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VLAN: 802.1q:


Logical network. A single layer 2 network portioned to create multiple broadcast domains. Which are mutually isolated, 4095 vlans (0 to 4095 are reserved). 32 bit will added to the Ethernet frame b/w source mac and etherType.

 It reduce the broadcasting by increasing the broadcasting domain.
16 bits
3 bits
1 bit
12 bits
TPID
TCI
PCP
DEI
VID
·         Tag protocol identifier (TPID): a 16-bit field set to a value of 0x8100 in order to identify the frame as an IEEE 802.1Q-tagged frame. This field is located at the same position as the EtherType/length field in untagged frames, and is thus used to distinguish the frame from untagged frames.
·         Tag control information (TCI)
·         Priority code point (PCP): a 3-bit field which refers to the IEEE 802.1p class of service and maps to the frame priority level. Values in order of priority are: 1 (background), 0 (best effort), 2 (excellent effort), 3 (critical application), ..., 7 (network control). These values can be used to prioritize different classes of traffic (voice, video, data, etc.).
·         Drop eligible indicator (DEI): a 1-bit field. (formerly CFI[note 1][4]) May be used separately or in conjunction with PCP to indicate frames eligible to be dropped in the presence of congestion.[5]

·         VLAN identifier (VID): a 12-bit field specifying the VLAN to which the frame belongs. The hexadecimal values of 0x000 and 0xFFF are reserved. All other values may be used as VLAN identifiers, allowing up to 4,094 VLANs.
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