Users Guide

Figure 144. 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 (mandatory)
• Assigning Interfaces to a VLAN (optional)
• Assigning an IP Address to a VLAN (optional)
• Enabling Null VLAN as the Default VLAN
Enabling Null VLAN as the Default VLAN
In a Carrier Ethernet for Metro Service environment, service providers who perform frequent reconfigurations for customers with
changing requirements occasionally enable multiple interfaces, each connected to a different customer, before the interfaces are fully
configured.
This presents a vulnerability because both interfaces are initially placed in the native VLAN, VLAN 1, and for that period customers are able
to access each other's networks. The system has a Null VLAN to eliminate this vulnerability. When you enable the Null VLAN, all ports are
placed into it by default, so even if you activate the physical ports of multiple customers, no traffic is allowed to traverse the links until
each port is place in another VLAN.
To enable Null VLAN, use the following command.
• Disable the default VLAN, so that all ports belong to the Null VLAN until configured as a member of another VLAN.
CONFIGURATION mode
default-vlan disable
Default: the default VLAN is enabled (no default-vlan disable).
Assigning an IP Address to a VLAN
VLANs are a Layer 2 feature. For two physical interfaces on different VLANs to communicate, you must assign an IP address to the
VLANs to route traffic between the two interfaces.
The shutdown command in INTERFACE mode does not affect Layer 2 traffic on the interface; the shutdown command only prevents
Layer 3 traffic from traversing over the interface.
Virtual LANs (VLANs)
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