Hi Tanmoy, I agree, this is an interesting use case. However, we must be careful to handle it correctly. Consider, for example, when the only path to the forwarding address is through the ASBR which has the R-bit cleared. Routers in the same area as the ASBR would be able to determine this without any trouble and not use the ASBR for transit. We need to consider that the ASBR may be advertising local routes as externals, and these should be reachable via the ASBR even when the R-bit is clear. If the forwarding address shares the same prefix, then it would also be reachable in this scenario. So how do other routers determine which external prefixes are reachable, or not, via the ASBR with the R-bit cleared? In particular, the routers which are in other areas?
I can think of a couple of ways. A simple one would be for the ASBR to advertise the FA with an infinite metric. This would allow routers to calculate another path to the FA, if one is available, while ensuring the the ASBR itself would not be used as the transit to the FA. While at the same time allowing reachability to prefixes local to the ASBR, of course the local prefixes would not be advertised with the same FA. Thanks, Michael ------ Original Message ------ Received: Mon, 14 May 2012 07:20:09 AM PDT From: Tanmoy <[email protected]> To: 'Michael Barnes' <[email protected]>, [email protected] Subject: RE: [OSPF] ABR/ASBR with clear R-bit? > Hi Michael, > > There seems to be at least 1 use case where it would be required to install > the ASE/NSSA routes advertised by a router with R bit clear. If the ASE/NSSA > routes have a forwarding address, then those destinations may be reachable > directly bypassing the advertising router and could prove to be useful. > > Regards, > Tanmoy. > > -----Original Message----- > From: [email protected] [mailto:[email protected]] On Behalf Of > Michael Barnes > Sent: Thursday, May 10, 2012 8:27 PM > To: [email protected] > Subject: [OSPF] ABR/ASBR with clear R-bit? > > Hello Folks, > > Something which is not discussed in RFC5340 is how to treat Inter-Area > or External advertisements from an ABR/ASBR which has the R-bit clear in > its Router LSA. My initial thinking was that other routers should simply > ignore those advertisements. > > However it later occurred to me that it might be desirable to reach > those destinations if they are on links directly connected to the > advertising router. And if those Inter-Area or External routes will be > installed in the routing tables of other routers, the ABR/ASBR should > stop advertising prefixes which are not on its own interfaces. > > I think this deserves some discussion and we should have consensus so > that all routers behave the same way. > > Regards, > Michael > _______________________________________________ > OSPF mailing list > [email protected] > https://www.ietf.org/mailman/listinfo/ospf > _______________________________________________ OSPF mailing list [email protected] https://www.ietf.org/mailman/listinfo/ospf
