""p b"" wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > Thanks. But this doesn't really answer my question. I realize > that area 0 is partitioned. I'm not looking for an answer to > "is there a rule that prevents this", but instead, "what breaks > if ABR_1 were to consider routes learned via a non-area-0 summary > LSA in its computation of it's routing table?"
CL: sorry to be inflexible on this, but in my mind what you are asking is "why doesn't OSPF behave in a way that it is not supposed to behave?" > > Note, I'm also not asking why ABR_1 should not flood ABR_2's > summary LSAs into ABR_1's area 0. > > So back to the scenario: all routers in area 1, including > ABR_1, receive summary LSAs from ABR_2 which contain the routes > from ABR_2's area 0. CL: no - becasue no adjacency can be formed between area 1 and area 2 routers. all adjacencies have to be formed between an area's ABR, which is connected to area zero. this changes if you either 1) unpartition area 0, with a tunnel or a virtual link or 2) set up a virtual link across either area 1 or area 2, ( which is probably the same as # 1 ) CL: you have an adjacency between area 1 and the area 0 it conects to, and area 2 and the area 0 it connects to. you do not get an adjacency between the area 1 and the area 2 routers. > > All non-ABR routers in area 1 will process the information > injected by ABR_2's summary LSAs. These routers will install > these routes into their routing table. These non-ABR routers > will not realize there is an area 0 parition and will have > reachability into both. (I've not tested this, but believe > this to be true.) > > Since ABR_1 is an ABR with a backbone connection, it's not > allowed to: > > - forward information from ABR_2's summary LSAs into it's area 0. > - consider any routes found in ABR_2's summary LSAs as candidates > for insertion into its routing table. > > My question is, what breaks if ABR_1 was to use the information > found in ABR_2's summary LSA and put these into it's routing > table? > > Note, it is possible for an ABR, which does not have an area 0 > connection (hence it's an ABR between 2 or more non-zero > areas) to consider and use summary LSAs in it's route > installation process. (see Zinin's "Cisco IP Routing", > page 491; and > http://www.ietf.org/internet-drafts/draft-ietf-ospf-abr-alt-05.txt) CL: I don't have the book you refer to. I did a quick read of the draft RFC in the link above. My quick read is that it looks to me that the authors are suggesting a reinterpretation of the definition and activity of an ABR to suit some particular situation that could also be solved other ways. Their examples do not match yours, so I won't comment further, except to wonder why it is that some folks want to take the Microsoft attitude - do whatever you want to don't bother with design. I mean, for goodness sake, if you want chaos, then set up using EIGRP ;-> > > Thanks > > > > > > The Long and Winding Road wrote: > > > > ""p b"" wrote in message > > [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > > > Consider the following topology: > > > > > > area_0---ABR_1----area_1-----ABR_2----area_0 > > > > > > There are two area 0's. > > > > CL: you have a partitioned area 0. can't have two area zeros in > > ospf. to > > quote from my favorite movie of all time, "There can be only > > one!!!!" > > > > > > > > > ABR_1 and ABR_2 will generate > > > type 3 summary LSAs for the respective area 0s and > > > flood the information into area_1. An internal > > > router in area 1 will see the summary LSAs from ABR_1 > > > and ABR_2, determine the best routes, and then insert > > > them into it's routing table. > > > > > > Now consider ABR_1. It sees and stores in it's area 1 > > > LSDB the summary LSAs it got from ABR_2. > > > > > > The OSPF spec indicates that ABR_1, however, should > > > not forward this routing information into it's own area 0 > > > connection. This is done to prevent routing loops. > > > > > > My question is this: What is the reason why ABR_1 can > > > not use the routing information learned via ABR_2's > > > summary LSA and install these routes into it's own > > > routing table? > > > > > > CL: there can be only one area zero. them's the rules. > > > > > > > > > > Note, I believe if there was a virtual link between ABR_1 > > > and 2, ABR_1 would learn via ABR_2 the same set of routes via > > > summary LSAs and would be allowed to enter them into it's > > > routing table. > > > > > > There must be a routing loop issue here, but don't see > > > it. > > > > CL: interarea routing must transit area 0. what you are not > > seeing is that > > you have a partitioned area zero, not two area zero's. you have > > broken ospf, > > and now you need to repair it. > > > > > > > > > > Thanks Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=58009&t=57990 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
