Subject: Question about installing path to external destination

From: Manivannan Devarajan <[email-masked]>
Date: Tue, 9 Dec 2003 12:48:31 -0500

Hi all,

          Area 0
          ---------
          cost 1
     RT1              RT2 -------o(External destination)
          cost 1        (ASBR)
          ---------
          Area 1

My question is, whether RT1 will have a ECMP to Ex destination
through Area0 & Area 1. Or RT1 will have only one route through
Area 1 to the external destination.

Reference
RFC2328 Section 16.4.1.  External path preferences
 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.

RFC2328 Section 16.4.  Calculating AS external routes
(3) ...
  In any case, among the remaining routing
  table entries, select the routing table entry with the least
  cost; when there are multiple least cost routing table
  entries the entry whose associated area has the largest OSPF
  Area ID (when considered as an unsigned 32-bit integer) is
  chosen.

Thanks in advance,
-<Mani

---

From: Anand Oswal <[email-masked]>
Date: Tue, 9 Dec 2003 10:25:12 -0800

Mani,

If you have RFC1583 Compatability "ON" cost is the first tie-breaker and
if the cost is same -
area-id is the tie-breaker.

So in your case assuming RFC1583 is "ON" you will have one route via
area 1

Thanks
Anand