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Dynamic Trunking Protocol (3.2.3)

The Dynamic Trunking Protocol (DTP) is used to negotiate forming a trunk between two Cisco devices. DTP causes increased traffic, and is enabled by default, but may be disabled.

Introduction to DTP (3.2.3.1)

Ethernet trunk interfaces support different trunking modes. An interface can be set to trunking or nontrunking, or to negotiate trunking with the neighbor interface. Trunk negotiation is managed by the Dynamic Trunking Protocol (DTP), which operates on a point-to-point basis only, between network devices.

DTP is a Cisco proprietary protocol that is automatically enabled on Catalyst 2960 and Catalyst 3560 Series switches. Switches from other vendors do not support DTP. DTP manages trunk negotiation only if the port on the neighbor switch is configured in a trunk mode that supports DTP.

The default DTP configuration for Cisco Catalyst 2960 and 3560 switches is dynamic auto as shown in Figure 3-20 on interface F0/3 of switches S1 and S3.

Figure 3-20

Figure 3-20 Initial DTP Configuration

To enable trunking from a Cisco switch to a device that does not support DTP, use the switchport mode trunk and switchport nonegotiate interface configuration mode commands. This causes the interface to become a trunk but not generate DTP frames.

In Figure 3-21, the link between switches S1 and S2 becomes a trunk because the F0/1 ports on switches S1 and S2 are configured to ignore all DTP advertisements, and to come up in and stay in trunk port mode. The F0/3 ports on switches S1 and S3 are set to dynamic auto, so the negotiation results in the access mode state. This creates an inactive trunk link. When configuring a port to be in trunk mode, there is no ambiguity about which state the trunk is in; it is always on. With this configuration, it is easy to remember which state the trunk ports are in; if the port is supposed to be a trunk, the mode is set to trunk.

Figure 3-21

Figure 3-21 DTP Interaction Results

Negotiated Interface Modes (3.2.3.2)

Ethernet interfaces on Catalyst 2960 and Catalyst 3560 Series switches support different trunking modes with the help of DTP:

  • switchport mode access: Puts the interface (access port) into permanent nontrunking mode and negotiates to convert the link into a nontrunk link. The interface becomes a nontrunk interface, regardless of whether the neighboring interface is a trunk interface.
  • switchport mode dynamic auto: Makes the interface able to convert the link to a trunk link. The interface becomes a trunk interface if the neighboring interface is set to trunk or desirable mode. The default switchport mode for newer Cisco switch Ethernet interfaces is dynamic auto. Note that if two Cisco switches are left to the common default setting of auto, a trunk will never form.
  • switchport mode dynamic desirable: Makes the interface actively attempt to convert the link to a trunk link. The interface becomes a trunk interface if the neighboring interface is set to trunk, desirable, or auto mode. This is the default switchport mode on older switches, such as the Catalyst 2950 and 3550 Series switches.
  • switchport mode trunk: Puts the interface into permanent trunking mode and negotiates to convert the neighboring link into a trunk link. The interface becomes a trunk interface even if the neighboring interface is not a trunk interface.
  • switchport nonegotiate: Prevents the interface from generating DTP frames. You can use this command only when the interface switchport mode is access or trunk. You must manually configure the neighboring interface as a trunk interface to establish a trunk link.

Table 3-8 illustrates the results of the DTP configuration options on opposite ends of a trunk link connected to Catalyst 2960 switch ports.

Table 3-8 DTP Negotiated Interface Modes

Dynamic Auto

Dynamic Desirable

Trunk

Access

Dynamic Auto

Access

Trunk

Trunk

Access

Dynamic Desirable

Trunk

Trunk

Trunk

Access

Trunk

Trunk

Trunk

Trunk

Limited connectivity

Access

Access

Access

Limited connectivity

Access

The default DTP mode is dependent on the Cisco IOS Software version and on the platform. To determine the current DTP mode, issue the show dtp interface command, as shown in the following output.

S1# show dtp interface f0/1
DTP information for FastEthernet0/1:
  TOS/TAS/TNS:                               TRUNK/ON/TRUNK
  TOT/TAT/TNT:                               802.1Q/802.1Q/802.1Q
  Neighbor address 1:                        0CD996D23F81
  Neighbor address 2:                        000000000000
  Hello timer expiration (sec/state):        12/RUNNING
  Access timer expiration (sec/state):       never/STOPPED
  Negotiation timer expiration (sec/state):  never/STOPPED
  Multidrop timer expiration (sec/state):    never/STOPPED
  FSM state:                                 S6:TRUNK
  # times multi & trunk                      0
  Enabled:                                   yes
  In STP:                                    no
<output omitted>
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