PLANET ISW-1022MPT - Manual (Page 81)

User’s Manual of ISW-1022M Series and ISW-1033MT
81
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
New Tagged Packet
Original Ethernet
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."