At 08:36 AM 7/10/2003 -0700, Zsombor Papp wrote: >Hi, > >the L1/L2 behavior can be configured on a per interface basis. The >question why you would want an interface to be both L1 and L2, and >especially why you would want a router to form both L1 and L2 adjacency >with one of its neighbors, is a good one.
In general, I don't think you would want this. I can attest to have never intentionally designing a network of that nature. Unless you wish to connect L1 domains to a backbone, or are in the process of a migration from one topology to another, minimizing adjacency state and its related overhead is a good thing. >One (exotic) example would be if an L1L2 router has L1, L2, and L1L2 >neighbors as well on the same interface like, this: > >|--L2 >| >|--L1L2 >| >|--L1L2 >| >|--L1 > >In this case the L1L2 routers' interface must be configured for both L1 >and L2 if we want the L1 router to be able to "get out". Consequently, the >two L1L2 routers will form both L1 and L2 adjacency with each other, but >this is more a coincidence than a requirement, IMHO. I would agree. There is no benefit to this as I see it. >A more realistic scenario would be like this: > >L1--| > |--L1L2(A)--L2 > | > |--L1L2(B)--L2 >L1--| In this case, you L1 areas will not usually be the same and the L1 adjacency between the two "core" routers will not form. If the area is the same, the L2 adjacency is superfluous. Many large networks are single area, or single level (ie L1 everyone in one area, or L2 everywhere where area isn't very relevant.) Pete >I am not sure however if there is any advantage of having A and B form >both L1 and L2 adjacencies with each other. It appears to me that L1 would >be just fine. I, too, would be happy to hear some comments on this. > >Thanks, > >Zsombor > >At 02:40 PM 7/10/2003 +0000, Peter van Oene wrote: >>At 03:40 AM 7/10/2003 +0000, wj chou wrote: >> >Hi.. >> > >> >a basic ISIS question... >> > >> >I know that by default, an IS is L1-L2, so it can form a L1L2 adjacency >> with >> >its neighbors. But what's the benefit of it? and under what kind of >> >situation in real world people want to configure it this way? >> >>L1L2 routers are required to interconnect L1 areas via an L2 backbone. An >>L1L2 router acts much like an OSPF ABR. >> >> >thanks! >> > >> >Ellie Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=72118&t=72081 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
