Planet IGSW-2840 User Manual page 173

Adding an IEEE802.1Q Tag
Dest. Addr. Src. Addr.
Dest. Addr. Src. Addr.
■ 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."
■ Assigning Ports to VLANs
Before enabling VLANs for the switch, you must first assign each port to the VLAN group(s) in which it will participate. By default
all ports are assigned to VLAN 1 as untagged ports. Add a port as a tagged port if you want it to carry traffic for one or more
VLANs, and any intermediate network devices or the host at the other end of the connection supports VLANs. Then assign ports
on the other VLAN-aware network devices along the path that will carry this traffic to the same VLAN(s), either manually or
dynamically using GVRP. However, if you want a port on this switch to participate in one or more VLANs, but none of the
Length/E. type Data
E. type Tag Length/E. type
Priority CFI VLAN ID
Original Ethernet
Old CRC
Data New CRC
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User's Manual of IGSW-2840
New Tagged Packet