At 05:14 PM 7/9/2003 +0000, Priscilla Oppenheimer wrote: >Zsombor Papp wrote: > > > > The original question (as I understood) was about a single LSA > > that is > > larger than 1500 bytes (think Type 1 LSA for a router with 200 > > interfaces). > > I can't see how such an LSA could be divided into multiple OSPF > > messages so > > the only logical (implementation independent) solution seems to > > be to > > fragment the packet at the IP layer. Am I missing something? > >OSPF could send multiple packets.
How would the receiver know that the second packet holds the second half of the LSA whose first half was transmitted in the first packet? OSPF doesn't have a way of coalescing fragments of an LSA, does it? > That's what IP RIP would do. It used to be >pretty common to see bunches of RIP packets every 30 seconds. Even more >common for IPX RIP, (every 60 seconds). RIP doesn't have a concept of LSAs. A good analogy would be to say that RIP could advertise a single prefix distributed into multiple packets, which is not true. > > If you are asking about how LSAs that are individually smaller > > than 1500 > > byte are grouped together, then my (moderately educated :) > > answer is this: > > IOS defines a constant called MAXOSPFPACKETSIZE to be 1500 > > bytes and > > another constant called MAX_OSPF_DATA to be MAXOSPFPACKETSIZE - > > IPHEADERBYTES - OSPF_HDR_SIZE. The code that transmits the LSAs > > keeps > > packing the LSAs into the same packet as long as their total > > length is > > below MAX_OSPF_DATA, the net result being that the size of the > > IP packet > > can be up to 1500 bytes (and will in fact be close to it if the > > individual > > LSAs are not too big) if there are enough LSAs, regardless of > > the MTU. So > > for example if you set the IP MTU on an Ethernet interface to > > 500 bytes, > > and you have a large enough OSPF database, then you should see > > a lot of > > fragmented OSPF packets, regardless of how big the individual > > LSAs are. > >Thanks for the info. > >As another example, say that the MTU is 1500 and there is so much info to >advertise (links, routers, routes, depending on the type) that it requires >more than 1500 bytes. Then OSPF would just send multiple packets, wouldn't >it? Yes. >And there wouldn't be any IP fragmentation? Unless there is a single LSA larger than 1500 bytes, there wouldn't be any. In case it confused anyone, MAXOSPFPACKETSIZE (ie. 1500 bytes) is *not* the size of the largest OSPF packet that IOS can generate. > I think that was the original question. Well, if the term "router-lsa whose size exceed 1500 bytes" refers to a set of LSAs whose size individually does *not* exceed 1500 bytes (as opposed to a single Type 1 LSA whose size does exceed 1500 bytes), then I misunderstood the question. :) Thanks, Zsombor >According to Howard, if I understood him correctly in his message, that's >how Nortel, Bay, Wellfleet do it (send multiple messages). But is that what >Cisco does? > >I think it is what the RFC recommends too when it says this: "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." > >Sorry to beat this to death, but I'm not sure we have an answer yet. > >Priscilla > > > > > > I didn't write the code though, so take all this with a grain > > of salt. :) > > > > Thanks, > > > > Zsombor > > > > At 12:40 AM 7/9/2003 +0000, Howard C. Berkowitz wrote: > > >At 10:46 PM +0000 7/8/03, Zsombor Papp wrote: > > > >The LSA will be fragmented at the IP layer. > > > > > >Do you know for certain this is what Cisco's implementation > > does? > > >The OSPF code is aware of the MTU and can build OSPF packets > > for it. > > >I don't think you're really going to simplify it by relieving > > it of > > >the need to keep track of lengths. > > > > > >On the other hand, if you send a LSupdate that is at the MTU, > > the > > >receiving router can immediately start checking and installing > > it in > > >the LSDB, without waiting for fragments. This allows some > > concurrency > > >between OSPF packet transmission and OSPF protocol processing. > > > > > > >At 11:39 AM 7/8/2003 +0000, hebn9999 wrote: > > > >>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)? Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=72068&t=72024 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
