Subject: [Isis-wg] question on MTU

From: Rick Han <[email-masked]>
Date: Wed, 31 May 2000 16:14:02 +0000

Hi, iso10589 (8.2.3 (c)) says that IIH should be padded (at least for the
first one) to the maxsize-1. This is to make sure that allISs in the domain
use the same MTU. Does this mean that MTU check is part of HELLO checking
before further processing the PDU? My understanding is that there will be
no adjacency established if an IS uses different MTU as other ISs are
configured. Is this correct? It seems Cisco allows adjacency setup even the
two routers configured different MTU, why is that? 

Thanks

Rick 
-- 
Rick Han
Iron Bridge Networks, Inc.
[email-masked]
(781)372-8180

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From: Jeff Learman <[email-masked]>
Date: Wed, 31 May 2000 16:56:26 -0400

Remember that ISO 10598 requires point-to-point links to
be reliable, so that failure to deliver an IS-IS PDU would
cause the link to go down.  Note that 8.2.2 does not require
the IS to check the size of the received IIH.

I think the idea was to make sure that whatever your idea of
the MTU size is, that you can transmit a packet of that size
without worrying about it being dropped by the link layer.
If you're transmitting IIHs that are too large to be received
at the other end, your data-link connection will go down and
the adjacency will never come up.  But this only works if the
link is reliable (that is, if the link goes down if it can't
deliver a packet).

Since Integrated IS-IS is usually used over unreliable links
(ones where failure to deliver doesn't cause anything special),
the guarantee that you can actually deliver a packet of what
you think is the maximum size doesn't work any more.

On the other hand, isn't MTU size negotiation handled by LCP/NCP?
If IS-IS is run over PPP with MTU size negotiation, this IS-IS
guarantee may no longer be important.

Regards,
Jeff

>two routers configured different MTU, why is that? 
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