hebn9999 wrote: > > hello,everyone: > OSPF use raw socket (datagram) to communicate with peers. In > general, layer 2 frame has a MTU of 1500 bytes. > how does cisco router propagate router-lsa whose size exceed > 1500 bytes(more than 122 links in one area)?
Well, I don't have a definite answer, but I'll discuss it with you in the hopes of lighting a fire under one of the OSPF experts on this list. Howard? Chuck? Peter? Where's Pamela when we need her? :-) OSPF runs directly above IP. I don't know if that could be called "raw socket" which is a UNIX thing? My perception is that with Cisco IOS, OSPF calls IP with a set of parameters and lets IP handle the rest. So maybe that's sort of raw. I can say this: The OSPF packets I have seen coming out of Cisco routers have the IP fragmentation bit set to "May Fragment." This makes me think that Cisco's OSPF relies on IP to push the bytes into the data-link-layer frame and fragment if necessary. The OSPF RFC (RFC 2178) says this: "OSPF does not define a way to fragment its protocol packets, and depends on IP fragmentation when transmitting packets larger than the network MTU. If necessary, the length of OSPF packets can be up to 65,535 bytes (including the IP header). The OSPF packet types that are likely to be large (Database Description Packets, Link State Request, Link State Update, and Link State Acknowledgment packets) can usually be split into several separate protocol packets, without loss of functionality. This is recommended; IP fragmentation should be avoided whenever possible." Unfortunately, that's not very clear. It implies that the recommended method is for OSPF to split its own protocol packets. But that the method for doing this is undefined and that's OK because OSPF can depend on IP to do fragmentation. Cisco routers tell each other their MTU in database description packets, per RFC 2178. Until recently, if the routers didn't agree on the MTU, they wouldn't become adjacent. A recent IOS version supports telling a router to ignore the other side's MTU so they can still become adjacent. That doesn't answer your question, but maybe there are some hints in the article that discusse the "ip ospf mtu-ignore" feature here: http://www.cisco.com/warp/public/104/12.html _______________________________ Priscilla Oppenheimer www.priscilla.com > ______________________________________ > > =================================================================== > [EMAIL PROTECTED] (http://bizsite.sina.com.cn) > > Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=72043&t=72024 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
