To fully understand the operation and configuration of port based VLANs, it is essential to understand the meanings of several key terms.
VLAN ID
A unique number (between 1 and 4095) that identifies a particular VLAN.
VLAN Name
A 32-character alphanumeric name associated with a VLAN ID. The VLAN Name is intended to make user-defined VLANs easier to identify and remember.
Tag Header (VLAN Tag)
A field within a frame that identifies the VLAN the frame has been classified into. The Tag Header is inserted into the frame directly after the Source MAC address field. Twelve bits of the Tag Header are the VLAN ID. The remaining bits are other control information.
Tagged Frame
A data frame that contains a Tag Header. The Tag Header can be added to the data frame by a VLAN aware switch to any frame received from a port that is a member of a VLAN.
Untagged Frame
A data frame that does not have a Tag Header inserted into it.
Port VLAN ID (PVID)
An identification that encompasses a particular switch port’s identification (port 6, module 2) and that port’s VLAN membership. This identification is used to classify incoming untagged frames when they are received.
Default VLAN
The VLAN to which all ports are assigned upon initialization. The Default VLAN has a VLAN ID of 1.
Forwarding List
A list of the ports on a particular device that are eligible to transmit frames for a selected VLAN. The Forwarding List identifies what ports are associated with a single VLAN for frame transmission purposes.
Egress List
A per port list of all eligible VLANs that can be forwarded out one specific port and the frame format of transmissions for that port.The Egress List specifies what VLANs are associated with a single port for frame transmission purposes.
Filtering Database
A database structure within the switch that keeps track of the associations between MAC addresses, VLAN eligibilities, and interface (port) numbers. The Filtering Database is referred to when a VLAN aware switch makes a forwarding decision on a frame.
1Q Trunk
A connection between 802.1Q switches that passes only traffic with a VLAN Tag Header inserted in the frame.
1d Trunk
A connection from a switch that passes only untagged traffic.
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