This is what I captured from this link
http://www.cisco.com/univercd/cc/td/doc/product/iaabu/pix/pix_sw/v_63/config/fixup.pdf
MGCP messages are transmitted over UDP. A response is sent back to the source address (IP address and UDP port number) of the command, but the response may not arrive from the same address as the command was sent to. This can happen when multiple call agents are being used in a failover configuration and the call agent that received the command has passed control to a backup call agent, which then sends the response.
Under which circumstance the primary call-agent will pass the control to backup agent? Usually we''ll have both ccm as agents. But only when primary agent down,gw will send request to backup, not when primary is active.
