Auto listens and desirable initiates
PAgP is Cisco’s protocol for negotiating EtherChannel membership. A port in desirable mode starts PAgP negotiation. A port in auto mode responds to PAgP messages. Desirable can form a channel with auto or desirable when the ports are compatible. Two ports waiting in auto mode do not start an exchange.
Check the settings of all candidate members, including speed and the Layer 2 access or trunk configuration. Cisco also supports a static on mode, which bypasses negotiation. PAgP and LACP are different protocols and cannot negotiate one bundle with each other.
Try it in a lab
- On two PAgP-capable switches, configure compatible links and select auto with non-silent negotiation on both sides. Verify that the channel does not negotiate.
- Change one side to desirable and check that the peer is discovered and the compatible links join the EtherChannel. Compare the negotiated members with the physical links that report up.