5.5 Redundant Links
5.5.2 Port aggregation protocol
The Port Aggregation Protocol (PAgP) adds additional features to Fast EtherChannel technology. PAgP aids in the automatic creation of Fast EtherChannel links. PAgP packets are sent between Fast EtherChannel technology-capable ports. The protocol learns the neighbors and their group capability dynamically and informs the neighbors of its local group capability. After the protocol determines correctly paired, bidirectional, point-to-point links, the protocol groups the ports that have the same neighbor device ID and neighbor group capability into a channel. Then the channel is added to Spanning Tree as a single bridge port.

Some restrictions have been deliberately introduced into PagP, which will not form a bundle on ports that are configured for dynamic VLANs. PAgP requires that all ports in the channel belong to the same VLAN or are configured as trunk ports.

Because dynamic VLANs can force the change of a port into a different VLAN, dynamic VLANs are not included in Fast EtherChannel participation. When a bundle already exists and a VLAN of a port is modified, all ports in the bundle are modified to match that VLAN. PAgP does not group ports that operate at different speeds or port duplex. If speed and duplex are changed when a bundle exists, PAgP changes the port speed and duplex for all ports in the bundle.