Dynamic Trunking Protocol

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Dynamic Trunking Protocol (DTP), is a proprietary protocol developed by Cisco Systems for the purpose of negotiating trunking on a link between two switches participating in a VLAN, and for negotiating the type of trunking encapsulation (802.1Q) to be used. It exists on layer 2 of the OSI model.

VLAN trunks formed using DTP can be either dot1q VLAN Trunk or ISL.

[edit] Modes of Operations

DTP is on by default in all (modern) Cisco Catalyst switches. When two switches supporting DTP are connected together, they will automatically attempt to form a trunk, regardless of the number of VLANs on each switch. This is due to the default setting of "switchport mode dynamic desirable" for each switchport. In addition, if two switches are connected together via two or more cables, they will automatically form a Link Aggregate Group and VLAN trunk over that due to LACP. The following options exist:

switchport mode dynamic desirable - will form a trunk if connected to a 'mode dynamic' or a 'mode trunk' running DTP
switchport mode dynamic auto - will automatically form a trunk if connected to a 'mode dynamic desirable or a 'mode trunk' switchport
switchport mode access - will only be an access port (prevents DTP from working)
switchport mode trunk - will always be a trunk, and by default will notify the other end via DTP
switchport mode trunk w/ switchport nonegotiate - will always be a trunk, but will not run DTP to notify the other end

[edit] DTP vs VTP - Two Totally Different Protocols

Many people confuse DTP and VTP (Vlan Trunking Protocol) as two protocols that perform the same basic function, like LACP and PAgP. They are not at all related in this way! VTP is a protocol used to share vlan information between switches, but despite the "Trunk" in its name, it does not ever form trunks. A set of switches that run only VTP will automatically create (or otherwise maintain) VLANs across the network, but will be unable to trunk between switches without manual setup. A set of switches that runs only DTP will trunk all VLANs automatically, but an administrator must create these VLANs on each switch individually. Switches running both can accomplish both tasks. By default, Cisco switches run only DTP; this may be a source of confusion among novice Network Engineers as setting up VTP gives the appearance of trunk creation between switches. This is only because DTP is on by default.