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Catalyst 3750 Metro Switch Software Configuration Guide
78-15870-01
Chapter 9 Configuring Interface Characteristics
Understanding Interface Types
Port-Based VLANs
A VLAN is a switched network that is logically segmented by function, team, or application, without
regard to the physical location of the users. For more information about VLANs, see Chapter 10,
“Configuring VLANs.” Packets received on a port are forwarded only to ports that belong to the same
VLAN as the receiving port. Network devices in different VLANs cannot communicate with one another
without a Layer 3 device to route traffic between the VLANs.
VLAN partitions provide hard firewalls for traffic in the VLAN, and each VLAN has its own MAC
address table. A VLAN comes into existence when a local port is configured to be associated with the
VLAN, when the VLAN Trunking Protocol (VTP) learns of its existence from a neighbor on a trunk, or
when a user creates a VLAN.
To configure normal-range VLANs (VLAN IDs 1 to 1005), use the vlan vlan-id global configuration
command to enter config-vlan mode or the vlan database privileged EXEC command to enter VLAN
database configuration mode. The VLAN configurations for VLAN IDs 1 to 1005 are saved in the VLAN
database. To configure extended-range VLANs (VLAN IDs 1006 to 4094), you must use config-vlan
mode with VTP mode set to transparent. Extended-range VLANs are not added to the VLAN database.
When VTP mode is transparent, the VTP and VLAN configuration is saved in the switch running
configuration, and you can save it in the switch startup configuration file by entering the copy
running-config startup-config privileged EXEC command.
Add ports to a VLAN by using the switchport interface configuration commands:
• Identify the interface.
• For a trunk port, set trunk characteristics, and if desired, define the VLANs to which it can belong.
• For an access port, set and define the VLAN to which it belongs.
• For a tunnel port, set and define the VLAN ID for the customer-specific VLAN tag. See Chapter 13,
“Configuring IEEE 802.1Q and Layer 2 Protocol Tunneling.”
Switch Ports
Switch ports are Layer 2-only interfaces associated with a physical port. Switch ports belong to one or
more VLANs. A switch port can be an access port, a trunk port, or a tunnel port. You can configure a
port as an access port or trunk port or let the Dynamic Trunking Protocol (DTP) operate on a per-port
basis to determine switchport mode by negotiating with the port on the other end of the link.You must
manually configure tunnel ports as part of an asymmetric link connected to an 802.1Q trunk port. Switch
ports are used for managing the physical interface and associated Layer 2 protocols and do not handle
routing or bridging.
Configure switch ports by using the switchport interface configuration commands. For detailed
information about configuring access port and trunk port characteristics, see Chapter 10, “Configuring
VLANs.” For more information about tunnel ports, see Chapter 13, “Configuring IEEE 802.1Q and
Layer 2 Protocol Tunneling.”
Access Ports
An access port belongs to and carries the traffic of only one VLAN (unless it is configured as a voice
VLAN port). Traffic is received and sent in native formats with no VLAN tagging. Traffic arriving on
an access port is assumed to belong to the VLAN assigned to the port. If an access port receives a tagged
packet (Inter-Switch Link [ISL] or 802.1Q tagged), the packet is dropped, and the source address is not
learned.
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