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Intel® Blade Server Ethernet Switch Module IXM5414E 29
Port VLAN ID
Packets that are tagged (are carrying the 802.1Q VID information) can be transmitted from one
802.1Q compliant network device to another with the VLAN information intact. This enables
802.1Q VLANs to span network devices (and indeed, the entire network, if all network devices are
802.1Q compliant).
Not all network devices are 802.1Q compliant. These devices are referred to as tag-unaware. 802.1Q
devices are referred to as tag-aware.
Before the adoption of 802.1Q VLANs, port-based and MAC-based VLANs were in common use.
These VLANs relied upon a port VLAN ID (PVID) to forward packets. A packet received on a
given port would be assigned that port PVID and then be forwarded to the port that corresponded to
the packet destination address (found in the switch forwarding table). If the PVID of the port that
receives the packet is different from the PVID of the port that is to transmit the packet, the switch
module will drop the packet.
A switch port can have only one PVID but can have as many VIDs as the switch module has
memory in its VLAN table to store them.
Tagging and untagging
Every port on an 802.1Q compliant switch can be configured to admit or discard packets that are
received without a tag. Untagged packets that are admitted will be tagged with the port’s PVID.
Every port on an 802.1Q compliant switch can also be configured to transmit packets with or
without tags. Ports with tagging enabled will leave the 802.1Q tag received with the packet or
inserted by the ingress port unchanged. Ports with untagging enabled will strip the 802.1Q tag from
all packets that it transmits. Untagging is used to send packets from an 802.1Q-compliant network
device to a noncompliant one.
Egress rules
If the packet is not tagged with VLAN information, the ingress port will tag the packet with its own
PVID as a VID (if the port is configured to accept untagged packets) and pass it to the forwarding
function.
Dest. Src.
Length/EType
Data
Dest.
Src.
EType
Tag
Length/EType Data
Priority
VLAN ID
VLAN ID
Adding an IEEE 802.1Q Tag
Adding an IEEE 802.1Q Tag
Old
CRC
Old
CRC
New
CRC
New
CRC
Orginal Ethernet
Packet
Orginal Ethernet
Packet
New Tagged
Packet
New Tagged
Packet