724-746-5500 | blackbox.com Page 35
Chapter 4: Basic Concepts and Management
What if VLAN tagging is applied?
VLAN tagging is a 4-byte long data immediately following the MAC source address. When tagged VLAN is applied, the Ethernet
frame structure will change slightly.
Collision domain 1
Collision domain 2
Collision
domain 3
Collision
domain 4
Collision
domain 5
Figure 4-6.
Only two fields, VLAN ID and Tag control information, are different when compared to the basic Ethernet frame. The rest of the
fields are the same.
The first two bytes are the VLAN type ID, with the value of 0x8100 indicating the received frame is a tagged VLAN. The next two
bytes are Tag Control Information (TCI). They provide user priority and VLAN ID, explained next.
Table 4-6. User priority and VLAN ID.
Bits 15–13 User priority 7–0, 0 is lowest prio rity
Bit 12
CFI (Canonical Format Indicator)
1: RIF field is present in the tag header.
0: No RIF field is present.
Bits 11 –0
VID (VLAN identifier)
0x000: Null VID. No VID i s present and only user priority is p resent.
0x001: Default VID
0xFFF: Reserved
NOTE: RIF is used in Token Ring networks to provide source routing and consists of two fields: Routing Control and Route
Descriptor.