The setup of a Cisco router as ISATAP server one needs to define a tunnel interface as follows:

interface Tunnel0
no ip address
no ip redirects
ipv6 address 2003:abc:def:123::/64 eui-64
no ipv6 nd suppress-ra
tunnel source FastEthernet0
tunnel mode ipv6ip isatap

If an ISATAP supporting client is set up to know about this Cisco router it will be able to automatically configure its interface with the prefix above. The last 64 bits will be based on the EUI-64 address and include the IPv4 address of the client as specified in the ISATAP draft [TGTT05]. According to present experience with ISATAP it does not matter which interface is specified as the tunnel source as long as it has an IPv4 address reachable by the clients. One can also specify an address directly.