Hi Steward,
That's forced first hop won't help; I can always add an extra node against such
tricks. :)
4 4
[Src]-----[A]------[B]
| |
2| |1
| |
[C]-----[D]-----[Dest]
1 3
You cannot solve this without "directed forwarding" (forcing B with some magic
to forward packets to A instead of to Dest). Think on that phenomenon like
this: until you have only P chance to protect a failure, protecting it in both
directions will always be P*P<P until P<1. :)
Gabor
-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of
Stewart Bryant
Sent: Wednesday, November 09, 2011 6:56 PM
To: András Császár
Cc: [email protected]
Subject: Re: Charter Update (Discussion)
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
>> _______________________________________________
>> rtgwg mailing list
>> [email protected]
>> https://www.ietf.org/mailman/listinfo/rtgwg
>>
--
For corporate legal information go to:
http://www.cisco.com/web/about/doing_business/legal/cri/index.html
_______________________________________________
rtgwg mailing list
[email protected]
https://www.ietf.org/mailman/listinfo/rtgwg
_______________________________________________
rtgwg mailing list
[email protected]
https://www.ietf.org/mailman/listinfo/rtgwg
