Thanks Sina

I think I explained quite badly one key element which
you pointed out re ASExternal advertisements. I guess
the key for RTR5 is not the ASExternal for the route
it is the cost that RTR4 advertises to reach RTR1 in
the Type4 it generates. If it advertises the cost
based on the non-backbone cost then this is why I see
the non-backbone to non-backbone routing occuring.

Do I have this right ?

Regards

Paul
--- Sina Mirtorabi <sinaCISCO.COM> wrote:
> Paul,
>
> Paul B wrote:
>
> > Can anyone help clarify what should happen in this
> > scenario please with and without 1583
> compatibility.
> >
> > Which path is correct to this external destination
> > which is advertised as a type 1 external from ASBR
> > RTR1. Specifically I am considering the
> non-backbone
> > intra-area vs backbone intra-area rule.
> >
> > My question relates to which next hop RTR4 should
> > install and advertise a metric for.
> >
> > This is the scenario.
> >
> > 48.0.1.0/24 (External Route)
> > |
> > |
> > RTR1------Area 0 ---RTR2---Area1
> > | \ ---- |
> > | \ |
> > Area 2 Area0 RTR5
> > | \- |
> > | \ |
> > RTR 3--Area2----Rtr4---Area1-|
> >
> > >From the perspective of RTR5
> > the cost to 48.0.1.0 is 9 via RTR4-RTR1
> > 10 via RTR2-RTR1
> > 13 via RTR4-RTR3-RTR1
> >
> > Is it correct for RTR4 to advertise a metric based
> on
> > a path via RTR3 because this subscribes to the
> > non-backbone intra-area rule from its perspective
> or
> > should it advertise the backbone path/metric via
> RTR1
> > ?
>
> RTR4 is not advertising any thing, type 5 is
> generated by ASBR ( I guess
> RTR1 in your figure ) and are flooded untouched AS
> wide ( except Stub
> and NSSA )
> so RT4 will just flood it to the area 1 and RT5 will
> pick up one of the
> path through RTR4 or RTR2 based on the cost
> however once a packet reach RTR4 it will chose one
> of the path based on
> the 1583 compatibility, if its disabled it will
> choose the intra-area
> non backbone path which is through area 2
>
> >
> >
> > I am not clear as to whether it is possible to
> route
> > from non-backbone to non backbone.
> >
> > Because RTR4 is an ABR in area 1 & Area 2 is it
> > correct for RTR4 to subscribe to the intra-area
> > non-backbone rule and advertising the route/cost
> via
> > RTR3 through area 2 ? i.e. should RTR4 advertise
> the
> > cost to RTR5 based on going through the
> non-backbone
> > area 2, and not its backbone area connection even
> > though it is receiving on a non-backbone interface
> ?
> >
>
> again RTR4 is not the one who generates the LSA
>
> >
> > I am not clear as to whether it is legal to route
> > through multiple non-backbone areas if you receive
> a
> > packet from a non-backbone area.
> >
>
> if you have 2 non backbone area attached to an ABR
> they could
> communicate directly through this ABR depending on
> the path cost
> however you cannot communicate between multiple non
> backbone area if
> they are not attached to the same ABR because an ABR
> consider only the
> summary from the backbone and not from a non
> backbone area
>
> Sina
>
> >
> > Thanks
> >
> > Paul
> >
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