Cisco Certification Exam Review:

Trunking Modes On 2950 Cisco Switches

By Chris Bryant, CCIE #12933

Trunking is getting easier and easier with Cisco switches; most new Cisco switches have their FastEthernet ports in dynamic desirable mode by default, which means that the ports are actively attempting to trunk.

Over time, though, the port settings may be changed by other engineers, and then one day you have to form a trunk between ports that have never trunked before. You connect the ports with a crossover cable, and...nothing. That's when you better know the different trunking modes and exactly what's going on with each!

SW1(config-if)#switchport mode dynamic ?
auto Set trunking mode dynamic negotiation parameter to AUTO
desirable Set trunking mode dynamic negotiation parameter to DESIRABLE

dynamic desirable: Default for many new Cisco switches, a port in this mode is actively attempting to form a trunk via Dynamic Trunking Protocol (DTP).

dynamic auto: A port in this mode will form a trunk, but is not actively attempting to negotiate trunking, so DTP frames are not transmitted.

SW1(config-if)#switchport mode trunk ?
<cr>

trunk: Port trunks unconditionally.

SW1(config-if)#switchport nonegotiate ?
<cr>

nonegotiate: Negotiation frames (DTP frames) are not sent. This mode can only be used if the interface is already in trunk mode; trying to put a port into nonegotiate mode if it's in a dynamic state (either dynamic desirable or dynamic auto) will result in an error message.

SW1(config-if)#switchport mode access

access: Port is placed into nontrunking mode.

To your success,

Chris Bryant

CCIE #12933

chris@thebryantadvantage.com