The MPLS WG Archive

[Yangguang Xu <xuyg@lucent.com>]

Kireeti,

> First of all, that means that signalling must include a mechanism
> for "avoid lsp-id x-y".  Even if that is possible, how does w
> compute a path that avoids lsp x-y?  There is no info in the IGP
> about LSP ids.  Finally, it is much preferable to do the path
> computation in one step rather than two steps.
>
> Angela's approach works better: if the x->y LSP is announced as an
> FA with path info, and there was a mechanism to say "compute an LSP
> that avoids this path", then w has the info to compute the w-z LSP.
> However, this requires some extensions to the signalling protocol,
> and moreover, this is still a two-step approach.
> 

After optical network create a LSP x-y, LSP source node x (in optical domain)
will maintain the LSP path information. New path w-z source node w can query x
for path LSP x-y ER hop list info and run explicit routing to avoid LSP x-y. 

IGP maintain topology information. It doesn't need to know which LSP use which
topology. LSP information is maintain separately from topology information.

LSP w-z is created with the "constrain" saying avoid LSP x-y. Please refer to
"Path selection section" of the document I sent to you for details.


> > 2) A can send one request for two disjointed paths (x-y) and (w-z) together.
> 
> This is a better approach, except that there is no mechanism for
> this in signalling protocols.  Also, two independent OXCs, x and
> w have to coordinate -- this idea doesn't scale very well.  Or
> there is a third party for path computation, in which case, forget
> routers and switches computing paths; just do the TE offline.

Because it's one request, the OXC who receives the request for (example x) can
perform explicit routing for both paths, then using MPLS signaling to set up
these two paths (it has to notify W with ER hop lists). No scalability problem,
no need for co-ordination at all.

> 
> > The key point is whatever routers can do to optical network, optical switches
> > can do by themselves (as easy as router can do). Why bother?
> 
> This is not a router vs. optical switch issue.  The issue is
> that the *originator* of the LSP has all the info.  The originator
> of the LSPs can do a full path computation, can do both paths
> simultaneously, has no coordination issues, no extensions to
> signalling or link state protocols.
> 

Using the second solution I mention above, it's as simple as have router do it.
Yet, it applies to all business models. The solution you mentioned only works
for certain service provider.


> Kireeti.
> 
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