Subject: Re: [OSPF] ABR/ASBR with clear R-bit?

From: "Michael Barnes" <[email-masked]>
Date: Mon, 14 May 2012 12:04:49 -0700

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-masked]>
To: 'Michael Barnes' <[email-masked]>, [email-masked]
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-masked] [mailto:[email-masked]] On Behalf Of
> Michael Barnes
> Sent: Thursday, May 10, 2012 8:27 PM
> To: [email-masked]
> 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-masked]
> https://www.ietf.org/mailman/listinfo/ospf
>

---

From: "Mike Dubrovskiy (mdubrovs)" <[email-masked]>
Date: Mon, 14 May 2012 16:36:14 -0700

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-masked] [mailto:[email-masked]] On Behalf Of
Michael Barnes
Sent: Monday, May 14, 2012 12:05 PM
To: [email-masked]
Cc: [email-masked]
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-masked]>
To: 'Michael Barnes' <[email-masked]>, [email-masked]
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.
> From: [email-masked] [mailto:[email-masked]] On Behalf 
> Of Michael Barnes

> 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.
>

---

From: Tanmoy <[email-masked]>
Date: Tue, 15 May 2012 19:23:10 +0530

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. 

@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. 
 [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. 

Thanks,
- Tanmoy

-----Original Message-----
From: Mike Dubrovskiy (mdubrovs) [mailto:[email-masked]] 
Sent: Tuesday, May 15, 2012 5:06 AM
To: Michael Barnes; [email-masked]
Cc: [email-masked]
Subject: RE: [OSPF] ABR/ASBR with clear R-bit?

Hi Michael,

How about this one:

Mike

-----Original Message-----

Hi Tanmoy,

Thanks,
Michael

------ Original Message ------
Received: Mon, 14 May 2012 07:20:09 AM PDT
From: Tanmoy <[email-masked]>
To: 'Michael Barnes' <[email-masked]>, [email-masked]
Subject: RE: [OSPF] ABR/ASBR with clear R-bit?

ASE/NSSA
be useful.