xStack DGS-3400 Series Fast Ethernet Switch
The EtherType and VLAN ID are inserted after the MAC source address, but before the original EtherType/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.
Figure 7- 3. Adding an IEEE 802.1Q Tag
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 VLANs to span network devices (and indeed, the
entire network, if all network devices are 802.1Q compliant).
Unfortunately, not all network devices are 802.1Q compliant. These devices are referred to as tag-unaware. 802.1Q devices
are referred to as tag-aware.
Prior to the adoption of 802.1Q VLANs, port-based and MAC-based VLANs were in common use. These VLANs relied
upon a Port VLAN ID (PVID) to forward packets. A packet received on a given port would be assigned that port's PVID
and then be forwarded to the port that corresponded to the packet's destination address (found in the Switch's forwarding
table). If the PVID of the port that received the packet is different from the PVID of the port that is to transmit the packet,
the Switch will drop the packet.
Within the Switch, different PVIDs mean different VLANs (remember that two VLANs cannot communicate without an
external router). So, VLAN identification based upon the PVIDs cannot create VLANs that extend outside a given switch
(or switch stack).
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 PVID is not used to make packet-forwarding decisions, the VID is.
Tag-aware switches must keep a table to relate PVIDs within the Switch to VIDs 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 VIDs 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 VIDs 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.
Tagging and Untagging
Every port on an 802.1Q compliant switch can be configured as tagging or untagging.
Ports with tagging enabled will put the VID number, priority and other VLAN information into the header of all packets
that flow into and out of it. If a packet has previously been tagged, the port will not alter the packet, thus keeping the
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