Re: [mpls] Doubts on draft-helvoort-mpls-tp-ring-protection-switching-03

[Huub van Helvoort <[e-mail]>]

Dear Naveen,

You write:

> Considering the topology below, I have a question.
>
>                 +-----+              +-----+
>                 |  F  |--------------|  A  |-------- LSP 1
>                 +-----+              +-----+
>                    /                     \
>                   /                       \
>                  /                         \
>              +-----+                     +-----+
>              |  E  |                     |  B  |
>              +-----+                     +-----+
>                   \                        /
>                    \                      /
>                     \                    /
>                   +-----+            +-----+
> LSP 1 ------------|  D  |------------|  C  |
>                   +-----+            +-----+
>
> In case of an associated bidirectional path, the forward and reverse
 > path LSPs may span between the same end pointsbut via different
 > transit nodes.

In Transport Networks in general co-routed bidirectional path are used.
This has the advantage that the propagation delay is the same in
forward and backward direction. In general the shortest path will be
configured around the ring.

> Let's consider the forward path of the LSP 1 as: A->B->C->D (Clockwise
> direction) and, the reverse path of the LSP 1 as: D->E->F->A (Clockwise
 > direction).
> With this topology, if the link between B->C fails, then, only the
 > forward path LSP will be affected and APS PDUs will be sent via the
 > protection path from B->C and C->B to inform each other of the signal
 > failure and eventually, the traffic on the forward path LSP will be
 > switched to the protection path.
> However, the reverse path LSP, D->E->F->A, will not be affected by the
 > link failure between B->C

Indeed.

> In this scenario, will there be a necessity to switch the traffic on
 > the reverse path (unaffected by the signal failure between B->C) LSP
 > to the protection path?

In your scenario the traffic on the reverse path will not be switched
because the wrapping of all working traffic to the protection LSPs and
back will take place only in nodes B and C.

> Or, will it be sufficient to switch just the traffic on the forward
 > path LSP to the protection path and leave the traffic on the reverse
 > path LSP as it is?

I would not say it is sufficient, but is is due to the way traffic
is wrapped. Not only LSP1 but all working traffic between B and C
is wrapped.

> If traffic on both the forward and reverse path LSPs are switched to
 > the protection path, then, it is bidirectional switching.

Indeed, but again: only the forward and reverse working traffic between
nodes B and C is switched to the protection LSPs.

> But, if only the traffic on the affected LSP is switched on to the
 > protection path, then, it is unidirectional switching.

No, the switching is still bi-directional, but in your scenario
only the traffic of LSP1 that passes nodes B and C is switched.

> Does this draft recommend unidirectional switching?

No, it recommends bi-directional switching.
Together with the preference for co-routed bi-dirtectional LSPs
I will capture this in the next rev. of the draft.

Cheers, Huub.


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