A virtual LAN (VLAN) is an administratively defined broadcast domain logically segmented by function, team, or application. It enhances performance by limiting traffic to stations in the same VLAN; traffic to other VLANs is blocked. By default, a switch is configured with a single VLAN that contains only static-access ports.
To see the port members of a VLAN, choose VLAN > VLAN and use the VLAN window. This window lets you:
You cannot use the VLAN Membership window to assign ATM ports to VLANs; see the Catalyst 2900 Series ATM Modules Install and Configure Guide for command-line interface instructions.
By default, all ports are static-access ports assigned to VLAN 1. If you change the VLAN ID, use an ID in the range 1 to 1001 or 1006 to 4094.
Notes:
Static-access ports cannot be assigned to multiple VLANs, so if you plan to move a port connection from one switch to another, configure the port for dynamic VLAN membership or as a trunk port to avoid reconfiguring it.
On Catalyst 2912MF, 2924M, and 3500 XL series switches, you can configure up to 250 port-based VLANs and up to 64 instances of the Spanning Tree Protocol (see Configuring Spanning Tree Protocol). On other Catalyst 1900, 2820, and 2900 switches, you can create up to 64 port-based VLANs.
Note: Before you assign ports to a VLAN, you must first create the VLAN and decide whether to use VLAN Trunking Protocol (see Managing VTP for details).
To assign static ports to a VLAN, choose VLAN > VLAN and use the VLAN window.
Dynamic-access ports are assigned to a VLAN by the VLAN Membership Policy Server (VMPS). Therefore, you must configure the VMPS server before you configure dynamic ports (see Selecting VMP Configuration Settings for details).
A dynamic-access ports cannot be:
To configure a port as a dynamic-access port, choose VLAN > VLAN and use the VLAN window.
Connect dynamic-access ports only to end stations, and be sure to configure the network so that STP does not put a dynamic-access port into an STP blocking state (see Configuring Spanning Tree Protocol for details).
Caution: To avoid loss of connectivity, do not connect dynamic-access ports to switches or routers running bridging protocols.
A multi-VLAN port can belong to more than one VLAN, which creates overlapping VLANs. Only ports connected to routers or servers should be defined as multi-VLAN ports. When the multi-VLAN port is connected to a router, all traffic is forwarded within the boundaries of the VLANs, but the two (or more) VLANs establish connectivity through the router.
A multi-VLAN port functions normally in all its VLANs. For example, when an unknown MAC address is received on a multi-VLAN port, it is learned by all VLANs in which the port is a member. Multi-VLAN ports also respond to the STP messages generated by the different instance of STP in each VLAN. Because the multi-VLAN port is a member of more than one VLAN, flooded traffic received from the multi-VLAN port is forwarded to ports in all VLANs assigned to the multi-VLAN port, unless the VLAN is pruned (see Configuring a Trunk Port).
Caution: To avoid loss of connectivity, do not connect multi-VLAN ports to hubs or switches.
To assign a multi-VLAN port to a VLAN, choose VLAN > VLAN and use the VLAN window.
A trunk is a point-to-point link between two switches or between switches and routers. Trunks carry the traffic of multiple VLANs and extend VLANs from one switch to another.
You can configure two types of trunk ports: ISL and IEEE 802.1Q. On ISL trunk ports, the switch encapsulates all received and transmitted packets with an ISL header, and it filters native frames received from an ISL trunk port. On an 802.1Q trunk port, the switch receives both untagged traffic and traffic containing 802.1Q tags.
Follow these guidelines when configuring a trunk port:
To configure a trunk port, choose VLAN > VLAN and use the VLAN window.
Trunk ports are automatically assigned to all VLANs in the VTP domain where the switch is a member. You can use the VLAN window to restrict the VLANs assigned to a trunk port.
VLAN information is advertised to network devices by means of the VLAN Trunking Protocol (VTP). To save network bandwidth, VTP prunes to protect VLANs from unnecessary traffic. If you create a new trunk on a switch that belongs to a VTP domain where pruning is in effect, pruning is automatically enabled. If the trunk is created in a VTP domain where pruning is not in effect, pruning is disabled.
All VLANs are placed on a pruning eligibility list so that pruning will occur if it is in effect. However, in some network configurations, pruning will block VLAN traffic to ports that need it. Pruning should not occur in these VLANs. To disable pruning on a VLAN, you must remove the VLAN from the pruning eligibility list.
"Configuring
VLANs," Catalyst 2950 Desktop Switch Software Configuration Guide
"Configuring
VLANs," Catalyst 2900 XL and Catalyst 3500 XL Software Configuration
Guide
"Configuring
VLANs," Catalyst 3550 Multilayer Switch Software Configuration Guide