With a CA, a peer authenticates itself to the remote peer by sending a certificate to the remote peer and performing some public key cryptography. Each peer sends its own unique certificate which was issued and validated by the CA. This process works because each peer's certificate encapsulates the peer's public key, each certificate is authenticated by the CA, and all participating peers recognize the CA as an authenticating authority.
Check the URL: Managing VPN Remote Access:
http://www.cisco.com/en/US/docs/security/pix/pix63/configuration/guide/basclnt.html
Configuring IPSec and Certification Authorities:
http://www.cisco.com/en/US/docs/security/pix/pix63/configuration/guide/ipsecint.html