The MPLS WG Archive

[Yakov Rekhter <yakov@cisco.com>]

Angela,

> Some followup discussions in line.

more in line...
  
> Regards,
> Angela
> 
> -----Original Message-----
> From: owner-mpls@UU.NET [mailto:owner-mpls@UU.NET]On Behalf Of Kireeti
> Kompella
> Sent: Monday, October 23, 2000 1:58 PM
> To: kireeti@juniper.net; sc@tellium.com; xuyg@lucent.com; yxue@UU.NET
> Cc: ip-optical@lists.bell-labs.com; mpls@UU.NET
> Subject: RE: [IP-Optical] RE: Optical link bundling. Was Re:
> DraftMinutes From Pittsburgh
> 
> 
> > I don't see why TE and protection require the routers to specify explicit
> > routes.
> > The routers can simply specify to the optical layer what type of optical
> > layer protection
> > it requires.
> 
> Suppose router A wants to get to router B, and wants to take two
> different ingress and egress points in the optical domain, X->Y
> for the primary LSP, and W->Z for the backup.  A does not require
> optical protection for the X->Y path, nor for the W->Z path.  A
> *does* require that the X->Y path and the W->Z path do not share
> common links.  How is this to be done?
> 
> If A did the full path computation, this is simplicity itself.
> 
> [AC] I think you have a good point here. I also heard the same kind if
> reasoning (i.e., have a layer-3 like protection switching) for supporting
> the peer model. But after discussing with others, it seems that overlay
> model should be able to provide the same capability. 

Not really... for more on this see below...

> Normally, the primary
> LSP X->Y is set up first, and becomes a forwarding adjacency (FA) according
> to your LSP Hierarchy draft. Then the associated information of the FA X->Y
> including its exact path and SRLG information should be propagated via IGP
> extensions, same as with any other link in the network. Thus if router A
> sends a request to OXC W to set up a backup lightpath from W->Z to be
> diversely router from the existing FA X->Y, OXC W should already have the
> right information to perform proper routing.

It is a known fact that for computing disjoint paths the approach
you outlined above may result in a situation where no backup
path will be found, despite the fact that that it is possible
(using some other approach) to find two disjoint paths. 

> Comparing with the peer model solution where routers need to know all the
> SRLG information of the optical domain as well as all relevant physical
> impairments in the optical signal in the case of transparent optical
> network, it is still not clear to me which one is simpler.
> 
> I think it is very good to have this kind of technical discussion openly on
> the list. Hope others can provide more technical and business (after all
> carriers need to pay for these features) evidences for the need of each
> model. Some other reasoning I heard includes that peer model can improve the
> IGP scalability in terms of the number of neighbors a router needs to peer
> with. But since large ISPs today seem to cope well with the IGP scalability
> today, I don't see why the problem will get significant worst when optical
> networks come into play.

In the end it is not the discussion on this list, but the competition
in the marketplace that will determine the viability of different
models.

Yakov.