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From: Sina Mirtorabi (sinaCISCO.COM)
Date: Thu Jul 12 2001 - 04:44:28 CDT
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.
> 18.104.22.168/24 (External Route)
> RTR1------Area 0 ---RTR2---Area1
> | \ ---- |
> | \ |
> Area 2 Area0 RTR5
> | \- |
> | \ |
> RTR 3--Area2----Rtr4---Area1-|
> >From the perspective of RTR5
> the cost to 22.214.171.124 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
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