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User’s Manual of GS-5220 LCD Series
189
Adding an IEEE802.1Q Tag
Dest. Addr. Src. Addr. Length/E. type Data Old CRC
Dest. Addr. Src. Addr. E. type Tag Length/E. type Data New CRC
Priority CFI 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 allows 802.1Q VLAN to span
network devices (and indeed, the entire network – if all network devices are 802.1Q compliant).
Every physical port on a switch has a PVID. 802.1Q ports are also assigned a PVID, for use within the switch. If no
VLAN 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 VLAN 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.
Tag-aware switches must keep a table to relate PVID within the switch to VID on the network. The switch will
compare the VID of a packet to be transmitted to the VID of the port that is to transmit the packet. If the two
VID are different the switch will drop the packet. Because of the existence of the PVID for untagged packets and
the VID for tagged packets, tag-aware and tag-unaware network devices can coexist on the same network.
A switch port can have only one PVID, but can have as many VID as the switch has memory in its VLAN table to
store them.
Because some devices on a network may be tag-unaware, a decision must be made at each port on a tag-aware
device before packets are transmitted – should the packet to be transmitted have a tag or not? If the
Original Ethernet Packet
New Tagged Packet