At 07:54 AM 7/12/2003 +0000, Hemingway wrote: >""hebn9999"" wrote in message >news:[EMAIL PROTECTED] > > 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)? > > >I've browsed through the other responses, and I did not see this particular >piece of information, but it being late perhaps I missed it. I understand >this question to mean "what if there are lots of routes, so many that the >LSA would end up larger than the MTU"
For the sake of clarity: OSPF, being a link-state protocol, doesn't advertise routes, and the size of the LSAs doesn't depend on the number of routes. Apologies if this is obvious; from the above statement and based on the previous discussion I thought it might not be. I would also like to mention that LSAs are not exchanged only between neighbors, they are flooded throughout the OSPF domain (depending on type and area configuration, as I am sure everybody knows :). I think this simple fact has far-reaching consequences as far as the nature and handling of LSAs are concerned. >As I read the RFC ftp://ftp.rfc-editor.org/in-notes/rfc2328.txt, beginning >on page 194 of said document, OSPF knows the link MTU, and would contruct >it's LSA's based on that information. My understanding is that the only thing that influences how the LSAs are constructed is the topology. I would be curious to see where the RFC says otherwise. LSAs are not equivalent to DD packets. (And FWIW, page numbers in the RFCs are on the bottom of the pages... :) As for the OSPF *packets* being constructed based on MTU, that is surely a possibility. The IOS *implementation* however doesn't care about the MTU, as far as I can tell. > Within the database description >packet, there is the "M" bit, which indicates whether or not there are >additional database description packets following. > >The receiving router would see that a particular DDP M bit is marked "on" >and would expect more. When the last DDP is received ( M bit marked "off" ) >then the current DD sequence number becomes the reference number for the >link state database. Future LSA's would have to have a higher sequence >number in order to be considered updates. Which part of the RFC says that the DD sequence numbers have something to do with the identification of LSAs? How will this identification method work if the same (instance of an) LSA reaches the router from two directions (see flooding)? IMHO, DDPs constitute the transport mechanism, while LSAs are the data to be transported, so what you are saying above is alike to claiming that, for example, web pages are identified by TCP sequence numbers. Thanks, Zsombor >Howard? > >I "think" this answers the original question, although one never can tell. > >-Hem- > > > > > > ______________________________________ > > > > =================================================================== > > [EMAIL PROTECTED] (http://bizsite.sina.com.cn) Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=72184&t=72024 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
