What form are PAgP keepalives? Multicast/Broadcast?
Is there any in depth info on how they are used to keep track of the etherchannel bundle?
PagP messages are always sent to well known cisco multicast address 01-00-0C-CC-CC-CC with protocol type code, which I think is 0104 (not sure).
Please note that this multicast mac is also used by other protocols such as CDP, VTP, DISL, DTP with approriate protocol type values.
PAGPworks by comparing a switch's capabilities (device ID, port group etc) with that of its neighbor device. It then groups these capable ports into a channel.
The above link also explains the different channel modes such as deseriable, auto etc.