Subject: [OSPF] FWD: RE: OSPF NSSA ABR behaviors

From: "Pat Murphy - (650)329-4044" <[email-masked]>
Date: Tue, 09 Feb 2010 18:01:03 -0700 (PDT)

James,

You seem to be questioning whether the tie breakers rules ever apply. Don't 
forget that the calculation is linear; plus it can be updated incrementally. If 
the NSSA partitions separating the R2 translator and its R3 ASBR from R1, then 
R1 will install the Type-5 translation. Later, when the full NSSA is restored, 
R1 will have to make an incremental decision about what to do with the 
installed Type-5 LSA as soon as it receives the newly arrived Type-7 LSA. In 
this case the already installed Type-5 LSA will be pruned by the tie breaker 
rules. Indeed anomalous topologies can probably tweak all of the tie breaker 
rules.

Also don't forget in Scenario 1 that aggregations always add +1 to their E2 
metrics.

In Scenario 2, the broken configuration, a Type-5 LSA is preferred over a 
Type-7 LSA with a zero P-bit simply because a zero P-bit indicates there is no 
intent that the Type-7 path be a transit path for the full OSPF domain. 

Pat

[James Huang]

I have a few scenarios. Let's assume the non-zero forwarding address is X, 
which is part of a subnet N, in the following discussion.

(Scenario 1)

Assuming multi-area network is not allowed (i.e. N is part of the NSSA area 
only). There are two ABRs, R1 and R2, in this NSSA and R1 is this router doing 
the route computation. An ASBR R3 within the NSSA originates a type-7 P=1 
E2-cost LSA.

(case 1.1) R2 translates type-7 LSA to type-5 LSA without aggregation:

In this case, R1 may receive one <type-7, P=1, FA=X, ADV=R3> LSA from NSSA and 
another <type-5, FA=X, ADV=R2> LSA from the transit topology. The routing table 
for N is intra-area associated with the NSSA. So the type-5 LSA will not pass 
the check in the last paragraph of step (3) of sec 2.5.

(case 1.2) R2 translates type-7 LSA to type-5 LSA with aggregation (aggregating 
with some other LSAs):

In this case, R1 may receive one <type-7, P=1, FA=X, ADV=R3> LSA from NSSA and 
another <type-5, FA=0, ADV=R2> LSA from the transit topology. The routing table 
for N is intra-area associated with the NSSA. If the routing table for R2 is 
non-backbone intra-area and the cost from R1 to N and from R1 to R2 are the 
same, then R1 will install both LSAs into the routing table.

So neither (1.1) nor (1.2) requires the tie-breaking rule in sec 2.5 (6e).

(Scenario 2)

Assuming multi-area network is allowed. Let's say, subnet N is part of this 
NSSA area 1 and another type-5 capable area 2. R1 is this ABR router in area 1 
and R2 (an ASBR within area 1) originates a <type-7, P=1, FA=X, ADV=R2, 
E2-cost> LSA. R3 (an ASBR within area 2) also originates a <type-5, FA=X,
ADV=R3, E2-cost> LSA. This type-5 LSA may traverse the transit topology to 
reach R1

There are two sub-cases here:

(case 2.1) R1 is not connected to area 2: The route for N is intra-area through 
the NSSA area 1 (intra-area route is preferred over inter-area route). So the 
type-5 LSA will not pass the last paragraph of step (3) of sec 2.5.

(case 2.2) R1 is also connected to area 2: The route for N is intra-area 
through the area 1 and area 2. In this case, the tie-breaking rule in sec 2.5 
(6e) will be used. According to rule (6e), R1 should prefer the type-7, P=1, 
FA=X, ADV=R2, E2-cost> LSA over the <type-5, FA=X, ADV=R3, E2-cost> LSA. But if 
the type-7 LSA has P=0, R1 should prefer the type-5 LSA over type-7 LSA.

Why is it so?

I know that RFC 2328 does not support multi-area network. So case (2) is not 
valid in that sense. But I just cannot find other cases where the rule in sec 
2.5 (6e) will be used for tie-breaking with good rationales.

Thanks,
James Huang =20

>Hope this helps.
>Pat

---

From: "James Huang" <[email-masked]>
Date: Wed, 10 Feb 2010 10:48:37 -0800

Pat,

In your area partitioning scenario, the decision about whether to prune
a type-5 
or a functionally equivalent type-7 LSA can be done at the check in the
last 
paragraph of step (3) of sec 2.5.  There are three cases:
(1) network N (which contains the forwarding address X) and this ABR
router are in the 
    same partition: 
    In this case, the route to network N is intra-area through the NSSA.
    So step (3) of sec 2.5 will prune the type-5 LSA.
(2) network N and this ABR are in different partitions:  
    In this case, the route to network N is inter-area. 
    So step (3) of sec 2.5 will prune the type-7 LSA.  
    Later when the partitioning is repaired and the route to network N
is changed back to 
    intra-area, step (3) of sec 2.5 will prune the type-5 LSA and let
the type-7 LSA pass.  
(3) both the NSSA area and the network N itself are partitioned:
    OSPF cannot handle this case. The part of Network N in the other
partition is always 
    unreachable.

So I still do not see when the tie-breaker rules at sec 2.5 step (6e)
will ever get applied.
As for incremental updates, I assume that this ABR will perform a full
SPF computation when 
a new router LSA or a new network LSA is received (as per RFC 2328).  
But even if iSPF is performed for intra-area routes instead, we still
have to reexamine all 
external routes (at least those with a forwarding address in N) after
iSPF computation for 
intra-area routes completes. So the above arguments are still valid.

-- James Huang      

>-----Original Message-----
>From: Pat Murphy - (650)329-4044 [mailto:[email-masked]]
>Sent: Tuesday, February 09, 2010 5:01 PM
>To: James Huang
>Cc: [email-masked]
>Subject: FWD: RE: [OSPF] OSPF NSSA ABR behaviors
>
>James,
>
>Also don't forget in Scenario 1 that aggregations always add +1 to
their
>E2
>Type-7 LSA with a zero P-bit simply because a zero P-bit indicates
there
>is no
>only). There are two ABRs, R1 and R2, in this NSSA and R1 is this
router
>doing
>for N is intra-area associated with the NSSA. So the type-5 LSA will
not
>pass
>FA=X, ADV=R2, E2-cost> LSA over the <type-5, FA=X, ADV=R3, E2-cost>
LSA.
>But if
>I know that RFC 2328 does not support multi-area network. So case (2)
is
>not