Hi Acee, Got it now. We don't know in advance whether there are other ASBRs which, while having connectivity to FA, have external routing information.
Thank you for explanation! > 1 марта 2019 г., в 16:41, Acee Lindem (acee) <[email protected]> написал(а): > > Hi Alexander, > If the ASBR is not reachable, the AS-External LSA could be stale and the > route it originally advertised could be unreachable or administrative > prohibited irrespective of the reachability of the forwarding address. > Thanks, > Acee > > On 3/1/19, 8:16 AM, "Lsr on behalf of Alexander Okonnikov" > <[email protected] on behalf of [email protected]> wrote: > > Hi WG, > > I don't know whether the problem below is known already, but I have not > found neither update nor corresponding errata for RFC 2328. RFC 2328, section > 16.4 says: > > "(3) ... 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. ..." > > and > > "... 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. ..." > > > In case when AS-external LSA has non-zero FA, why do we need to look up > the routing table entries for ASBR, that originated the LSA? It is possible > that FA is available via other ASBR(s). This is valid case when there are > more than one ASBRs, and all may originate AS-external LSA with the same FA > and metric for some external destination, but only one (with highest RID) > will originate such LSA (RFC 2328, section 12.4.4.1). Another case with > Type-7 to Type-5 translation doing by ABRs in Candidate mode (RFC 3101). > > Per my understanding a calculating router should initially analyse FA > value and only then check availability of ASBR or FA, depending on whether FA > is zero or not. Am I missing something? > > Thanks. > _______________________________________________ > Lsr mailing list > [email protected] > https://www.ietf.org/mailman/listinfo/lsr > > _______________________________________________ Lsr mailing list [email protected] https://www.ietf.org/mailman/listinfo/lsr
