Final answer:
Auto/Desirable refers to configuration options for the Port Aggregation Protocol (PAgP), a Cisco proprietary protocol used to bundle multiple physical ports into one logical channel.
Step-by-step explanation:
Auto/Desirable refers to configurations options available in the Port Aggregation Protocol (PAgP), which is used to automate the creation of Cisco proprietary EtherChannels. PAgP facilitates the bundling of several physical ports into a single logical channel. When a port is set to 'Auto', it passively waits for PAgP packets from a partner before forming an EtherChannel. Conversely, when set to 'Desirable', the port actively sends PAgP packets and attempts to negotiate an EtherChannel.