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Re: LFAs and OSPF with multiple intra-area AS-external or summary-routes

Subject: Re: LFAs and OSPF with multiple intra-area AS-external or summary-routes
From: Alia Atlas
Date: Wed, 09 Feb 2005 15:40:28 -0500
At 03:17 PM 2/9/2005, Alex Zinin wrote:
> Here's an example.  In this case --- are area 1 and **** are area 2.  S is
> connected via a virtual link to ABR1 in area 1 and by a virtual link to
> ABR2 in area 2.  Y isn't connected to the backbone; it could also have
> virtual links in the two areas to ABR1 and ABR2 if needed.

>                10        1
>        [ A1 ]-----[ S ]*****[ P2 ]
>          |          |          *
>          |        5 |          * 1
>          |          |     1    *
>       10 |        [ P1 ]-----[ Y ]
>          |          |          *
>          |        5 |          * 13
>          |     5    |          *
>         [ABR1]----[ B ]     [ ABR2 ]
>           |                    |
>        10 |                    | 10
>           |                    |
>        Summary Prefix 1   Summary Prefix 1

In this scenario, assuming the failure is that of P1 node, Y would lose its
connectivity in area 1 completely, wouldn't it?
True. One could easily add a link between Y and another node in area 1 so
that doesn't happen. There could be a link between S and Y of greater cost
- so that Y tries to use S as a loop-free link-protecting alternate.

>>How about we look at it this way: what we're interested in is protection,
>>whether this is ECMP or LFA. Removing increased memory consumption from
>>consideration as something we'll have to deal with anyways: if an
>>is constrained to support N next-hops only, and all of the slots for a
>>route are
>>taken, well then one option is to not install LFAs at all--the route is
>>protected with ECMP after all. Should the topology change such that some slots
>>become available--one can add LFAs there.

> True - but the ECMP may not give protection in the multi-area paths.

I'm sorry, I'm not sure I understand correctly what you mean here. Could you
explain, please?
Because the path in area 2 might cross back to area 1, the path in area 2
may not be link or node protecting with regard to the path in area 1. In
the above example, P2 offers node-protection, but not link-protection to
the path via P1. If a broadcast link (with the different costs) connected
S, P1 and Y, then P2 wouldn't be link-protecting either.
Does that clear things up or shall I do more ascii art? ;-)


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