Anton, Actually the original primary purpose of the rule was to make sure that the Type-5 translation of a Type-7 LSA is never preferred since both would have the same non-zero forwarding address. As you state a Type-5 LSA with an NSSA forwarding address should never be installed, ala Step (3)'s
If the forwarding address is non-zero look up the forwarding address in the routing table. For a Type-5 LSA the matching routing table entry must specify an intra-area or inter-area path through a Type-5 capable area. For a Type-7 LSA the matching routing table entry must specify an intra-area path through the LSA's originating NSSA. I long ago toyed with the idea of removing (e). Unfortunately, whether or not a prefix belongs to a specific area is a supposition that is broken all the time. Hence it remains. Pat Date: Wed, 18 Jul 2007 23:02:09 +0200 From: Anton Smirnov <[EMAIL PROTECTED]> Subject: Re: [OSPF]Path prefrence for External LSA In-reply-to: <[EMAIL PROTECTED]> To: "Pat Murphy - (650)329-4044" <[EMAIL PROTECTED]> Cc: [EMAIL PROTECTED], [email protected] Message-id: <[EMAIL PROTECTED]> 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 Pat Murphy - (650)329-4044 wrote: >> 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 > > > _______________________________________________ > OSPF mailing list > [email protected] > https://www1.ietf.org/mailman/listinfo/ospf _______________________________________________ OSPF mailing list [email protected] https://www1.ietf.org/mailman/listinfo/ospf
