DTP

DTP — Dynamic Trunking Protocol.

DTP is a Cisco proprietary point-to-point protocol, for full-duplex switchlinks.

An older feature intended to automate parts of network setup, you could set one switch to dynamic desireable and it will form trunks automatically.

The best practice is to disable this feature on trunks links with switchport nonegotiate

DTP is normally every 30 seconds.

Modes

switchport mode dynamic auto

• Send DTP
• Usually Default
• Become a trunk if the neighbor is a trunk
• Become a trunk if the neighbor is set to desireable

switchport mode access

• Probably doesn’t¹ send DTP frames
• If it does, it asks the neighbor to become an access port

switchport mode trunk

• Send DTP
• Asks the neighbor to become a trunk port

switchport mode dynamic desirable

• Become a trunk only if the neighbor can be convinced to become a trunk
• Works with trunk, desirable or auto

switchport nonegotiate

• Disable DTP
• Only works with access or trunk

Commands

show dtp
debug dtp packets

Verification

Performed on a C3560CX, running 15.2(7r)E.

No DTP with switchport mode access

switch# show dtp interface tenGigabitEthernet 1/0/7 | i Enabled
  Enabled:                                  no

References

Dynamic Trunking Protocol - Wikipedia

VLAN Configuration Guide, Cisco IOS XE 17.15.x (Catalyst 9500 Switches) - Configuring VLAN Trunks Cisco Catalyst 9500 Series Switches - Cisco

Solved: Switchport Mode Access question - Cisco Community

1. The IOS-XE guide says “negotiates” but in lab, I don’t see DTP frames on these ports. I checked on IOSv, IOL, and a C3560CX running 15.2(7r)E.
   Consensus online is this turns off DTP. ↩