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How to verify trunk modes in Cisco Catalyst switches that run CatOS or Cisco IOS system software

Core issue

Fast Ethernet and Gigabit Ethernet trunk ports support five different trunk modes. Some Fast Ethernet and Gigabit Ethernet ports also allow the encapsulation type to be specified. These are the encapsulation types that the trunk can use:

  • Inter-Switch Link (ISL) encapsulation
     
  • 802.1Q encapsulation
     
  • An automatically negotiated type of encapsulation

The trunk modes must match across the trunk link. If one side of the link is configured as an ISL trunk, the other side of the link must also be configured as ISL. Similarly, if one side of the link is configured as an 802.1Q trunk, the other side of the link must also be configured as 802.1Q.

These are the different types of trunk modes:

  • ON: This mode puts the port into permanent trunk mode and negotiates to convert the link into a trunk link. The port becomes a trunk port even if the adjacent port does not agree to the change.
     
  • OFF: This mode puts the port into permanent non-trunk mode and negotiates to convert the link into a non-trunk link. The port becomes a non-trunk port even if the adjacent port does not agree to the change.
     
  • Desirable: This mode causes the port to actively attempt to convert the link into a trunk link. The port becomes a trunk port if the adjacent port is set to on, desirable, or auto mode.
     
  • Auto: This mode enables the port to convert the link into a trunk link. The port becomes a trunk port if the adjacent port is set to on or desirable mode. This is the default mode for Fast and Gigabit Ethernet ports.
     
  • Nonegotiate: This mode puts the port into permanent trunk mode, but does not allow the port to generate Dynamic Trunking Protocol (DTP) frames. The adjacent port must be configured manually as a trunk port to establish a trunk link.

Resolution

In Catalyst OS (CatOS), issue the show trunk command to view trunk information for the switch.

In Cisco IOS Software, issue the show interfaces privileged EXEC command to view the administrative and operational status of all interfaces or a specified interface.

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