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.
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=53053&t=53047 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
