Reviving an oldie but a goodie, based on some work I was doing today. I wanted to check something that required 3 routers, and all I had were 2 routers and the 3550 switch. well, that's ok. L3 and all. BTW, have we decided which is better - an L3 switch or a router? Hint - the L3 switch is FAR superior to a 2503 router :->
OK, so I set up ospf among the routers ( switch ports configured as router interfaces ) and no ospf neighbor relationships are forming. this is BS. Done this in my sleep. start looking at the debugs and I keep seeing something weird from the switch side. keeps reporting the interface down. makes no sense. I check the speed and duplex, but you know, I know this has worked in the past. fiddle some more. fiddle some more. finally look at things from one of the routers' perspective, and the debug says something about a mismatched MTU. Oh yeah, I was doing some vlan tunneling practice and to do so, you have to set the switch MTU higher to accommodate the larger 801.q frame. can't change the MTU size on an interface by interface basis. quickly, I issue the interface command ip ospf mtu-ignore ( Cisco proprietary? IIRC? ) on the interfaces in question, and viola! everything is dandy. quick look at the command reference, and I see this command was introduce in 12.0.3 - so that puts it into the time frame of the early days of the 65xx and the MSFC. Dare I hazard a guess that the command was introduced in anticipation of exactly this kind of situation - the L2 part of the switch requiring a larger MTU for whatever reason, and the L3 part of the switch running Ospf and running into exactly this problem? Geez, some days I really appreciate the time I spend on this group. Amazing the stuff I remember. Chuck -- TANSTAAFL "there ain't no such thing as a free lunch" ""Kane, Christopher A."" wrote in message news:[EMAIL PROTECTED] > 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 Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=63936&t=63936 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
