Subject: Re: [OSPF] OSPF NSSA ABR behaviors

From: "Pat Murphy - (650)329-4044" <[email-masked]>
Date: Thu, 11 Feb 2010 08:32:34 -0700 (PDT)

James,

Regarding OSPF RFC 2328 Section 16.6, "Incremental updates -- 
AS-external-LSAs", and NSSA RFC 3101 Section 2.6, "Incremental updates", 
there is some efficiency to simply first running the NSSA Section 2.5 
calculation on the newly arrived external LSA to see if it actually 
replaces the current route to its destination N; or it simply updates 
the route's current list of next hops and/or installed LSAs; or it is 
not installed. This is only slightly different than what would happen if 
the newly arrived external LSA were simply the last LSA to pass through 
the calculation of NSSA Section 2.5.

Note that when a newly arrived external LSA for destination N is 
functionally equivalent (i.e., same destination, cost and non-zero 
forwarding address) to an installed LSA in the current path set, it 
could invalidate the current route if it involves a comparison between 
items 1 and 2 of Section 2.5 Step (6)(c), ala

  1. A Type-7 LSA with the P-bit set.

  2. A Type-5 LSA.

  3. The LSA with the higher router ID.

If an installed Type-7 LSA with the P-bit set is functionally equivalent 
to a newly arrived Type-5 LSA, then a recalculation is not required. If 
a newly arrived Type-7 LSA is functionally equivalent to an installed 
Type-5 LSA, then a recalculation is required. Here current route can 
simply be invalidated and recalculated; or the current list of next hops 
can be CAREFULLY adjusted. Note that if item 3 applies it does not 
change the set of next hops. It only potentially changes the set of 
installed LSAs.

Again these efficiencies are implementation specific. The current RFC 
2328 and RFC 3101 specs simply suggest that the current route to 
destination N be invalidated and recalculated.
 
Hope this helps. 

Pat

---

From: "James Huang" <[email-masked]>
Date: Tue, 16 Feb 2010 12:18:16 -0800

Hi Pat,

Please see my comments below.

Regards,
James Huang

>-----Original Message-----
>From: Pat Murphy - (650)329-4044 [mailto:[email-masked]]
>Sent: Thursday, February 11, 2010 7:33 AM
>To: James Huang
>Cc: [email-masked]
>Subject: Re: [OSPF] OSPF NSSA ABR behaviors
>
>James,
>
>to a newly arrived Type-5 LSA, then a recalculation is not required. 

[James Huang] 
I guess you mean that there will be no change to the installed route in
this case.

>If a newly arrived Type-7 LSA is functionally equivalent to an
installed
>can be CAREFULLY adjusted. 

[James Huang] 
I think this may happen after recovery of the NSSA area partitioning.
When such an event happens (i.e. detection of a new router with B-bit
set 
in its router-LSA), will ASBR's in NSSA be triggered to refresh their 
type-7 LSAs immediately?  Or it has to wait for the next refresh timer 
expiration?

There are two cases to consider at the ABR:
(1) The type-7 LSA is received after the ABR re-computes the NSSA 
intra-area routes: 
The route to the forwarding address points toward a router in the NSSA.
In this case, the ABR would have re-examined external LSAs with a
forwarding address 
in the NSSA network after the intra-area routes are recomputed. 
So the installed route from type-5 LSA would have been invalidated at
that time.     

(2) The type-7 LSA is received before the ABR re-computes the NSSA
intra-area routes: 
The route to the forwarding address still points toward a router in the
transit topology.
This means that the ABR has not yet received the intra-area LSA that
will change the route 
towards the forwarding address.  
In this case, the type-7 LSA will not pass the check at the last
paragraph of 
Section 2.5 Step 3. 
Later after the ABR receives an intra-area LSA that changes the route
towards the 
forwarding address, it will re-examined all external LSAs with a
forwarding address 
in the NSSA network. The type-7 LSA will then replace the installed
route from type-5 LSA
(according to the check at the last paragraph of Section 2.5 Step 3). 

There is a small window in case (1) after the route from the type-5 LSA
is invalidated.
Within the window, no route exists at the ABR for those external
destinations. 
So should the ABR postpone invalidation of the route from the type-5 LSA
until the type-7 LSA 
is received instead?  (If so, this will be the only use that I can find
for the rules in 
Section 2.5 Step (6)(e).)

>Note that if item 3 applies it does not

[James Huang] 
Doe it really matter that the LSA with highest router-ID is installed?
After all, they all have the same non-zero forwarding address.
So the same next-hop(s) will be computed regardless which one of the
functional 
equivalent LSAs is installed.