Hi Michael, A couple of ideas...
Firstly we should prevent that other routers do not use the R-bit clear Router as transit for reaching the FA. To ensure this, the Router with R-bit clear MUST not originate any inter-area network prefixes. It is allowed to originate only inter-area host prefixes (128 prefix length) that correspond to its own interface addresses. [This make sense too since this router does not want to service transit traffic so there is no need to advertise the attached networks]. Additionally : The R-bit clear ASBR can originate the AS-external route only when there is a good chance that there is an alternate path to the FA. To do this, the ASBR can examine the network LSA of the interface corresponding to the FA and check that there is at least one other router with the R-bit set, in which case the LSA can be originated. This part is fully optional and if not followed, may result in AS-External LSAs that are not usable. Anyway, the solution presented here is simple and not foolproof for all cases (e.g. there is another router with the R-bit set, but it is reachable to a set of routers in the topology only through the router with R-bit clear). More robust but complex solutions are possible ;-) With these restrictions/conditions, the following cases need to be examined: Case 1 : R-bit Clear router is the only ABR for the other area. Based on the restriction proposed, the R-Bit clear router will originate only directly interface routes as Inter-area routes(with 128 prefix length). This will ensure the other router cannot use it as a transit to reach the FA, since the network is not visible at all. Case 2 : There is another ABR in the area. In this case, since the other ABR router will originate the inter-area route for the FA's network prefix, all routers will consider the other router as the transit router and will calculate reachability only through the other router. Regards, - Tanmoy -----Original Message----- From: Michael Barnes [mailto:[email protected]] Sent: Tuesday, May 15, 2012 8:37 PM To: [email protected] Cc: [email protected] Subject: RE: [OSPF] ABR/ASBR with clear R-bit? Hi Tanmoy, ------ Original Message ------ Received: Mon, 14 May 2012 11:24:00 PM PDT From: Tanmoy <[email protected]> To: "'Mike Dubrovskiy (mdubrovs)'" <[email protected]>, 'Michael Barnes' <[email protected]>Cc: [email protected] Subject: RE: [OSPF] ABR/ASBR with clear R-bit? > Hi Micheal/Mike, > > @Michael : Considering your suggestion I have one question, during SPF all > router would add the Router with R-Bit clear as Stub node. Hence there is no > chance where this router being used as transit for forwarding address. This is true when the router is in the same area as the ASBR. However, if the router is in another area then it does not have a way to know if the inter-area route transits the ASBR or is reachable through another router in that area. > @Mike : we shall not consider this router to give up the ABR/ASBR status. As > Michael said, the router might need to advertise its own directly connected > routes as Inter or ASE/NSSA routes. > > I have one suggestion, since the Originator will know BEST about the routes > its originating, we shall put some restrictions there. > Such as.. > [1] Router with R-bit clear is allowed to originate AS-External/NSSA LSA > for directly connected routes ONLY. > [2] Router with R-bit clear is allowed to originate inter aera LSA to other > areas for the directly connected routes ONLY. The two above are essentially the same as what I suggested in my first e-mail of the thread. :-) > [3] For all other AS-External/NSSA destination where there is a valid > Forwarding address can be derived, the LSA should be originated. > All other sources MUST NOT be originated by the Router with R-Bit Clear. How does the ASBR know if the FA is reachable not via itself? Should it do special type of SPF calculation to see if there is some other path? Thanks, Michael > Thanks, > - Tanmoy > > > > > > > -----Original Message----- > From: Mike Dubrovskiy (mdubrovs) [mailto:[email protected]] > Sent: Tuesday, May 15, 2012 5:06 AM > To: Michael Barnes; [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
