Also note that if you test this there is an underlying difference between the "mtu" command and the "ip mtu". Also FastE doesn't support changing the physical MTU, so you'd need to test this on GigE, TenGigE, or POS. Seeing the problem is less likely on POS though, since some encapsulations don't work if the MTU doesn't match in the first place.

Brian McGahan, CCIE #8593 (R&S/SP/Security)
bmcgahan_at_INE.com
 
Internetwork Expert, Inc.
http://www.INE.com

On Mar 29, 2011, at 8:08 PM, "Carlos G Mendioroz" <tron_at_huapi.ba.ar> wrote:

> Brian McGahan @ 29/03/2011 20:23 -0300 dixit:
>> Test it out and see for yourself. It's always been a known design problem for OSPF,
>> that's the the MTU check is there to begin with.
>
> AFAIK, it's a design *feature*.
> At least, that's Moy point of view, citing from "Anatomy of an internet
> protocol":
> ...
> Over these data links, it is possible that two neighboring routers will
> disagree on the largest packet that can be sent over the link, which
> causes problems in forwarding. As one router sends a packet that is too
> big for the other to receive, it becomes impossible to deliver large
> packets over certain paths. (One might think that IP fragmentation
> would deal with this situation, but although fragmentation nicely
> handles links with differing MTUs, it assumes that all routers attached
> to a given link agree on that link's MTU.) As a result, OSPF was
> modified to detect and avoid links having MTU mismatches.
>
> i.e. it's done on purpouse.
>
> Given that PMTU discovery can be hard to get done because of security
> wizards, having the routing protocol choose routes over a consistent MTU
> path sounds right. We should not resort to tcp mss tinkering if it was
> always that way.
>
> -Carlos
>
> --
> Carlos G Mendioroz <tron_at_huapi.ba.ar> LW7 EQI Argentina

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Received on Tue Mar 29 2011 - 23:44:34 ART