[c-nsp] IP LFA in ring topology

[Adam Vitkovsky]

Right there have to be some compromises 

Not that I would want to mess with it but..
Would it be possible with use of multi-topology isis 
 -create one topology/point of view per node in ring?


adam

-----Original Message-----
From: cisco-nsp-bounces at puck.nether.net
[mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of Saku Ytti
Sent: Wednesday, November 28, 2012 6:45 PM
To: cisco-nsp at puck.nether.net
Subject: Re: [c-nsp] IP LFA in ring topology

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
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