LAN-to-LAN IPsec Tunnel Between Two Routers Configuration Example
IPv6 Tunnel through an IPv4 Network
http://www.cisco.com/en/US/products/hw/routers/ps259/prod_configuration_examples_list.html
http://www.cisco.com/en/US/products/hw/routers/ps259/tsd_products_support_eol_series_home.html
DHCP client + NAT
https://learningnetwork.cisco.com/message/81597
http://forums.speedguide.net/showthread.php?225673-Cisco-2621-on-Comcast-Cable
http://forums.whirlpool.net.au/archive/1395877
IPv6 (IOS 12.4)
http://www.cisco.com/en/US/docs/ios/ipv6/configuration/guide/12_4/ipv6_12_4_book.html