PAgP (Port Aggregation Protocol) is Cisco’s proprietary protocol for dynamically negotiating an EtherChannel between switch ports. EtherChannel combines multiple physical links into one logical link, allowing the links to remain in STP forwarding state without creating Layer 2 loops. section 16.2.1 section 16.5
PAgP uses two negotiation modes:
- desirable actively attempts to form an EtherChannel. section 16.5
- auto waits for a neighboring port in desirable mode and does not initiate negotiation. section 16.2.1
- desirable plus desirable and desirable plus auto form a channel; auto plus auto does not. section 16.2.1
Two-link PAgP trunk
On SW1, configure G0/0–G0/1 with channel-group 1 mode desirable. On SW2, configure the same ports with channel-group 1 mode auto. A logical Port-channel1 interface is created automatically. Configure switchport mode trunk on the port-channel interface; the member ports inherit the setting. Verify the result with show etherchannel summary, where the protocol appears as PAgP and member ports show P.
