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

From: "Pat Murphy - (650)329-4044" <[email-masked]>
Date: Mon, 20 Mar 2006 08:59:07 -0700

Sorry Acee,

>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.

I wasn't suggesting any change to RFC 3101. I thought it already said 
that in Section 2.3 via the following wording:

   Normally the next hop address of an installed AS external route
   learned by an NSSA ASBR from an adjacent AS points at one of the
   adjacent AS's gateway routers. If this address belongs to a network
   connected to the NSSA ASBR via one of its NSSAs' active interfaces,
   then the NSSA ASBR copies this next hop address into the forwarding
   address field of the route's Type-7 LSA that is originated into this
   NSSA, as is currently done with Type-5 LSAs. (See [OSPF] Section
   12.4.4.1.)  For an NSSA with no such network the forwarding address
   field may only be filled with an address from one of the its active
   interfaces or 0.0.0.0...

The non-zero-forwarding address of any Type 7 LSA originated into a 
specific NSSA must come from one of that NSSA's active interfaces. 
Perhaps it could be better worded. But, if you read it carefully ("into 
this NSSA"), that is what it says.

So if a Type-7 LSA is originated for the same prefix into two different 
directly attached NSSAs with its P-bit set, then these two LSAs MUST 
have different non-zero forwarding addresses chosen from their 
respective NSSA's active interfaces.

As for my recommendation, it is an old errata that John and I discussed 
and agreed to years back. I think he had planned to add it to a list of 
minor errata to RFC 2328. Perhaps that already happen. I added the 
sentence...

   ...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.

to RFC 3101's Section 2.4, Paragraph 2, as an interim measure. Otherwise 
an NSSA with multiple ABRs can black hole traffic bound for the Type-5 
LSA's prefix.

I will send the recommendation under a different posting leaving out any 
reference to RFC 3101.

Pat

---

From: Acee Lindem <[email-masked]>
Date: Tue, 21 Mar 2006 14:37:34 -0500

Hi Pat,

>   field may only be filled with an address from one of the its active
I was referring to the case covered in the next paragraph. If I had my 
choice today, I'd qualify
the text to limit the choice to NSSA addresses. By implied, I meant that 
it was covered in
the paragraph you quote.

   When a router is forced to pick a forwarding address for a Type-7
   LSA, preference should be given first to the router's internal
   addresses (provided internal addressing is supported).  If internal
   addresses are not available, preference should be given to the
   router's active OSPF stub network addresses.  These choices avoid the
   possible extra hop that may happen when a transit network's address
   is used.  When the interface whose IP address is the LSA's forwarding
   address transitions to a Down state (see [OSPF] Section 9.3), the
   router must select a new forwarding address for the LSA and then re-
   originate it.  If one is not available the LSA should be flushed.

Thanks,
Acee