Failover IPv6 with multiple PA prefixes (Was: IPv6 fc00::/7 - Unique local addresses)

Valdis.Kletnieks at Valdis.Kletnieks at
Mon Nov 1 13:15:21 CDT 2010

On Tue, 02 Nov 2010 03:46:55 +1030, Mark Smith said:

> How about algorithmically generating these addresses, so that
> they're near unique, instead of having the overhead of a central
> registry, and a global routability expectation?

Go re-read RFC4193, section 3.2.3:

3.2.3.  Analysis of the Uniqueness of Global IDs

   The selection of a pseudo random Global ID is similar to the
   selection of an SSRC identifier in RTP/RTCP defined in Section 8.1 of
   [RTP].  This analysis is adapted from that document.

   Since Global IDs are chosen randomly (and independently), it is
   possible that separate networks have chosen the same Global ID.  For
   any given network, with one or more random Global IDs, that has
   inter-connections to other such networks, having a total of N such
   IDs, the probability that two or more of these IDs will collide can
   be approximated using the formula:

      P = 1 - exp(-N**2 / 2**(L+1))

   where P is the probability of collision, N is the number of
   interconnected Global IDs, and L is the length of the Global ID.

   The following table shows the probability of a collision for a range
   of connections using a 40-bit Global ID field.

      Connections      Probability of Collision

          2                1.81*10^-12
         10                4.54*10^-11
        100                4.54*10^-09
       1000                4.54*10^-07
      10000                4.54*10^-05

   Based on this analysis, the uniqueness of locally generated Global
   IDs is adequate for sites planning a small to moderate amount of
   inter-site communication using locally generated Global IDs.

Works great if you're creating a conglomerate of even a few thousand private
networks that would otherwise have been RFC1918 collision city.

Global usage is another story. Last week's 'Weekly Routing Table' posting
listed 332,569.  Feel free to do the computation above for 300K networks and
let us know how you'd feel about debugging the resulting routing table.

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