Hi,
Cisco recommends an explicit trunk configuration of desirable at both ends. In this mode, network operators can trust syslog and command line status messages that a port is up and trunking, unlike on mode, which can make a port appear up even though the neighbor is misconfigured. In addition, desirable mode trunk provides stability in situations where one side of the link cannot become a trunk or drops trunk state. Issue this command in order to set desirable mode:
set trunk mod/port desirable ISL | dot1q
For more information, Just have a look into the below URL
http://www.cisco.com/en/US/products/hw/switches/ps663/products_tech_note09186a0080094713.shtml
Thanks
Samy