kannan,

sdf sdaf wrote:

> hi.
> In RFC 2328 16.2 first para
> "If the router has active attachments to multiple
> areas, only
> backbone summary-LSAs are examined."
> what is the meaning of this..
> If the router has acive attachements to multiple areas
> it will be a ABR and it must be attached to backbone
> area.so back bone routes will be INTRA area route in
> the routing table of ABR.then what is the need of
> examining the summary LSA?
> can anybody can give a clarification about this..
> thanks
> kannan

the RFC definition of ABR is, if a router has an active interface in at
least 2 area the router is ABR. this means the router consider itself ABR
and set the B bit in its Router LSA even without having connection to the
backbone ( as soon as it is attached to 2 area). however it's required for
the proper connectivity that all the area be connected to the backbone the
reason being that an ABR consider only a summary from the backbone and not
from a non-backbone area
imagine the following topology

R1----BB----R2----- area 2 ----
 \ /
    \ area 1 /
        \ /
            R3

R2 will generate summary for destinations in area 2 into the backbone and
area 1, R1 receive this summary through the backbone via R2, this is an
interarea route and it generate a summary for area 1 for the same
destination. also R1 being part of area 1 receive the summary advertised by
R2 into area 1 but it will ignore it since an ABR will only consider a
summary from the backbone otherwise you could easily create routing loop

now consider this topology, this time the link between R1-R2 is not part of
the backbone but area 1

  -- --BB------ R1------------R2----- area 2 ----
                         \ /
                            \ area 1 /
                                \ /
                                   R3

R2 is ABR since its attached to area 1 and area 2 so it will generate
summary for area 2 into area 1, this will allow all the internal ( non-ABR )
routers in area 1 to have connectivity to area 2 however R1 being an ABR
although receive the summary from R2 but it ignore it since this is not a
summary received from backbone
and therefore will not announce this summary into the backbone so you will
not have connectivity to area 2 from other area ( except the internal
routers in area 1 )
to fix that you would need to connect the area 2 to the backbone by
configuring a virtual link through area 1

one last note, we said that an ABR will consider only summary from the
backbone the only exception is when we have a virtual link and the
TransitCapbility of the area is set to true in this case the ABR will
consider the summary from the Transit area ( non-backone ) see section 16.3

Cisco ABR implementation is different and it requires that one of the area
be backbone for more information regarding this please refer to

http://search.ietf.org/internet-drafts/draft-ietf-ospf-abr-alt-04.txt

Sina

>
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