Subject: Re: Nssa (P-bit Policy Paradox)

From: "Pat Murphy - (650)329-4044" <[email-masked]>
Date: Wed, 13 Nov 2002 10:20:27 -0700

Krishna,

Regarding

> Hi,
> Is there any proposed solution to overcome this
> paradox.
>

Recall the example in Appendix E.

                              AS 4156
                                |
            Area 2              |
                                |
              A2                A0   Area 0      C0-----Internet
              |                 |                |      Default
              |                 |                |
              |                 |                |
              +-----------------B0---------------+
                                /\
                               /  \
                              /    \
         Internet------------A1    B1------AS 4156 (P-bit clear)
         Default (P-bit set)
                                 NSSA 1

A2 wants access to the networks of AS 4156 through A0. A1 wants access
to the networks of AS 4156 through B1. Traffic from both A2 and A1 to AS
4156 must pass through B0. The point of the paradox is that the vanilla
OSPF deployment will make a hijacking forwarding choice on B0 unless one
compromises A1's preferred path to AS 4156 though B1. Any proposed
solution would require a forwarding decision based on the inbound area
source, whereas OSPF forwards traffic based on destination address only.

This simple example could be resolved by configuring a preferred tunnel
adjacency between A1 and B1 through B0 and then tweaking the external
metrics (as Sina suggests) so that A0's Type 5 LSAs are more preferred
on B0 than B1's Type 7 LSAs. This is not a solution, just a cluge.

Pat