Bull 1GB Intel Ethernet Switch Module, Installation And User Manual

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NovaScale Blade 1 GB Intel® Ethernet Switch Module: Installation and User’s Guide
IEEE 802.1Q VLAN tags
The following illustration shows the 802.1Q VLAN tag. Four additional octets are inserted between
the source MAC address and the packet’s EtherType field. Their presence is indicated by a value of
0x8100 in the two bytes following the MAC address, in the VLAN tag’s EtherType field, indicating
that the packet carries an 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) and 12 bits of VLAN ID
(VID). The 3 bits of user priority are used according to the protocols defined in IEEE 802.1p (now
part of IEEE 802.1D). The VID is the VLAN identifier and its use is defined 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.
Ingress
Rules Ingress
Rules
Forwarding
Process Forwarding
Process
Filtering
Database Filtering
Database
Egress
Rules Egress
Rules
Packet
Transmit Packet
Transmit
Port State Port State
802.1Q Packet Forwarding 802.1Q 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