The forwarding address can be set to either 0.0.0.0 (the RID of ASBR) or
a connected network on the ASBR... Note [24] mentions that if the FA is
non-zero, the router it points to should be part of a different routing
Autonomous System (and the network itself must be reachable via an intra
or inter-area LSA)

----------------------
16.4. Calculating AS external routes
            ...
        (3) Call the destination described by the LSA N. N's address is
            obtained by masking the LSA's Link State ID with the
            network/subnet mask contained in the body of the LSA. Look
            up the routing table entries (potentially one per attached
            area) for the AS boundary router (ASBR) that originated the
            LSA. If no entries exist for router ASBR (i.e., ASBR is
            unreachable), do nothing with this LSA and consider the next
            in the list.

            Else, this LSA describes an AS external path to destination
            N. Examine the forwarding address specified in the AS-
            external-LSA. This indicates the IP address to which
            packets for the destination should be forwarded.

            If the forwarding address is set to 0.0.0.0, packets should
            be sent to the ASBR itself. Among the multiple routing table
            entries for the ASBR, select the preferred entry as follows.
            If RFC1583Compatibility is set to "disabled", prune the set
            of routing table entries for the ASBR as described in
            Section 16.4.1. In any case, among the remaining routing
            table entries, select the routing table entry with the least
            cost; when there are multiple least cost routing table
            entries the entry whose associated area has the largest OSPF
            Area ID (when considered as an unsigned 32-bit integer) is
            chosen.

            If the forwarding address is non-zero, look up the
            forwarding address in the routing table.[24] The matching
            routing table entry must specify an intra-area or inter-area
            path; if no such path exists, do nothing with the LSA and
            consider the next in the list.
---------------------------

alex lerin wrote:
>
> Hi..
> I have a basic doubt about ASExternalLSA.
> This LSA contain a field called forwarding address.
> What is this ...
> Can this field be a Router ,other than ASBR or is a
> Network..
> If this can be a network what will be the field
> indicate(DestinationID or linkstateid of
> networkLSA)..?
> Please clarfy my doubt..
>
> Thanks
> Alex
>
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