PAgP

PAgP, or Port Aggregation Protocol, is a Cisco proprietary protocol used to automate the creation and maintenance of EtherChannel links. By grouping multiple physical Ethernet links between two network devices into a single logical link, PAgP increases available bandwidth and provides redundancy in the event of a link failure.

The protocol operates at Layer 2 and exchanges PAgP frames between neighboring switch interfaces that are configured as part of the same channel group. Negotiation determines whether the member links should form an EtherChannel. PAgP supports several modes that control how aggressively a port participates in the aggregation: Desirable and Auto are the primary negotiation modes, while On forces the port to join the channel without negotiation. Desirable actively attempts to form a channel, Auto is passive and will form only if the neighbor is in Desirable or Auto mode. If both sides are Auto, the channel will not form. When a channel is established, all member links must have compatible speed and duplex settings.

PAgP is Cisco proprietary, which means it is best suited for networks consisting largely of Cisco devices. In mixed-vendor environments, or where interoperability with non-Cisco switches is required, the IEEE 802.3ad/LACP standard is commonly preferred, as it provides a vendor-agnostic mechanism for link aggregation. Administrators should ensure consistent configuration across connected devices and consider using LACP for multi-vendor deployments, while PAgP can be appropriate for homogeneous Cisco deployments seeking automatic EtherChannel formation with minimal manual configuration.