A VLAN is a group of devices on one or more LANs that you can configure so that the devices can communicate as if they were
attached to the same wire. When in fact, they are located on several different LAN segments. Because VLANs are based on logical
instead of physical connections, they are flexible for user and host management, bandwidth allocation, and resource optimization.
The IEEE 802.1Q protocol standard addresses the
problem of dividing large networks into smaller parts so broadcast and
multicast traffic does not consume more bandwidth than necessary. The standard
also helps provide a higher level of security between segments of internal
The 802.1Q specification establishes a standard
method for inserting VLAN membership information into Ethernet frames.
Cisco IOS XR software supports VLAN subinterface configuration on 40Gigabit, HundredGig, FourHundredGig, and bundle interfaces.
802.1Q Tagged Frames
The IEEE 802.1Q tag-based VLAN uses an extra tag in the MAC header to
identify the VLAN membership of a frame across bridges. This tag is used for
VLAN and quality of service (QoS) priority identification. The VLANs can be
created statically by manual entry or dynamically through Generic Attribute
Registration Protocol (GARP) VLAN Registration Protocol (GVRP). The VLAN ID
associates a frame with a specific VLAN and provides the information that
switches must process the frame across the network. A tagged frame is four
bytes longer than an untagged frame and contains two bytes of Tag Protocol
Identifier (TPID) residing within the type and length field of the Ethernet
frame and two bytes of Tag Control Information (TCI) which starts after the
source address field of the Ethernet frame.