Member ports must have same values for:

• Speed & duplex
• trunking/access mode, DTP mode
• same access VLAN
• same trunk type, allowed & native vlans
• same STP cost per VLAN
• no SPAN configured

Switch shuts down all physical ports when no interface port-channel [n] to avoid loop.
spanning-tree etherchannel guard misconfig for the same purposes (BPDUs received via both ports).
PAGP: 8 links max; 01:00 :0c:cc:cc:cc
LACP: 16 links max, 8 active max, 8 standby; 01:80:C2:00:00:02
channel-protocol: interface command. refuse any modes keywords other than configured under this command