Dan,
It has to be setup as a trunking port - DTP is Dynamic Trunking Protocol, a Cisco proprietary protocol in the VLAN group, and is for negotiating trunking on a link between two devices and for negotiating the type of trunking encapsulation (802.1Q) to be used. The "Dynamic" part is it will automaticly
But, both ends of a trunk cable had better agree they're trunking, or they're going to be interpreting trunk frames as normal frames and will not know what to do with the trunking header info.
Here are the different modes -
Mode and what the Mode Does
on - Forces the link into permanent trunking, even if the neighbor doesn't agree
off - Forces the link to permanently not trunk, even if the neighbor doesn't agree
desirable - Causes the port to actively attempt to become a trunk, subject to neighbor agreement (neighbor set to on, desirable, or auto )
auto - Causes the port to passively be willing to convert to trunking. The port will not trunk unless the neighbor is set to on or desirable . This is the default mode. Note that auto-auto (both ends default) links will not become trunks.
nonegotiate - Forces the port to permanently trunk but not send DTP frames. For use when the DTP frames confuse the neighboring (non-Cisco) 802.1q switch. You must manually set the neighboring switch to trunking.
Hope this helps!
E.A. Broda
CCNA, CCDA, CCAI, Network +
