Subject: Re: [OSPF]Path prefrence for External LSA

From: "Pat Murphy - (650)329-4044" <[email-masked]>
Date: Wed, 18 Jul 2007 13:12:12 -0700 (PDT)

>In RFC 3101 (NSSA option), section 2.5.6 (e) for calculating External 
>LSA states;

 (e) If the current LSA is functionally the same as an
     installed LSA (i.e., same destination, cost and non-zero
     forwarding address) then apply the following priorities in
     deciding which LSA is preferred:

         1. A Type-7 LSA with the P-bit set.
         2. A Type-5 LSA.
         3. The LSA with the higher router ID.
     [NSSA]
 
>My doubt is:
>If the both LSAs have zero FA or different FA then should
>the above preference rule be skipped and both LSAs be
>considered for SPF?

That is correct. Once the (6)a through (6)e preference rules are applied 
to the installed LSAs and the current LSA under external calculation, 
some of the installed LSAs may be replaced in the installed set by the 
current LSA. Alternatively the current LSA may not be installed and the 
existing installed set may remain intact, or the LSA may be added to the 
existing installed set. If the forwarding addresses of the installed set 
are all different, then this preference rule simply does not prune any 
LSAs from the combined set. If an LSA has a zero FA then this preference 
rule is skipped.

Pat

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From: Anton Smirnov <[email-masked]>
Date: Wed, 18 Jul 2007 23:02:09 +0200

Hi Pat,
    if this rule is supposed to be applied only to LSAs with the same fw 
address then it can be reached only when two Type-7 LSAs originated 
within the same area are being compared. Because if we are comparing 
Type-5 and Type-7 with equal fw address then at least one of LSAs will 
be rejected at the step 3. Fw address is reachable either thru NSSA area 
(then Type-5 is rejected) or through 'normal' area (and then Type-7 is 
rejected).

---
    Thanks,

Anton