> > > The most frequently mismatched parameters relevant for OSPF > > > configuration > > > seem to be dead intervals & mtu sizes. > > > > OSPF doesn't care about MTU size. > > Uh, excuse me? Go read RFC 2178 (OSPF v2), section G.9: > > "When two neighboring routers have a different interface MTU for their > common network segment, serious problems can ensue: large packets are > prevented from being successfully transferred from one router > to the other, > impairing OSPF's flooding algorithm and possibly creating > "black holes" for > user data traffic. > > This memo [RFC2178] provides a fix for the interface MTU > mismatch problem by > advertising the interface MTU in Database Description > packets. When a router > receives a Database description packet advertising an MTU > larger than the > router can receive, the router drops the Database Description > packet. This > prevents an adjacency from forming, telling OSPF flooding and > user data > traffic to avoid the connection between the two routers. For more > information, see Sections 10.6, 10.8, and A.3.3. >
Wow. The learning continues. I have never actually run into this problem. I have checked the RFC. That's RFC 2328 by the way, it obsoletes RFC 2178. Indeed, its during the Database Describtion Packet exchange that the MTU size is checked. The Database Description Packet format includes an "Interface MTU" field. But, why wait until the DDP phase of the neighbor/adjacency development? Why wouldn't this thing be a 'must match' situation and be included in the Hello packet? I just config'd it in my lab on a Point-to-Point and the neighbor state makes it to EXSTART and then stops. The router with the smaller MTU size reports the following in it's debug: Nbr x.x.x.x has larger interface MTU Only the router with the smaller MTU is upset by this. The router with the interface that has the larger MTU makes no mention of any problems. Quick search on CCO shows that Cisco has a work around for this: http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fipr rp_r/1rfospf.htm#xtocid24 Again, learn something new everyday. Since MTU is never mentioned in the Hello packet, I thought it didn't matter. Sorry about posting inaccurate information. I appreciate the feedback pointing out my error. -chris Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=41759&t=41613 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
