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Combination of EtherChannel (PAgP) operating modes
Cisco’s proprietary EtherChannel negotiation mechanism PAgP has two modes to set up the interface.
Enable PAgP unconditionally. Start sending negotiation messages to other ports.
Enable PAgP only when a PAgP device is detected. In other words, it waits for an on-the-other request and responds to a request for PAgP negotiation. This reduces the transmission of PAgP packets.
Auto and Auto do not start negotiating PAgP from the above mode.
A combination of Desirable/Desirable and Desirable/Auto is required to use PAgP.
Therefore, the combination that prevents formation is ④.