[c-nsp] IP LFA in ring topology

[Saku Ytti]

On (2012-11-28 18:24 +0100), Adam Vitkovsky wrote:

> Is it possible to tune metric on links in ring to get the most nodes on the
> ring protected by the LFA?

It's pretty easy to see how many boxes will be protected by just pen and
paper. Just cross out link, and see if you'd return traffic back.

Imagine following topology with identical metric everywhere.

 P1
 |      
 R1-R2-R3
 |      |
 R5----R4


Ir R5 <-> R4 gets outage, R5 does not have LFA to P1, as R4 would send
towards R5.

So no matter what metrics you choose in this topology, you have to
compromise something as not everyone can have own best path to P1.


Otoh If you have

P1-----P2
|      |
PE2---PE3

PE boxes can get 100% coverage, as long as the connection between PE boxes
is strictly backup or at most used to reach each end PE, not for transiting
in non-fault scenario.

-- 
  ++ytti