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

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

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

> they're near unique, instead of having the overhead of a central
> registry, and a global routability expectation?

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.

-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 227 bytes
Desc: not available
URL: <http://mailman.nanog.org/pipermail/nanog/attachments/20101101/780a8da5/attachment.bin>
```