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Interoperating: Cisco

• Cisco supports IPsec-over-UDP NAT traversal.
• Cisco VPN Client appears to use nonstandard IPsec and does not work with Openswan. This message concerns Cisco VPN Client 4.01.
• A Linux Openswan-Cisco connection may close after some time. Here is a workaround, and here is another comment on the same subject.
• Older Ciscos purchased outside the United States may not have 3DES, which Openswan requires.
• RSA keying may not be possible between Cisco and Openswan.
• In ipsec.conf, VPN3000 DN (distinguished name) must be in binary (X.509 only)

SANS Institute HOWTO (PSK). Detailed, with extensive references.
Short HOWTO (PSK)
French page with configs for Cisco IOS, PIX and VPN 3000 (X.509)
Dave McFerren's sample configs (PSK)
Wolfgang Tremmel's sample configs (PSK road warrior)
Old doc from Pete Davis, with William Watson's updated Tips (PSK)
Short NAT-T Openswan <> (possible NAT-T) Cisco PIX firewall howto

Some PIX specific information:

Waikato Linux Users' Group HOWTO. Nice detail (PSK)
John Leach's configs (PSK)
Greg Robinson's settings (PSK) (broken link) use this
Scott's ipsec.conf for PIX (PSK, Openswan side only) (broken link)
Rick Trimble's PIX and Openswan settings (PSK) (broken link)

Note, you can't take a Freeswan page, s/freeswan/openswan/ and expect URLs to still work!

Cisco VPN support page
Cisco IPsec information page