Hi András
Yes, it is true that for that topology fragment there is no
bidirectional LFA solution.
However you were talking about rings, and surely it's unusual for a ring
topology to have non-uniform costs.
If we consider tunnel solution with forced first hop the solution would
work.
C tunnels to A with forced first hop (C-Dest), decap and A to Src.
Src LFAs to A, A sends to B, B sends to Dest
- Stewart
On 09/11/2011 10:59, András Császár wrote:
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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