Switch Interface Overview
The 3550 supports many different interface types through the use of switch ports. Switch ports can be either an access port or a trunk port. You are also given the option of using Dynamic Trunking Protocol (DTP) on a per-port basis to automatically determine if the port should be an access or a trunk, by negotiating with the port on the other end. Because switch ports are Layer 2 ports, they cannot route or bridge traffic.
You use an access port to carry traffic from a single VLAN. This type of port can be a member of only one VLAN. Traffic is sent and received in native formats with no VLAN tagging. Because the traffic is untagged, any traffic that arrives on an access port is assumed to belong to the VLAN ...