Curtis Call wrote:

>
> > imagine the following topology
> >
> >
> > R1------area 1 -------R2----- area 2 ------
> > \ /
> > \ area 3 /
> > \ /
> > R3
> >
> > so if I understand your idea, R2 will generate a summary for area 2 into
> area 1
> > and area 3 and will add an Area _Path ( 2 ) then R1 receiving this summary
> will
> > generate a summary to area 3 with the Area_Path ( 3,2 ) also R1 being part
> of area
> > 3 receive the summary from R2 with Area_Path ( 2 )
> > so what is the rule for R1 to install the summary for itself ? if it's based
> on
> > the cost you could easily go into routing loop since R1 might use the path
> through
> > area 3 but R3 use the path through R1.
> > by having a common area 0 and considering only summary from backbone we will
> avoid
> > this situation.
> > I guess your idea works fine for not generating a summary back into an area
> if the
> > area number is present in the Area_Path but we need some how to tell the ABR
> which
> > path they have to choose themselves
> >
>
> It would use the same logic as BGP does. I.e. the shortest Area Path would be
> chosen. In this case R1 would have two possible paths to the destination, one
> through Area 1 and one through Area 3, both with an Area Path of 2 (R1 would
> ignore any summaries created by itself, just as is the current practice). The
> tie breaker for same length Area Paths could be the lower area number, in
> which case R1 would send packets through Area 1 in order to reach Area 2.
> This would not cause a routing loop since all routers in Area 1 will use the
> Summary created by R2 to reach Area 2 (it has a shorter Area Path).

if the criterion is based on the shortest Area_Path in this case we could have
sup optimal routing

ABR------ area 1----------------------------ABR
    \ /
        \ /
           area 2----------ABR ------- area 3

imagine the path through area 1 will go through much more number of router and the
cost would be much higher than the path through area 2 and 3
with this we basically some how fall back to the hop count metric.
contrary to BGP in which the shortest path ( optimal ) is not the main criteria
and path choice is based on policy in IGP this is very important to have optimal
routing

Sina

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