Cisco CCNP Certification Training:

Dynamic Trunking Protocol (DTP)

By Chris Bryant, CCIE #12933

With many newer Cisco switches, trunks are dynamic - just plug in a crossover cable and the trunk's up in seconds. However, trunking questions on your BCMSN and CCNP exams aren't going to be quite that easy! The good news is that the CCNP-level switching questions will build on the foundation you built during your CCNA studies.

You know that IEEE 802.1Q ("dot1q") and ISL are your two choices of trunking protocols, and you know the main differences between the two. What you might not have known is that there's a third trunking protocol that's running between your Cisco switches, and while it's a transparent process to many, you had better know about it for your BCMSN and other CCNP exams!

The Cisco-proprietary Dynamic Trunking Protocol (DTP) actively attempts to negotiate a trunk link with the remote switch. This sounds great, but there is a cost in overhead - DTP frames are transmitted every 30 seconds. If you decide to configure a port as a non-negotiable trunk port, there's no need for the port to send DTP frames.

DTP can be turned off at the interface level with the switchport nonegotiate command, but as you see below, you cannot turn DTP off until the port is no longer in dynamic desirable trunking mode. (Dynamic desirable is the default mode for most Cisco switch ports.)

SW2(config)#int fast 0/8
SW2(config-if)#switchport nonegotiate
Command rejected: Conflict between 'nonegotiate' and 'dynamic' status.

SW2(config-if)#switchport mode ?
access Set trunking mode to ACCESS unconditionally
dynamic Set trunking mode to dynamically negotiate access or trunk mode
trunk Set trunking mode to TRUNK unconditionally

SW2(config-if)#switchport mode trunk
SW2(config-if)#switchport nonegotiate

When you're working with Cisco switches in a home lab or rack rental environment, run IOS Help regularly to see what options are available for the commands you're practicing with. Cisco switch ports have quite a few options, and the best way to find them is with one simple symbol - the question mark!

To your success,

Chris Bryant

CCIE #12933

chris@thebryantadvantage.com