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From: Sina Mirtorabi (sina_at_CISCO.COM)
Date: Thu Oct 17 2002 - 11:38:34 CDT
> Hi Acee,
> With respect to an ABR with backbone connectivity the summary
> LSAs used for
> calculation in 16.2 will belong to backbone. Hence,
> -interpretation of 16.2 (5) is to see if the ASBR(destination N
> described by
> the summary LSA) is reachable through backbone intra-area paths.
> -interpretation of 16.2 (6) is to prefer backbone intra-area paths over
> inter-area paths
> Will such an interpretation not conflict with the preference
> rules of 16.4.1
you are correct, for backbone routers, Inter-area path to ASBR belongs to
the backbone and therefore could be compared to backbone intra-area path
in other word 16.2(6) and 16.4.1 are conflicting, although it could be
argued that 16.2 describe the 'general' steps and 16.4.1 explicitly describe
'external path preference' therefore over writing the general behavior ...
> | bkbone |
> | 12(cost) |
> |\ | !
> | \...............R2...!
> | 5(cost) |3(cost)
> | |
> | | non-bkbone
> R1 and R2 are ABRs.
> R1's intra-area (backbone) path cost to ASBR is 12.
> R1's intra-area (backbone) path cost to R2 is 5.
> R2's intra-area (non-backbone) path cost to ASBR is 3.
> By following 16.2 verbatim, will R1 not choose the intra-area path of cost
> 12? while applying 16.4.1 would have resulted
> in selection of inter-area path (of cost 8) through R2.
> I am unable to understand how you feel that there is no problem. Am I
> missing something?
> -----Original Message-----
> From: Acee Lindem [mailto:aceeREDBACK.COM]
> Sent: Thursday, October 17, 2002 2:30 AM
> To: OSPFDISCUSS.MICROSOFT.COM
> Subject: OSPF ASBR Path Selection
> As a piece of unfinished business, Alex Zinin asked that I
> review a question regarding ASBR path selection when an external prefix
> is advertised through an ASBR accessible via multiple areas. The crux of
> the issue is whether RFC 2328 section 16.2(6) and section 16.4.1 conflict
> with one another and can result in a routing loop. A link to
> one of the E-mails on the subject is included below:
> Excerpted from RFC 16.2
> (6) Else, if the paths present in the table are intra-area
> paths, do nothing with the LSA (intra-area paths are always
> Excerpted from RFC 16.4.1
> When multiple intra-AS paths are available to
> ASBRs/forwarding addresses, the following rules indicate
> which paths are preferred. These rules apply when the same
> ASBR is reachable through multiple areas, or when trying to
> decide which of several AS-external-LSAs should be
> preferred. In the former case the paths all terminate at the
> same ASBR, while in the latter the paths terminate at
> separate ASBRs/forwarding addresses. In either case, each
> path is represented by a separate routing table entry as
> defined in Section 11.
> This section only applies when RFC1583Compatibility is set
> to "disabled".
> The path preference rules, stated from highest to lowest
> preference, are as follows. Note that as a result of these
> rules, there may still be multiple paths of the highest
> preference. In this case, the path to use must be determined
> based on cost, as described in Section 16.4.
> o Intra-area paths using non-backbone areas are always the
> most preferred.
> o The other paths, intra-area backbone paths and inter-
> area paths, are of equal preference.
> I've looked at this and my opinion is that while RFC 2328
> could be more explicit, there is no problem. The reason is that
> section 16.2 (5) explicitly specifies that for a router route you must
> look up the routing table entry associated with the area whose summaries
> you are examining. Hence, the rule in (6) only applies to an intra-area
> path through the same area as the inter-area path.
> Excerpted from RFC 2328 Section 16.2:
> (5) Next, look up the routing table entry for the destination N.
> (If N is an AS boundary router, look up the "router" routing
> table entry associated with Area A). If no entry exists for
> N or if the entry's path type is "type 1 external" or "type
> 2 external", then install the inter-area path to N, with
> associated area Area A, cost IAC, next hop equal to the list
> of next hops to router BR, and Advertising router equal to
> Are there any implementations that calculate a single ASBR route
> one per area) and strictly prefer an intra-area path over an
> inter-area path
> (independent of the corresponding areas)?
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