Howard C. Berkowitz wrote: > > At 12:43 PM +0000 7/9/03, 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? > > I missed the point that the LSA was for the same router. > Without > testing it, however, I don't immediately see why it wouldn't > work to > have multiple LSAs for the same router, as long as no prefixes > were > duplicated.
Are you saying the router could send out one Link State Advertisement saying this router has link 1, 2, 3, etc. etc., for example. And then send out another LSA, saying this same router has link 101, 102, 103, etc.? That should work I would think, unless the recipient thought it was supposed to replace the old one with this new one. But that doesn't seem to be what Cisco does. I couldn't easily try the Hello with lots of neighbors in it that you mention below, but I did try a single router with gobs of loopbacks that it advertises to another router in a Type 1 LSA. It sends the info in one oversized message, that the IP layer fragmented, as Zsombor said it would. I had about 140 loopbacks all part of OSPF Area 0. The sending router sent this to another router in Area 0. The sending router's IP layer put it in two IP packets, one with 1500 bytes, and one with a few hundred. IP did the fragmentation. OSPF didn't divide it up. But I agree that it shouldn't have to work that way?? But it does, and I *think* that was the original question. I said that before, but now I'm much more sure that this was what the original poster wanted to know. :-) Priscilla >Certainly, you send out a new type 2 when an > additional > prefix activates -- I don't immediately see why you couldn't > send out > a new type 1 with the additional new prefix. Neither are in an > existing LSDB, so they shouldn't purge anything. > > Another argument about fragmentation hasn't been discussed. > Consider > Hello packets. IIRC, about 47 router entries can fit into an > OSPF > hello packet with a 1500 byte MTU. Consider the timing > complexities > of waiting to defragment before you can tell if another router > is > alive. Even scarier is if the load were heavy enough > (unlikely, but > possible) that you might hit the next hello update interval > before > you had finished sending (or at least processing) all the > segments. > > > > >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. > > > >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=72076&t=72024 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
