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Auto mode

As explained in Acing the CCNA Exam: Volume 1 Fundamentals and Protocols

Auto mode is a PAgP setting that passively responds to negotiation messages from a desirable-mode neighbor.

Auto mode is the passive PAgP setting. A port in auto mode does not initiate negotiation; it responds to PAgP messages from a neighbor configured for desirable mode. section 16.2.1 section 16.5

Remember these PAgP combinations:

  • Desirable plus desirable forms an EtherChannel. section 16.2.1
  • Desirable plus auto forms an EtherChannel. section 16.2.1
  • Auto plus auto does not form an EtherChannel; links operate independently, with STP blocking one link. section 16.2.1
  • Configure auto on member ports with channel-group group-number mode auto, then verify with show etherchannel summary or show pagp neighbor. section 16.5
Figure 16.5 An EtherChannel is formed between SW1 and SW2 using PAgP. SW1’s G0/0 and G0/1 ports are configured in PAgP desirable mode, and SW2’s G0/0 and G0/1 ports are configured in PAgP auto mode. switchport mode trunk is applied to each switch’s port channel interface to make the EtherChannel a trunk.

A two-switch diagram shows PAgP desirable on SW1, auto on SW2, and the resulting EtherChannel trunk. section 16.2.1

How Acing the CCNA Exam: Volume 1 Fundamentals and Protocols covers it

How deep it goes
Defines passive behavior, mode combinations, configuration, and verification, but does not descend into PAgP protocol internals.
Examples
Uses SW1 and SW2 configurations, two physical links, Port-channel1, and show etherchannel summary output.
What it assumes
Assumes familiarity with DTP’s desirable and auto keywords while defining their PAgP use.
What you can do after
Apply it via Cisco CLI using channel-group mode auto and diagnose the result with EtherChannel and PAgP neighbor commands.
How it connects
Connects auto mode to PAgP, EtherChannel, desirable mode, LACP, and Spanning Tree Protocol.
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