At 3:43 PM -0400 4/17/02, Kane, Christopher A. wrote: >In an attempt to find out why MTU is examined (more precisely, why it's >examined in the Database Description packets instead of the Hello packets) >one of my co-workers found this passage in IETF meeting minutes: > >"Editor's note: These minutes have not been edited. > >The OSPF Working Group met on Wednesday, December 11th from 1300-2500 at >the San Jose IETF. Minutes of the meeting follow: > >The second problem, reported by Dan Senie of Proteon, concerns MTU >mismatches between OSPF neighbors. This can cause flooding between >the two neighbors to fail, with large Link State Updates being >continually retransmitted. To fix this, we will report interface MTU >in Database Description packets. A router will discard received >Database Description packet which advertise an MTU that is larger >than the router can receive. In this way, adjacencies will not form >between routers having MTU mismatches. Tony Li expressed a desire >for a more general purpose mechanism. There was also a question >whether the same thing will have to be done for OSPF for IPv6 (we >think so)." > > >Very informative. Thank goodness for meeting minutes. Here's the link if >anyone is as hung up on this as I seem to be. :) > >http://www.ietf.org/ietf/ospf/ospf-minutes-96dec.txt
Hmmmm...I _think_ I was at that meeting...or at least one in SJ about that time. In a broader sense, I've run into other operational issues involving the MTU. There's been a weird interaction between Cisco and Bay RS OSPF, where Bay thinks Cisco's 1500 MTU is 1472. Don't know if it ever was fixed. Incidentally, Passport OSPF is a different implementation than Bay RS. While, in principle, OSPF supports fragmentation, it's one of those things that I avoid like the plague. It tends to exercise parts of the code that were rarely tested. When I was at Nortel, a sales type came running in announcing that some competitor could do, IIRC, 47 neighbors per hello. He wanted us to say we could do more, just because bigger numbers are better in sales. The sanity of having 47 neighbors on an interface was not considered. Anyway, I did a back-of-the-envelope calculation, and this number (might have been 46 or 48) was the maximum number of neighbors that could fit into a 1500 byte Hello packet. Good, practical restriction, that never should be approached in practice. -- "What Problem are you trying to solve?" ***send Cisco questions to the list, so all can benefit -- not directly to me*** ******************************************************************************** Howard C. Berkowitz [EMAIL PROTECTED] Chief Technology Officer, GettLab/Gett Communications http://www.gettlabs.com Technical Director, CertificationZone.com http://www.certificationzone.com "retired" Certified Cisco Systems Instructor (CID) #93005 Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=41788&t=41788 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
