[Tony Li] : RE : RE : Last call for draft-ietf-tewg-interarea-mpls-te-req-01.txt

["Adrian Farrel" <adrian@olddog.co.uk>]

Further non-member post
> Subject: Re: RE : RE : Last call for draft-ietf-tewg-interarea-mpls-te-req-01.txt 
> Date: Fri, 28 May 2004 01:19:16 -0700
> To: "LE ROUX Jean-Louis FTRD/DAC/LAN" <jeanlouis.leroux@francetelecom.com>
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> 
> >> Combine this with the fact that the primary reason to use this
> >> information
> >> is to ensure an optimal exit from the L1 area, and there doesn't seem
> >> like
> >> a real need to worry about instantaneous capacity.
> >
> > But an optimal exit, as regards cost, may not allow to find a feasible 
> > e2e path, if there is not enough capacity from
> > this exit to the destination.
> >
> 
> 
> Certainly true.  However, in a practical sense, the area is connected 
> via redundant
> L1L2 routers.  The redundancy is there so that neither is a single 
> point of failure.
> Thus both ABRs can pass the full traffic load of the area into the 
> backbone.  In short,
> engineering is sufficient to guarantee that we have enough fiber.
> 
> 
> >> You're making the assumption that the head end of the LSP is computing
> >> the entire
> >> path.
> >> Again, I'm advocating a somewhat different position (ala
> >> Nimrod): the path
> >> is computed with more refinement as you get closer to the
> >> destination.
> >> The
> >> head end might be able to provide an explicit path through the
> >> originating L1
> >> area, and based on its L2 topology information, it might select the
> >> exit point
> >> from that area.
> >
> > Based on which information does the Head End select the ABR ?
> 
> 
> L1 TE information plus L2 topology.
> 
> 
> >> By chaining together 3 locally optimal paths, we will NOT
> >> achieve global
> >> optimality, but we will get a good first approximation for a fraction
> >> of the cost.
> >
> > Not so sure that such scheme will get a good first approximation for a 
> > fraction
> > of the cost.
> > The ABR selected by the Head-End based on its L1 and L2 information 
> > may be the worst ABR, as regards the end-to-end path.
> > Actually you can definitely not control that. Further more such scheme 
> > cannot avoid cranckback risks,
> > and it seems from your last comment that you want to avoid that...
> >
> > Let's take the following example
> > A1, A2, A3, A4 are ABRs
> > Area0 topology is leaked into Area1 and Area2.
> > All metrics are set to 10 except A1-A2 metric that is set to 1
> > Avaialble bandwidth is 100M on all links except on link A2->R5, 
> > A1->A3, and A2->A4 where available bw=50M
> >
> >  Area1          Area0       Area2
> >   --------A1--------A2--------
> > R1         |         |        R5
> >   --------A3--------A4---------
> >
> > We want to setup a TE-LSP from R1 to R5 with bandwidth 60M
> > R1 selects an ABR based on Area1 and Area0 topology info. With your 
> > scheme it may select A1
> > Then A1 will select A2, and A2 will find that there is no path to the 
> > destination,
> > Then Crankback on A1 => No path to destination.
> > Then Crankback on R1 that selects A3....
> 
> 
> a) Per the above, this is an unacceptable network design in the first 
> place.
> b) R1 has metric information for Area 0, so it would naturally select 
> A3.
> 
> 
> >>> Not so sure, there are schemes that allow computing an optimal
> >>> inter-area path without adding any byte in LSA/LSP...
> >>>
> >> Which would take us back down the crankback path.  No thank you.
> >
> > I was refering to PCE approaches where the inter-area path is selected 
> > thanks to
> > a recursive CSPF computation on ABRs. Such schemes allow computing an 
> > OPTIMAL path, without any IGP extension and without crankback...
> 
> Sorry, I'm not sure I'm understanding you.  Are you suggesting off-line 
> path computation?
> 
> Tony
> 
> 
> 
>