Chapter 4: Web configuration
NS3552-8P-2S-V2 User Manual 109
802.1Q tag
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
Port VLAN ID
Packets that are tagged (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 as well as 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 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 VID, not
the PVID, is used to make packet forwarding decisions.
Tag-aware switches must keep a table to relate PVID within the switch to VID on the
network. The switch compares the VID of a packet to be transmitted to the VID of the
port that is to transmit the packet. If the two VIDs are different, the switch drops 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 VIDs as the switch has
memory in its VLAN table to store them.
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