Hello,
here is what the Cisco Error Message Decoder says:
1. %SPANTREE-2-BPDU_SKEWING: BPDU skewed with a delay of [dec] seconds (max_age/2)
This message indicates that a BPDU for one or more VLANs or instances when MISTP is running has been skewed for a time longer than max_age/2. [dec] is the number of seconds.
Recommended Action: No action is required. To determine which VLAN has been skewed, use the show spantree bpdu-skewing command.
Recommended documents:
Spanning-Tree Protocol Enhancements using Loop Guard and BPDU Skew Detection Features
http://www.cisco.com/warp/public/473/84.html
HTH,
Georg