Hi Stewart, this phenomenon occurs with symmetric costs with LFA.
Trying to sketch an example:
2 2
[Src]-----[A]------[B]
| |
1| |1
| |
[C]-------------[Dest]
2
Now for Src->Dest traffic, the link failure of Src-C can be remedied with LFA,
as Src may pass the packet to A which is an LFA. For backwards traffic (e.g.
TCP Acks) this link failure cannot be solved by C with LFA, so ultimately the
Src-Dest traffic is screwed.
András
> -----Original Message-----
> From: [email protected] [mailto:[email protected]]
> On Behalf Of Stewart Bryant
> Sent: 2011. november 8. 18:54
> To: [email protected]
> Subject: Re: Charter Update (Discussion)
>
> On 08/11/2011 17:20, András Császár wrote:
> > I think we should first see if there is a solution which
> provides full coverage and practical (=reasonable complexity).
> >
> > One problem with non-full-coverage solutions is
> bidirectional coverage. I.e. even if a flow is covered for a
> failure in one direction it may not be covered in the reverse
> direction which is, for most applications, equal to not being
> covered at all. This property is often neglected in coverage
> estimates. As an example consider the LFA unfirendly
> sub-topologies like longer-than-triangle rings. In those
> rings it might seem that the failures on the opposing side to
> the exit point are covered by the LFA. But they are not
> covered in the reverse direction. Might be a factor to
> consider for PQ and co too.
> Can you give me an example here?
>
> I would expect a ring to have symmetric properties except
> when there are asymmetric costs, but normally assymetric
> costs are a configuration accident.
>
> Stewart
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