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those interfaces. Different VLANs can communicate between each other by means of IP routing. Because
traffic is only broadcast or flooded to the interfaces within a VLAN, the VLAN conserves bandwidth. Finally,
you can have multiple VLANs configured on one switch, thus segmenting the device.
Interfaces within a port-based VLAN must be in Layer 2 mode and can be tagged or untagged in the VLAN ID.
VLANs and Port Tagging
To add an interface to a VLAN, the interface must be in Layer 2 mode. After you place an interface in Layer 2
mode, the interface is automatically placed in the Default VLAN.
The system supports IEEE 802.1Q tagging at the interface level to filter traffic. When you enable tagging, a tag
header is added to the frame after the destination and source MAC addresses. That information is preserved
as the frame moves through the network. The following example shows the structure of a frame with a tag
header. The VLAN ID is inserted in the tag header.
Figure 139. Tagged Frame Format
The tag header contains some key information that the system uses:
•
The VLAN protocol identifier identifies the frame as tagged according to the IEEE 802.1Q specifications
(2 bytes).
•
Tag control information (TCI) includes the VLAN ID (2 bytes total). The VLAN ID can have 4,096 values,
but two are reserved.
NOTE:
The insertion of the tag header into the Ethernet frame increases the size of the frame to more
than the 1,518 bytes as specified in the IEEE 802.3 standard. Some devices that are not compliant with
IEEE 802.3 may not support the larger frame size.
Information contained in the tag header allows the system to prioritize traffic and to forward information to
ports associated with a specific VLAN ID. Tagged interfaces can belong to multiple VLANs, while untagged
interfaces can belong only to one VLAN.
Configuration Task List
This section contains the following VLAN configuration tasks.
•
Creating a Port-Based VLAN
•
Assigning Interfaces to a VLAN
•
Assigning an IP Address to a VLAN
•
Enabling Null VLAN as the Default VLAN
(mandatory)
(optional)
(optional)
Virtual LANs (VLANs)
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Troubleshooting
