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NS3503-16P-4C User Manual
The Ether Type and VLAN ID are inserted after the MAC source address, but before the original Ether
Type/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.
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 transmitting port is connected to a tag-unaware device, the packet should be untagged. If the
transmitting port is connected to a tag-aware device, the packet should be tagged.
Default VLANs
The Switch initially configures one VLAN, VID = 1, called "default." The factory default setting assigns all
ports on the Switch to the "default". As new VLAN are configured in Port-based mode, their respective
member ports are removed from the "default."
Original Ethernet
New Tagged Packet
