Port Aggregation Protocol (PAgP) is a Cisco proprietary protocol used to negotiate and maintain LAG connections. EtherChannel on Cisco devices can be configured to use PAgP to negotiate and maintain aggregated bundles of physical links.
PAgP can be configured on Cisco devices using the desirable or Auto mode. At least one end of an LACP bundle must be configured as desirable in order for the aggregated link to form.
The other LAG protocol supported by Cisco devices is LACP.