The modes for PAgP are as follows:
On - This mode forces the interface to channel without PAgP. Interfaces configured in the on mode do not exchange PAgP packets
PAgP desirable - This PAgP mode places an interface in an active negotiating state in which the interface initiates negotiations with other interfaces by sending PAgP packets
PAgP auto - This PAgP mode places an interface in a passive negotiating state in which the interface responds to the PAgP packets that it receives but does not initiate PAgP negotiation
If all modes are disabled by using the no command, or if no mode is configured, then the EtherChannel is disabled
The modes must be compatible on each side