http://cio.cisco.com/univercd/cc/td/doc/product/lan/cat6000/sw_6_3/cmd_ref/set_f_l.htm#69273
cs-c6506-37a> (enable) set system highavailability enable
Failed to enable system high availability.
Feature not allowed while DOT1X is enabled.
cs-c6506-37a> (enable) set dot1x system-auth-control disable
dot1x system-auth-control disabled.
cs-c6506-37a> (enable) set system highavailability enable
System high availability enabled.
cs-c6506-37a> (enable)