Intel IXM5414E manual Port Vlan ID, Tagging and untagging, Egress rules

Adding an IEEE 802.1Q Tag

Dest.	Src.	Length/EType	Data	Old
				CRC

Orginal Ethernet

Packet

New Tagged

Packet

Dest.	Src.	EType	Tag	Length/EType	Data	New
						CRC

Priority

VLAN ID

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.

Intel® Blade Server Ethernet Switch Module IXM5414E 29