Dynamic Trunk Protocol (DTP) to dynamically learn whether the
device on the other end of the cable wants to perform trunking and, if so, which trunking
protocol to use.
- dynamic auto: The port will negotiate the mode automatically; however, it prefers to be an access port.
- dynamic desirable: The port will negotiate the mode automatically; however, it prefers to be a trunk port.
| Dynamic Auto | Creates the trunk based on the DTP request from the neighboring switch. |
| Dynamic Desirable | Communicates to the neighboring switch via DTP that the interface would like to become a trunk if the neighboring switch interface is able to become a trunk. |
| Trunk | Automatically enables trunking regardless of the state of the neighboring switch and regardless of any DTP requests sent from the neighboring switch. |
| Access | Trunking is not allowed on this port regardless of the state of the neighboring switch interface and regardless of any DTP requests sent from the neighboring switch. |
| Nonegotiate | Prevents the interface from generating DTP frames. This command can be used 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. |
Trunk + Trunk = Trunk
Trunk + Des = Trunk
Trunk + Auto = Trunk
Des + Des = Trunk
Des + Auto = Trunk
Auto + Auto = No Trunk
Any + Access = No Trunk
Default on swtich ports is Auto.
Preferred dynamic desirable and if both ports are dynamic but one is configured as auto and the other as desirable, the resulting operating mode will be trunk.
DTP also negotiates the type of encapsulation on the trunk should either of the two devices support both ISL and 802.1Q.If both devices support both trunk types, they will choose ISL. Should the DTP negotiation fail, any port in dynamic mode, either desirable or auto, will be operating as an access port
Catalyst 2950 and 3550 models default to dynamic desirable mode
Catalyst 2960, 3560 or 3750, default to dynamic auto mode.
While DTP and VTP are independent protocols, DTP carries the VTP domain name in its messages
Example:
! The administrative mode of dynamic desirable (trunking) and negotiate (trunking
! encapsulation) means that Switch1 attempted to negotiate to trunk, but the
! operational mode of static access means that trunking negotiation failed.
! The reference to "operational trunking encapsulation" of native means that
! no tagging occurs.
Switch1# show int fa 0/1 switchport
Name: Fa0/1
Switchport: Enabled
Administrative Mode: dynamic desirable
Operational Mode: static access
Administrative Trunking Encapsulation: negotiate
Operational Trunking Encapsulation: native
Negotiation of Trunking: On
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)
++++++++++++++++++++++++++++++++++++++++++++++++++++++
! Next, the show int gig 0/1 trunk command shows the configured mode
! (desirable), and the current status (n-isl), meaning negotiated ISL. Note
! that the trunk supports the extended VLAN range as well.
Switch1# show int gig 0/1 trunk
Port Mode Encapsulation Status Native vlan
Gi0/1 desirable n-isl trunking 1
++++++++++++++++++++++++++++++++++++++++++++++++++++++
! The TOS/TAS/TNS stand for Trunk Operating/Administrative/Negotiation Status
! The TOT/TAT/TNT stand for Trunk Operating/Administrative/Negotiation Type
! In the following output, Fa0/12 is configured as dynamic desirable
SW1# show dtp interface fa0/12
DTP information for FastEthernet0/12:
TOS/TAS/TNS: TRUNK/DESIRABLE/TRUNK
TOT/TAT/TNT: ISL/NEGOTIATE/ISL
++++++++++++++++++++++++++++++++++++++++++++++++++++++
Without any configuration, the default port settings on recent Catalyst switch
series such as 2960, 3560, 3750, 3650, and 3850 are as follows: mode set to dynamic auto,
native VLAN set to 1, access VLAN set to 1, trunk encapsulation set to auto (if both ISL
and dot1q supported) or dot1q, all VLANs allowed, and VLANs 2–1001 eligible for pruning.
On older 2950 and 3550 models, the default mode was dynamic desirable.
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