Some helpful sites to start with:

http://www.kame.net/newsletter/20001119b/

http://www.kame.net/newsletter/20001119/

http://www.debian-administration.org/?article=37

http://www.ipsec-howto.org/x299.html

http://www.tldp.org/HOWTO/Adv-Routing-HOWTO/

http://www.tldp.org/HOWTO/Adv-Routing-HOWTO/lartc.tunnel.gre.html

http://www.tldp.org/HOWTO/VPN-HOWTO/

http://www.littleyojik.co.uk/computers/debian_vpn.html

http://www.funknet.org/doc/tunnel/l2tp.xml

http://perlpimp.dk/perma/2005/03/21/setting_up_a_vpn_with_racoon/

http://www.sherman.ca/archives/2004/11/21/linux-26-ipsec-vpns/

http://www.fukt.bth.se/~teddy/debian-ipsec

http://www.onlamp.com/pub/a/bsd/2004/10/21/wifi_ipsec.html?page=1

http://www.rommel.stw.uni-erlangen.de/~hshoexer/ipsec-howto/HOWTO.html#LAN

http://vpn.ebootis.de/

http://www.shorewall.net/IPSEC-2.6.html#id2749213

http://eradman.com/article/gre1

http://linux-ip.net/gl/ip-tunnels/node2.html

Cisco

http://www.ciscopress.com/articles/article.asp?p=25477&rl=1

http://www.cisco.com/warp/public/471/ios-aes.html

http://www.cisco.com/univercd/cc/td/doc/product/software/ios113ed/113t/113t_3/ipsec.htm