I config the 6to4, the config is:
interface loopback 0
ip address 172.16.20.1 255.255.255.255
ip 192.168.1.1 255.255.255.252
tunel source loopback0
tunnel source ipv6ip 6to4
is it invalid config?
if yes, what is the source ip address of the tunnel?
192.168.1.1 or 172.16.20.1 or ipv6 2002::/16?
The tunnel address for a 6to4 tunnel should be a IPv6 address which is based on the IPv4 tunnel source address. In your case:
(where c0a8.0101 is the hex representation of your IPv4 tunnel source address). 6to4 tunnels assume the network between the IPv6 domains is NBMA and they find the destination IPv4 address using the embedded IPv4 address. The same address configuration should be applied to the other end of the tunnel.