ENSS FDDI MTU

Sat Feb 6 05:47:52 UTC 1993

At what point in the future are you planning to raise the FDDI MTU on the
ENSS from 4000 bytes to the default value of 4470 bytes? It has recently come
to my attention that we, and any other regional which uses a cisco router on
FDDI to peer with an ENSS, may be taking an enormous performance hit by using
the non-standard MTU - it seems that the FDDI fast-switching path on a cisco
router is automatically disabled if the interface is configured with other than
the default MTU. This may explain some of the problems we have been having
recently with high CPU utilization levels on our routers which peer with the
NSFNET over FDDI.

Can someone estimate for me the impact on an ENSS of fragmenting >4000 byte
packets? Given the relative scarcity of full-FDDI-MTU paths through the
Internet, the likelihood that packets larger than 4000 bytes will need to be
fragmented by an ENSS seems very low to me, so unless an ENSS does something
extremely stupid (such as drop packets) when faced with such a fragmentation
decision, my guess would be that we will want to optimize the common case and
run with default MTUs on our FDDI cisco routers. In fact, as a Friday-night
experiment, I have reset the MTUs on all of our FDDI cisco routers with no
apparent ill effect. Is this likely to cause problems for the NSFNET?

Please advise.

	Vince Fuller, BARRNet technical director

P.S. Note to BARRNet NOC folks: the FDDI MTU change, which I have applied to
     SU-A, SU-B, SU-CAMPUS, and XX1, has not been committed to NVRAM, so a
     reload (or crash) will cause it to be backed-out.