Priscilla Oppenheimer wrote: > > OSPF routers that don't agree on the MTU can get stuck in the > EXSTART phase and never succesfully exchange their database > description (DBD) packets, thus never becoming fully adjacent.
And I've actually seen this happen between a Cisco 6509 with a Flexwan and A3 Port adapter at one end, and at the other end was a Nortel BCN router with an ARE card. This was tested in a lab and the team who was implementing it got it working in the lab (it didn't work initially) by setting the 'mtu-ignore'. Unfortunately when it went to production the adjacency wouldn't come up because now the DBD's were too large. It turned out that in the Lab the adjacency came up because the initial descriptors were rather small, and hence the DBD's fell at less than a full MTU size, and came up ok in the lab once they told the Cisco to ignore the MTU mismatch. Fixed this in production by looking at what the Cisco box recorded in it's log that the mismatch size was, and set them appropriately. The Nortel box actually sent something different than what it was actually set for, and so that gave us a fit for a few minutes, until we saw what it was actually sending in the Cisco log. It's been in operation for over a year now. Have fun, Frank Merrill > > Neither router should have the MTU set to bigger than the > maximum as specified by the relevant standards for the data > link in use, but one of the routers could be set with an MTU > that is smaller than the max allowed. This router might be > unable to receive full-sized DBD packets from its neighbor. > > One fix is just to make sure the routers do agree on the MTU. > But what if the other router is Brand X router and doesn't > support such a change? > > In that case, you might want to use this new "ip ospf > mtu-ignore" command. > > Here's what Cisco says: > > "Cisco IOS ® Software Release 12.0(3) introduced interface MTU > mismatch detection. This detection involves OSPF advertising > the interface MTU in the DBD packets, which is in accordance > with the OSPF RFC 2178, appendix G.9. When a router receives a > DBD packet advertising a MTU larger than the router can > receive, the router ignores the DBD packet and the neighbor > state remains in exstart. This prevents an adjacency from > forming. To fix this problem, make sure the MTU are the same on > both ends of a link. > > In Cisco IOS Software 12.1(3), the interface-level ip ospf > mtu-ignore command was introduced to turn off the MTU mismatch > detection; however, this is only needed in rare instances." > > See this URL for the full story: > > http://www.cisco.com/warp/public/104/12.html > > Priscilla Oppenheimer > > Hello Goodbye wrote: > > > > There is a command 'ip ospf mtu-ignore' that makes > > ospf ignore the mtu at the interface for neighbor > > establishment. This may be a dumb question but since > > all the neighbors have to be on the same media to > > establish wouldn't the mtus be the same. Obviously > > there is not always the case or they wouldn't have the > > mtu-ignore command. > > > > Ben > > > > __________________________________________________ > > Yahoo! - We Remember > > 9-11: A tribute to the more than 3,000 lives lost > > http://dir.remember.yahoo.com/tribute > > > > > > Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=53057&t=53047 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
