Re: RE : RE : Last call for draft-ietf-tewg-interarea-mpls-te-req-01.txt

>> 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