IPSEC is encryption, usually using DES, 3-DES or AES, and on a Cisco router or Firewall refers to the VPN tunnel created after the key exchange (ISAKMP).

The command <b>clear ipsec sa peer < peer IP></b> removes the IPSEC entry for that particular peer, which means that the IPSEC tunnel needs to be re-negotiated when there is additional traffic to pass between the VPN peers. a bit like doing a shut/no shut on a router interface.