Black Box Gigabit Smart Switch Eco Fanless, LGB2118A, LGB2124A manual Collision

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 2

Collision domain 1

Collision	Collision	Collision
domain 3	domain 4	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 priority

CFI (Canonical Format Indicator)

Bit 12 1: RIF field is present in the tag header. 0: No RIF field is present.

VID (VLAN identifier)

0x000: Null VID. No VID is present and only user priority is present.

Bits 11–0

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.

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