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20.3 VLAN Operation
VLAN Operation Section 20: Implementing VLANs and Trunks A Cisco Catalyst switch operates in a network in a manner that is similar to a traditional bridge. Each VLAN that you configure on the switch implements address learning, forwarding and filtering decisions, and loopavoidance mechanisms as if the VLAN were a separate physical bridge. The Cisco Catalyst switch implements VLANs by restricting traffic forwarding to destination ports that are in the same VLAN as the originating ports. When a frame arrives on a switch port, the switch must retransmit the frame to only the ports that belong to the same VLAN. In essence, a VLAN limits the unicast, multicast, and broadcast traffic. Traffic originating from a particular VLAN is forwarded only to other ports in that VLAN. A port normally carries only the traffic for the single VLAN to which it belongs. For a VLAN to span multiple switches, a port must be configured as a trunk port and connected to a port on another switch that is also configured as a trunk port. The connection between these two switches is called a "trunk." A trunk can carry traffic for multiple VLANs. The figure shows a trunk carrying multiple VLANs between two switches. VLAN Membership Modes These are the VLAN membership mode characteristics of Cisco Catalyst switch ports: Static VLAN: An administrator statically configures the assignment of VLANs to ports. Dynamic VLAN: Cisco Catalyst switches support dynamic VLANs using a VMPS. Some Cisco Catalyst switches can be designated as the VMPS, but you can also designate an external server as the VMPS. The VMPS contains a database that maps MAC addresses to VLAN assignments. When a frame arrives at a dynamic port on the Cisco Catalyst access switch, the switch queries the VMPS server for the VLAN assignment based on the source MAC address of the arriving frame. A dynamic port can belong to only one VLAN at a time. Multiple hosts can be active on a dynamic port only if they belong to the same VLAN. Voice VLAN: Voice VLANs allow you to assign a second VLAN to a port. Typically you can only assign a single VLAN to a port. The two exceptions are when a port is configured as a trunk port or when you assign a voice VLAN. Only VoIP devices, such as phones, use voice VLANs. All other traffic uses the other assigned VLAN. Up Next: Trunking with ISL and 802.1Q