Port Vlan Id - D-Link DGS-3312SR Release 3.5 Manual

DGS-3312SR Stackable Gigabit Layer 3 Switch
Figure 4- 49. IEEE 802.1Q Tag
The EtherType and VLAN ID are inserted after the MAC source address, but before the original EtherType/Length or
Logical Link Control. Because the packet is now a bit longer than it was originally, the Cyclic Redundancy Check (CRC)
must be recalculated.
Figure 4- 50. Adding an IEEE 802.1Q Tag

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 allows 802.1Q VLANs to span network devices (and indeed, the
entire network – if all network devices are 802.1Q compliant).
Unfortunately, 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.
Prior to the adoption 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's PVID
and then be forwarded to the port that corresponded to the packet's destination address (found in the Switch's forwarding
table). If the PVID of the port that received the packet is different from the PVID of the port that is to transmit the packet,
the Switch will drop the packet.
Within the Switch, different PVIDs mean different VLANs. (remember that two VLANs cannot communicate without an
external router). So, VLAN identification based upon the PVIDs cannot create VLANs that extend outside a given Switch
(or Switch stack).
Every physical port on a Switch has a PVID. 802.1Q ports are also assigned a PVID, for use within the Switch. If no
VLANs are defined on the Switch, all ports are then assigned to a default VLAN with a PVID equal to 1. Untagged packets
are assigned the PVID of the port on which they were received. Forwarding decisions are based upon this PVID, in so far
as VLANs are concerned. Tagged packets are forwarded according to the VID contained within the tag. Tagged packets
are also assigned a PVID, but the PVID is not used to make packet forwarding decisions, the VID is.
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