Subject: Clarification on calculating Type 7 AS-external routes

From: Ashok Advani <[email-masked]>
Date: Tue, 29 Jul 2003 11:52:01 -0400

Hi!

I would appreciate some clarification on Section 2.5-(6)(e)
in RFC3101. The clause essentially extends the external
route computation specified in Section 16.4 of RFC 2328 and
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 understanding of this clause is that if there are multiple ABRs
between an NSSA area and the backbone, then it prevents the
(non-translating) ABR from installing equal cost routes for two LSAs
received for the same network, one the result of translation in the
backbone area, the other the result of the type 7 LSA received in
the NSSA area.

However, what would prevent this clause from being invoked when we
have 2 ASBRs attached to the same external router on the same network
intended for equal cost routing, i.e., if we had the following
network

                          [Cloud]
                            |
                          +-----+
                          | RTX |
                          +-----+
                             |
               -------------------------------------
                    |                     |
                 +------+             +-------+
                 |ASBR1 |             | ASBR2 |
                 +------+             +-------+
                    |                     |
                    |                     |
                    |     +------------+  |
                    ------|Internal Rtr|---
                          +------------+

In this case, it is conceivable that both ASBRs will advertise
"functionally equivalent" LSAs with the same destination, cost
and non-zero forwarding address, but it may be desirable to
equal-cost route between the 2 ASBRs.

Appreciate the clarification

thx
Ashok

---

From: "Pat Murphy - (650)329-4044" <[email-masked]>
Date: Tue, 29 Jul 2003 10:05:37 -0700

Ashok,

Several cases are covered here, but the primary function of
the tie breaker rule is to define preference for a Type-7 LSA
with the P-bit set over its Type-5 LSA translation. It also
prefers the Type-5 LSA when an NSSA's ABR originates both a
Type-5 LSA and a Type-7 LSA with the P-bit clear for the same
network (See Section 2.4 Par. 2).

As for your ?, note that functionally equivalent external LSAs
of the same type should be short lived (See RFC 3101 Section
3.2, Step (2) and RFC 2328 Section 12.4.4.1, pg. 141).
However, OSPF does compute multiple paths to external
destinations when there are multiple paths to an LSA's
non-zero forwarding address. Paths to the ASBR are consulted
only when the forwarding address is 0. Hence only one LSA need
be installed in your example, but it would have two paths.
Finally, your assertion is correct, while functionally
equivalent external LSAs of the same type should be short
lived, router ID can be used by the tie breaker to define
preference between two functionally equivalent external LSAs
that are of the same type and, when the type is 7, have the
same P-bit setting.

Pat