[c-nsp] IS-IS Topology database

[Chintan Shah]

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 ?

Regards,
Chintan