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?"
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. 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) 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=58006&t=57990 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
