IPv6 Automatic Tunneling and 6to4
Automatic tunneling with IPv4-compatible addresses and 6to4 both work through a route for a prefix that is on-link to interface #2. The 32 bits following the prefix are extracted and used as an IPv4 destination address for the encapsulated packet.
Automatic tunneling uses the ::/96 prefix, which is enabled by default. It can be disabled by removing the route for ::/96.
6to4 uses the 2002::/16 prefix, which is not enabled by default.