Dynamic Trunking Protocol
A Cisco-proprietary protocol called Dynamic Trunking Protocol (DTP) is used by Cisco Catalyst switches to automatically negotiate the trunk encapsulation type and trunk port status. Establishing and dynamically managing trunk lines requires DTP.
Key Functions and Characteristics of DTP
Negotiation of Trunk Status: Whether a link between two switches should be an access link or a trunk is decided by DTP. It only handles this negotiation if the nearby switch’s port is set up in a trunk mode that allows DTP.
Negotiation of Encapsulation Type: In earlier Cisco switches that support IEEE 802.1Q and Inter-Switch Link (ISL), DTP can negotiate the kind of encapsulation to be used. Dynamic Trunking Protocol favours ISL if both switches support it; if not, they utilise the protocol that both are compatible with. The more recent Cisco switches, however, favour 802.1Q over the outdated ISL trunking protocol.
VLAN 1 Traffic: Cisco Discovery Protocol (CDP), Port Aggregation Protocol (PAgP), VLAN Trunking Protocol (VTP), and DTP information are among the specific protocol traffic that switches use VLAN 1 to transport.
You can also read Access Port vs Trunk Port And Access Port Disadvantages
DTP Trunking Modes
The many trunking modes that Cisco Catalyst switches enable specify how a port establishes a trunk link with its peer port using Dynamic Trunking Protocol:
Access: The port is set up to function as an access port (non-trunk) at all times, and trunking is not allowed.
Trunk: The port is configured to function as a trunk port at all times and makes an effort to turn the nearby link into a trunk. The interface enters a permanent trunking state in this mode, and the distant port is negotiated to become a trunk.
Dynamic Desirable: The port dynamically determines whether to begin using trunking by actively initiating and responding to negotiation messages. If the nearby switch interface can become a trunk, ports in this mode will attempt to become a trunk by communicating via Dynamic Trunking Protocol.
Dynamic Auto: The port responds to trunk negotiation signals by passively waiting for them and deciding whether to employ trunking. Switches that have both ports configured to dynamic auto will not create a trunk; instead, they will negotiate to be in access mode. In dynamic auto, a port will only become a trunk again if the neighbouring switch is in trunk or dynamic desirable mode.
Nonegotiate: By using this command, Dynamic Trunking Protocol frame generation from the interface is stopped. It is only usable if the neighbour interface is manually configured as either access or trunk.
You can also read What Is IEEE 802.1Q? The Key To VLANs In Ethernet Networks
Default DTP Configurations
Default Dynamic Trunking Protocol settings differ depending on the type of switch:
- Older 2950 switches defaulted to
dynamic desirable, meaning they would initiate trunk negotiations. - Newer 2960 switches default to
dynamic auto, meaning the interface does not initiate trunk negotiations.
Configuration and Verification
- To manually configure a switch port for trunking, the
switchport mode trunkcommand is used. - On switches supporting both ISL and 802.1Q (e.g., Cisco Catalyst 3560), the
switchport trunk encapsulation {dot1q | isl | negotiate}command configures or negotiates the trunking type. - Dynamic Trunking Protocol information can be displayed using the
show dtp [interface <name>]command, which shows global DTP information or details for a specified interface, including its configured mode, trunk encapsulation, and packet statistics. Theshow interfaces [name] switchportcommand also provides administrative and operational trunking encapsulation details and negotiation status.
Recommendations and Troubleshooting
Cisco Recommendation: Cisco generally recommends manually configuring switch ports to switchport mode trunk rather than relying on Dynamic Trunking Protocol for automatic negotiation, partly due to DTP’s vulnerability to VLAN hopping attacks.
Troubleshooting: Common issues include mismatched DTP modes where neither side initiates negotiation (e.g., both dynamic auto), or when DTP is disabled on one side (switchport nonegotiate) while the other side expects negotiation. If a port is set to dynamic auto, it needs the remote port to be on or desirable to trunk. A switchport mode trunk command may fail if encapsulation is not statically set on switches that negotiate encapsulation.
Though manual configuration is frequently favoured for security and predictability, Dynamic Trunking Protocol streamlines trunk link setup by automatically negotiating modes and encapsulation types across Cisco switches.
You can also read Link Aggregation Control Protocol (LACP) & How LACP Works?