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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