Content

| 2.5 |  | VLAN Trunking and Dynamic Trunking Protocol (DTP) |
| 2.5.6 |  | DTP combinations |

All of these DTP modes and their various combinations can be somewhat confusing. Looking at some of the basic combinations can help clarify this.

Default: Desirable mode
By default, Ethernet interfaces on most Cisco switches are set to desirable mode. This is true for both the Catalyst 2950 and 3550 switches. Desirable mode will create a trunk link if the neighboring interface is set to desirable, trunk, or auto mode. Because both interfaces by default are in desirable mode, this means a link between two Cisco switches will automatically become a trunk link unless configured otherwise.

Combinations
Figure shows the various DTP trunking modes and the results of the different combinations. One combination that could result in traffic being blocked from transmitting the link is if one interface is in access mode and the neighboring interface is in trunk mode. In this case, the access mode interface will only want to do non-trunking, while the trunk mode interface will only want to do trunking. This is not a recommended combination and may create unexpected results. Traffic may or may not be able to transit the link depending upon several factors, such as the spanning tree root bridge location. In some situations the link will work as a non-trunking link even though one interface is in access mode and the other interface is in trunk mode. In other situations spanning tree will put the access mode port into blocking mode and traffic will not transit the link to the neighboring trunk mode interface. In order to get consistent results, regardless of the topology, it is recommended never to have a combination of access mode and trunk mode on the interfaces of two neighboring switches.

The only modes that actually negotiate trunking between adjacent switches are trunk, dynamic desirable and dynamic auto. Even though the switchport mode trunk will not modify its own interface to be anything other than trunking, it will send out DTP frames to attempt to negotiate the other end of the link to be a trunking interface.

In Figure the show interface switchport command shows the Negotiation of Trunking set to On. The interface will always attempt to negotiate trunking on the link unless the switchport nonegotiate command is configured on the interface.

Nonegotiate
The switchport nonegotiate command prevents the interface from generating DTP frames, turning the Negotiation of Trunking off. This can be viewed with the show interface switchport command. The switchport nonnegotiate interface command can only be used on an interface that is in access mode or trunk mode, previously configured with either the switchport mode access command or the switchport mode trunk command.

If an interface is configured with both the switchport mode trunk command and the switchport nonegotiate command, the interface on the neighboring switch must be configured as with switchport mode trunk command to establish a trunk link, otherwise the link will be a nontrunking link.

To enable trunking to a device that does not support DTP, the switchport mode trunk and switchport nonegotiate interface configuration commands should be used to cause the interface to become a trunk but to not generate DTP frames. The switchport mode access and switchport nonegotiate interface configuration commands should be used when configuring a non-trunking interface to another switch that does not support DTP.