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10.7. Configuring 6to4 Preference

By default, NATed IPv4 is preferred over 6to4-relayed connectivity:

Example 1:

  • Candidate Source Addresses: 2002:c633:6401::2 or 10.1.2.3
  • Destination Address List: 2001:db8:1::1 or 203.0.113.1
  • Result: 203.0.113.1 (src 10.1.2.3) then 2001:db8:1::1 (src 2002:c633:6401::2) (prefer matching label)

However, NATed IPv4 is now also preferred over 6to4-to-6to4 connectivity by default. Since a 6to4 prefix might be used natively within an organization, a site-specific policy entry can be used to cause native IPv6 communication (using a 6to4 prefix) to be preferred over NATed IPv4 as follows.

   Prefix        Precedence Label
   ::1/128               50     0
   2002:c633:6401::/48   45    14
   ::/0                  40     1
   ::ffff:0:0/96         35     4
   2002::/16             30     2
   2001::/32              5     5
   fc00::/7               3    13
   ::/96                  1     3
   fec0::/10              1    11
   3ffe::/16              1    12

Such a configuration would have the following effect:

Example 2:

  • Candidate Source Addresses: 2002:c633:6401:1::1 or 10.1.2.3
  • Destination Address List: 2002:c633:6401:2::2 or 203.0.113.1
  • New Result: 2002:c633:6401:2::2 (src 2002:c633:6401:1::1) then 203.0.113.1 (sec 10.1.2.3) (prefer higher precedence)

Since 6to4 addresses are defined to have a /48 site prefix, an implementation might choose to add such a row automatically on a machine with a native IPv6 address with a 6to4 prefix.

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