Paul

it's not RTR4 who generates the type 4 LSA but RTR1 since it is the ABR
type 4 is generated by the ABR attached to the area of an ASBR so its
always an intra area path from the originating ABR to the ASBR

Sina

Paul B wrote:

> 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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