Subject: OSPF ASBR Path Selection

From: Acee Lindem <[email-masked]>
Date: Wed, 16 Oct 2002 17:00:17 -0400

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:

<http://discuss.microsoft.com/SCRIPTS/WA-MSD.EXE?A2=ind9912&L=OSPF&P=R3674&I=-3>

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
             preferred).

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
             BR.

Are there any implementations that calculate a single ASBR route (vis-a-vis
one per area) and strictly prefer an intra-area path over an inter-area path
(independent of the corresponding areas)?

Thanks,
--
Acee

---

From: "Sudharsana V - CTD, Chennai." <[email-masked]>
Date: Thu, 17 Oct 2002 13:14:59 +0530

-----Original Message-----
From: Sudharsana V - CTD, Chennai.
Sent: Thursday, October 17, 2002 1:14 PM
To: Sudharsana V - CTD, Chennai.
Subject: RE: OSPF ASBR Path Selection

Sorry! the figure is not proper in the previous mail.

-------------------------------
        |       bkbone  |
        |       12(cost)    |
        R1..............ASBR....
        |\                      |    !
        | \...............R2...!
        |       5(cost) |3(cost)
        |               |
        |                       | non-bkbone
        -------------------------
                                |

-----Original Message-----
From: Sudharsana V - CTD, Chennai. [mailto:[email-masked]]
Sent: Thursday, October 17, 2002 1:07 PM
To: [email-masked]
Subject: Re: OSPF ASBR Path Selection

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
?

        -------------------------------
        |       bkbone  |
        |       12(cost)    |
        R1..............ASBR....
        |\                      |    !
        | \...............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?
Regards,
Sudharsana

-----Original Message-----
From: Acee Lindem [mailto:[email-masked]]
Sent: Thursday, October 17, 2002 2:30 AM
To: [email-masked]
Subject: OSPF ASBR Path Selection

<http://discuss.microsoft.com/SCRIPTS/WA-MSD.EXE?A2=ind9912&L=OSPF&P=R3674&I
=-3>

Excerpted from RFC 16.2

Thanks,
--
Acee