Hi Mike, Yes, there are a number of possible ways to address this.
This would be one way to go about it. However, I don't think it is desirable. Especially when considering that in some cases operators like to advertise prefixes for local interfaces as external routes on spoke routers. They could use the R-bit to make the spoke not pass transit traffic. If we chose this option, then all prefixes would have to be advertised as intra-area prefixes. Therefore I'm reluctant to use this method. Regards, Michael ------ Original Message ------ Received: Mon, 14 May 2012 04:36:26 PM PDT From: "Mike Dubrovskiy (mdubrovs)" <[email protected]> To: "Michael Barnes" <[email protected]>, <[email protected]>Cc: <[email protected]> Subject: RE: [OSPF] ABR/ASBR with clear R-bit? > Hi Michael, > > It seems that there's more than one way to skin a cat. > > How about this one: > > Router with R-bit cleared should be area internal router. > In case if R-bit is cleared on ABR or ASBR, the router must give up the > ABR/ASBR status. > > Mike > > -----Original Message----- > From: [email protected] [mailto:[email protected]] On Behalf Of > Michael Barnes > Sent: Monday, May 14, 2012 12:05 PM > To: [email protected] > Cc: [email protected] > Subject: Re: [OSPF] ABR/ASBR with clear R-bit? > > 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 _______________________________________________ OSPF mailing list [email protected] https://www.ietf.org/mailman/listinfo/ospf
