If I understand the concern correctly, the issue is that the PE is used as a remote LFA even though the PE is not connected to links with enough bandwidth.

A quick solution is to configure the protecting/repairing routers to only consider certain routers as rLFAs. Admin tags or routing policy can be used to achieve this goal. I believe this would be an internal implementation detail rather than a protocol modification

Thanks

Ahmed


On 1/17/2012 6:35 PM, stephane.litkowski <at> orange.com wrote:

Hi all,

 

I made some simulation based on real topologies on LFA and remote LFA.

I saw some issues related to topologies and I wanted to have some feedback on it.

 

We focus on P-P link failures.

In some ring topologies between Ps (which doesn't work very well with LFA),  alternate Ps are not eligible for LFAs (for some destinations) but some PEs are eligible providing link protection for the P-P link failure.

 

We tried to apply remote LFA algorithm for non LFA eligible destinations, and sometimes we have the same thing  : PEs are used as best rLFA because there is no eligible P.

 

PE are sometimes meshed with low BW link (compared to core links). So if P core link fails, traffic will be switched on PE link (during protection time) and so will potentially (high probability) make the link congestionned.

 

From P point of view : traffic is protected, even if some traffic is dropped due to congestion(CoS should prioritize traffic), it's better than dropping all.

 

From PE point of view : some destinations of the PE where maybe not using the P-P link that fails. But due to congestion of the link, they become impacted. So impact for the PE is maybe greater than not having LFA on the core (we are talking about few seconds of impact -> time for the P to converge).

 

Note : it's not a one to one mapping of traffic between the P-P link that fails and the P-PE link used as LFA. It will depend on how many destinations are using this PE as LFA (and quantity of traffic per destination). But we could clearly expect congestion as PE links are sized only for their usage.

 

Do you think this is a real concern ?

If yes, is there already a proposal/solution to deal this issue ?

    - like using TE informations (ISIS subTLV) to take account BW, and so basically not protecting a link by a lower BW link

    - preventing some links to be used as LFA

 

 

Thanks for your feedback

 

---

Stephane

 

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