[c-nsp] IS-IS Topology database

[Oliver Boehmer (oboehmer)]

Chintan Shah <> wrote on Sunday, September 28, 2008 10:05 AM:

> Hi Guys,
> 
> I have two topology and diff result in IS-IS topology.
> 
> 1. PE1-----PE2
>     |            |
>     |            |
>    P1--------P2
> 
> PE1--PE2 - IS-IS L1 Type , PE1-P1 = IS-IS L1 , PE2-P2 = IS-IS L1 ,
> P1-P2 ==L1/L2 both , P1/P2 connet to Core IP/MPLS network
> With this toplogy, when I do show isis topology either on PE1 or PE2 
> i see only one route for all L1 routes so what happes is  for any IGP
> i only have one route in IS-IS routig table and if say P1 to PE1 link
> fails, It has to get converge to get alternative routei via PE1-PE2-P2
> 
> 2. Now if i have PE1 to P2 ad PE2-P1 =L1 link ( dual uplink) and
> remove PE1-PE2. I see that PE1 ad PE2 both has two routes for each L1
> routes in topology and i see that there is backup route in show ip
> route so if primary link fails since second route via P2 is already
> there in IS-IS routing table , convergace is quite better.
> 
> The one thing i don't understad is , why in topology i don't have
> backup route , what is theroy behind that ?? and I belive topology 2
> would be much better tha 1. Is that correct ?

I've never really figured out what the backup routes in ISIS are good
for exactly (haven't digged deep into this either), and I don't bother
as you can achieve fast convergence either way by tuning the SPF- and/or
PRC-interval down. So either topology is able to converge equally fast.

Which one is better depends on your capacity, triangles are often
preferred (albeit more expensive in terms of # of interfaces/links) as
you can plan your capacity better..

	oli