Cisco Dynamic Trunking Protocol (DTP) Explained

DTP, Dynamic Trunking Protocol, is a trunking protocol that is developed and proprietary to Cisco which is used to automatically negotiate trunks between Cisco switches. Trunk negotiations are managed by DTP only if the port is directly connected to each other.

Ethernet trunk interfaces support various trunking modes. Those interfaces can be configured as a trunk or non-trunk, or to initiate negotiating trunking to a neighbor interface or is waiting to receive a trunking negotiation message from another directly connected interface. Most Cisco switches nowadays use IEEE 802.1Q as their trunking type of choice because of less overhead compared to Inter-Switch Link (ISL).


Dynamic Trunking Protocol (DTP) Cisco Administrative Modes

Here are the trunking/switchport modes that are used to establish trunking between Cisco Catalyst switches. The DTP, Dynamic Trunking Protocol, ‘switchport mode’ interface subcommand is used to define the administrative trunking mode that tells the interface whether to trunk always or not or negotiate to trunk or not.

switchport mode access – an access port does not act as a trunk interface and only allows one VLAN through that port. An interface in access mode becomes a nontrunk interface, regardless of whether the neighboring interface is a trunk interface.

switchport mode trunk – the trunk mode enables the interface to be set in permanent trunking mode and establishes negotiations to convert the neighboring link to become a trunk link though the switch interface becomes a trunk even if the neighbor interface is not.

switchport mode dynamic auto – this DTP mode makes the interface passively waits to receive a negotiation message to make itself a trunk, at which point the switch will respond and negotiate whether to use trunking. The switch interface becomes a trunk port if the neighboring interface is set to trunk or dynamic desirable mode. The switchport mode dynamic auto is the default mode for newer Cisco switches such as Catalyst 2960 and 3560 Series switches.

switchport mode dynamic desirable – this DTP mode lets the port to initiates trunking with another port by sending a negotiation message to dynamically choose whether to start using trunking. The interface becomes a trunk port if the neighboring interface is set to trunk mode, dynamic desirable mode, or dynamic auto mode. The switchport mode dynamic desirable is the default on older switches, such as the Catalyst 2950 and 3550 Series switches.

switchport nonegotiate – this command disables Dynamic Trunking Protocol.


Dynamic Trunking Protocol (DTP) Cisco Negotiated Interface Modes

DTP, Dynamic Trunking Protocol, negotiation involves the exchange of DTP frames between two neighboring interfaces. When an interface is configured in switchport dynamic auto or dynamic desirable modes, it will initiate DTP negotiation in order to select its own trunking operational mode.

When an interface is statically configured in access mode or trunk mode, it will normally participate in the DTP process by responding to DTP frames if it receives any.

The table below describes how an interface selects its trunking operational mode based on its own trunking administrative mode and the results of the DTP process.

Trunk Mode 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

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