Subject: Re: [OSPF] OSPF NSSA ABR behaviors

From: "Pat Murphy - (650)329-4044" <[email-masked]>
Date: Fri, 05 Feb 2010 15:19:57 -0700 (PDT)

James,

See comments inline.

P>Thus if one desires to aggregate the external route during Type-7
P>translation using a Type-7 address range then a Type-7 LSA with the P-bit
P>set must be originated.

P>Note the latter statement from RFC 3101 simply ensures that non-NSSA
P>traffic forwarded by a Type-5 LSA installation toward a forwarding address
P>inside an NSSA won't loop. I prefer originating the Type-7 LSA with the
P>P-bit set and simply letting the Type-7 translation process originate the
P>Type-5 LSA. Thus Type-5 LSAs would then never need to set a forwarding
P>address that belongs in a stub area or an NSSA, except during the Type-7
P>LSA translation process. Hence if the best forwarding address is in the
P>transit topology, it is better to originate both a Type-5 LSA and a Type-7
P>LSA with the P-bit clear. If the best forwarding address belongs to the
P>NSSA, it is better to simply originate a Type-7 LSA with the P-bit set and
P>let the Type-7 translation process originate the Type-5 LSA.

J>When you say a router originates a type-7 LSA, does it mean
J>that the type-7 LSA will be advertised?

Of course, all LSAs are advertised by an originating router.
See RFC 2328 Appendix A.4.1, "The LSA header".

J>Or it simply means that the type-7 LSA is generated internally
J>and it depends on the translation process to determine whether
J>it should be aggregated with some other external LSA(s) or be
J>advertised individually? 

Like all LSAs, Type-7 LSAs are only generated to be originated
by an advertising router. But once originated with the P-bit set
the Type-7 LSA may generate a Type-5 translation as described in
RFC 3101 Section 3.2, Translating Type-7 LSAs into Type-5 LSAs.

P>Let's assume that the redistribute is intended to feed the route
P>into all NSSAs and the non-NSSA topology. If (G) belongs to the non-NSSA
P>topology, originate a Type-5 LSA with forwarding address (G) into the
P>non-NSSA topology and also originate a Type-7 LSA with the P-bit clear
P>into each NSSA. Here the Type-7 LSAs can simply use a 0.0.0.0 forwarding
P>address. If (G) belongs to one of the NSSAs, then originate a Type-7 LSA
P>with the P-bit set into that NSSA with the forwarding address set to (G).
P>Finally originate a Type-7 LSA with the P-bit clear into the remaining
P>NSSAs. Note again that Type-7 translation will cause the appropriate
P>Type-5 LSA to be originated into the non-NSSA topology.

J>Agree - assuming address aggregation is not performed.

Type-7 ranges that subsume only a single prefix with the same
[address,mask] pair result in a normal translation. Hence, since
type-7 ranges can even subsume other type-7 ranges, one can negate
the effects of aggregation under configuration control. This is a
subtle point from RFC 3101 Section 3.2.

J>On a related question: When a MSSA ABR originates a aggregated
J>type-5 LSA, does it has to advertise a corresponding type-7 (p=0)
J>LSA into the NSSA area?  I cannot find any rule in RFC 3101 about this. 

The origination of a type-5 LSA generated by the translation of a
type-7 LSA NEVER forces the origination of another type-7 LSA.

J>I think the answer is no unless more than one locally sourced
J>external LSAs with the best forwarding addresses NOT in the NSSA are
J>aggregated into the type-5 LSA (otherwise, individual type-7 LSA
J>(p=0) should be advertised).   Is this correct?

The non-zero forwarding addresses of Type-7 LSAs are always
chosen from interfaces that belong to the NSSA. See Section 2.3.
A type-5 translation that aggregates a set of installed type-7
LSAs also NEVER forces the origination of another type-7 LSA.

J>One more question, RFC3101, sec 3.2 step (3):

          The path type and metric of the Type-7 address range is
          determined from the path types and metrics of those
          translatable Type-7 LSAs that best match the range plus any
          locally sourced Type-5 LSAs whose network has the same
          [address,mask] pair.

J>What is the reason aggregation can only aggregate those locally sourced
J>type-5 LSAs with the same [address, mask] as the configured address range?

Since this is an aggregation its metric must reflect the highest cost 
amongst the type-7 LSAs being aggregated. A locally source type-5 LSA
that has the same [address,mask] pair coupled with a lower cost could
negate the translation metrics. Similarly a lower cost in the Type-5
translation of a type-7 range would negate the high cost of a
locally sourced type-5 LSA with the same [address,mask] pair. Hence
a single advertisement is required.

J>If a locally sourced type-5 LSA is subsumed within the range, can it
J>be aggregated too?

No. The above excerpt is the only exception.

Pat

---

From: "James Huang" <[email-masked]>
Date: Fri, 5 Feb 2010 16:58:16 -0800

Hi Pat,

	I did some NSSA test on a router (with very new software
installed) from the #1 networking vendor.
Here is my observation about their NSSA ABR behavior with regarding to
external LSA aggregation:

(1) When an NSSA ABR does not have any locally sourced external routes
matching an address range, it will aggregate installed type-7 LSAs (p=1)
received from the NSSA matching the address range into a type-5 LSA and
advertise the LSA to the transit topology. 

(2) When an NSSA ABR has some locally sourced external routes subsumed
within an address range (may or may not be strictly), it will aggregate
installed type-7 LSAs (p=1) received from the NSSA matching the address
range and all those external routes into a type-5 LSA and advertise the
LSA into the transit topology. In addition, it will originate a
corresponding type-7 LSA (same destination, cost, and advertising router
as the aggregated type-5 LSA, p=0, forwarding address=0) to the NSSA
area. It will not originate a type-7 LSA individually for each of the
locally sourced external routes.

So their NSSA ABR behavior does not seem to comply with what was stated
in your previous email in two ways:
(1) origination of an aggregated type-5 LSA from at least one locally
sourced external route will always force the origination of a
corresponding type-7 LSA (p=0) into the NSSA, and
(2) locally sourced external routes subsumed within the address range
are aggregated into the type-5 LSA. They does not have to have the same
[address, mask] as the address range.

Now I am really confused.

-- James Huang

>-----Original Message-----
>From: Pat Murphy - (650)329-4044 [mailto:[email-masked]]
>Sent: Friday, February 05, 2010 2:20 PM
>To: [email-masked]
>Cc: James Huang
>Subject: Re: [OSPF] OSPF NSSA ABR behaviors
>
>James,
>
>See comments inline.
>
>P>translation using a Type-7 address range then a Type-7 LSA with the
P-
>bit
>P>NSSA, it is better to simply originate a Type-7 LSA with the P-bit
set
>and