Dell C1048P Port Extender Networking Configuration Guide for the C9000 Series - Page 949

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 (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). Virtual LANs (VLANs) 949