>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.
This is true. I vaguely remember reading some notes from an IETF meeting from one of the developers of OSPF. They were discussing checks for the MTU. Basically OSPF checks whether a neighbor is using the same maximum transimission unit (mtu) on a common interface. This check is performed when neighbors exchange (exchange stage) (DD's) database description packet. If the receiving MTU in the DD packet was higher then the IP MTU configured on the incoming interface, OSPF will not establish an adjacency. The DD packet were dropped. This was done on the DD phase because initially MTU mismatches could cause flooding between 2 neighbors to fail with large LSU's being continually retransmitted. -Mario >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) >Report misconduct >and Nondisclosure violations to [EMAIL PROTECTED] Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=72059&t=72024 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
