IBM BladeCenter - 802.1 Q VLAN Tags

Ingress
Rules
Ingress
Rules
Forwarding
Process
Forwarding
Process
Filtering
Database
Filtering
Database
Egress
Rules
Egress
Rules
Packet
Transmit
Packet
Transmit
Port StatePort State
802.1Q Packet Forwarding802.1Q Packet Forwarding
Tagging or
Untagging
Tagging or
Untagging
Ingress
Filtering
Ingress
Filtering
PVID to VID
VLAN Table
PVID to VID
VLAN
Table
Packet
Receive
Packet
Receive
802.1Q VLAN tags
The following illustration shows the 802.1Q VLAN tag. Four additional octets are
inserted after the source MAC address. Their presence is indicated by a value of
0x8100 in the EtherType field. When a packet EtherType field is equal to 0x8100,
the packet carries the IEEE 802.1Q/802.1p tag. The tag is contained in the
following 2 octets and consists of 3 bits of user priority, 1 bit of Canonical Format
Identifier (CFI)
1
and 12 bits of VLAN ID (VID). The 3 bits of user priority are used
by 802.1p. The VID is the VLAN identifier and is used by the 802.1Q standard.
Because the VID is 12 bits long, 4094 unique VLANs can be identified.
The tag is inserted into the packet header, increasing the length of the entire packet
by 4 octets. All of the information that was originally contained in the packet is
retained.
1. CFI is used for encapsulating Token Ring packets so that they can be carried across Ethernet backbones.
26 IBM BladeCenter 4-Port Gb Ethernet Switch Module: Installation and User’s Guide