Subject: Re: Question regarding the P bit in type-7 LSA

From: "Pat Murphy - (650)329-4044" <[email-masked]>
Date: Tue, 14 Mar 2006 09:00:12 -0700

Regarding the initial question

>Can someone throw some light on why the P bit must be clear in
>the case when an ASBR originates both a type 7 and type 5 LSA
>for the same network?

here is a minor thought. When a Type 5 LSA is originated the forwarding address 
is set based on the discussion in RFC 2328 Section 12.4.4.1. This need not (and 
should not) be an address from a directly attached NSSA and could even be 
0.0.0.0. As Acee and others have pointed out, a Type 7 LSA with the P-bit set 
must have a non-zero forwarding address from the NSSA. If one attempted to 
advertise such beasts simultaneously wouldn't the translation process of RFC 3101 
Section 3.2 Step (2)

          ...then a Type-5 LSA
          should be generated if there is currently no Type-5 LSA
          originating from this router corresponding to the Type-7 LSA's
          network, or there is an existing Type-5 LSA and either it
          corresponds to a local OSPF external source whose path type
          and metric is less preferred (see Section 2.5 step (6), parts
          (b) and (d)), or it doesn't and the Type-5 LSA's path type or
          cost(s) have changed (See Section 2.5 step (5)) or the
          forwarding address no longer maps to a translatable Type-7
          LSA.

cause these two originations to thrash? Keep in mind that every NSSA ABR is a 
potential translator. With the P-bit always clear, this is not an implementation 
issue.

I would like to see a minor change in Type 5 originations, namely that a  
non-zero forwarding address must be computed from the OSPF domain's "transit" 
topology. It should never be chosen from a stub area or NSSA. If the true 
forwarding address is in a directly attached stub area or NSSA, the Type 5 LSA's 
forwarding address is simply set to 0.0.0.0; The only exception to this rule 
happens during Type-7 translation. This would eliminate the requirement for the 
last line in

   When an NSSA border router originates both a Type-5 LSA and a Type-7
   LSA for the same network, then the P-bit must be clear in the Type-7
   LSA so that it isn't translated into a Type-5 LSA by another NSSA
   border router.  If the border router only originates a Type-7 LSA, it
   may set the P-bit so that the network may be aggregated/propagated
   during Type-7 translation.  If an NSSA's border router originates a
   Type-5 LSA with a forwarding address from the NSSA, it should also
   originate a Type-7 LSA into the NSSA.

If one really wants this behavior they need only set the P-bit and let 
translation originate the Type 5 LSA. Type 5 LSAs with a forwarding address from 
a directly attached NSSA are never installed (See the last paragraph of RFC 3101 
Section 2.3 Step (3)) on NSSA border routers. Thus they create the potential for 
black holes. I think this is a loose point in the base OSPF spec. While the NSSA 
spec handles it, given all of the available OSPF implementation knobs for 
filtering external advertisements, careful implementation is required. Since it 
is not necessary for feature, why have it?

Pat

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From: Acee Lindem <[email-masked]>
Date: Mon, 20 Mar 2006 09:02:08 -0500

Hi Pat,

I don't think we need any changes to the specifications to handle the 
situation.
Possibly, RFC 3101 section 2.3 could have explicitly stated that if an NSSA
ASBR must chose a forwarding address it must be one of the NSSAs interface
address but this is implied since the normal forwarding address must be 
part of
the NSSA. The router solely attached to two NSSAs should generate two
type 7 LSAs. Since it is connected to any regular areas, it will not 
generate a
type 5 and the P bit will be set. The ABRs doing the respective NSSA should
originate type 5 LSAs that are not equivalent since the forwarding 
addresses
and possibly metrics will be different.

Regarding all the non-standard area filtering techniques, we don't even 
want to
try and consider them in the specifications since one break all sorts of 
things by
using them.

Thanks,
Acee