xStack DES-3500 Series Layer 2 Stackable Fast Ethernet Managed Switch User Manual
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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 VLAN information
intact. Other 802.1Q compliant devices on the network to make packet-forwarding decisions can then use the VLAN information
in the tag.
Ports with untagging enabled will strip the 802.1Q tag from all packets that flow into and out of those ports. If the packet doesn't
have an 802.1Q VLAN tag, the port will not alter the packet. Thus, all packets received by and forwarded by an untagging port
will have no 802.1Q VLAN information. (Remember that the PVID is only used internally within the Switch). Untagging is used
to send packets from an 802.1Q-compliant network device to a non-compliant network device.
Ingress Filtering
A port on a switch where packets are flowing into the Switch and VLAN decisions must be made is referred to as an ingress port.
If ingress filtering is enabled for a port, the Switch will examine the VLAN information in the packet header (if present) and
decide whether or not to forward the packet.
If the packet is tagged with VLAN information, the ingress port will first determine if the ingress port itself is a member of the
tagged VLAN. If it is not, the packet will be dropped. If the ingress port is a member of the 802.1Q VLAN, the Switch then
determines if the destination port is a member of the 802.1Q VLAN. If it is not, the packet is dropped. If the destination port is a
member of the 802.1Q VLAN, the packet is forwarded and the destination port transmits it to its attached network segment.
If the packet is not tagged with VLAN information, the ingress port will tag the packet with its own PVID as a VID (if the port is
a tagging port). The switch then determines if the destination port is a member of the same VLAN (has the same VID) as the
ingress port. If it does not, the packet is dropped. If it has the same VID, the packet is forwarded and the destination port transmits
it on its attached network segment.
This process is referred to as ingress filtering and is used to conserve bandwidth within the Switch by dropping packets that are
not on the same VLAN as the ingress port at the point of reception. This eliminates the subsequent processing of packets that will
just be dropped by the destination port.
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 VLANs are configured in Port-based mode, their respective member ports are removed from the "default."
Packets cannot cross VLANs. If a member of one VLAN wants to connect to another VLAN, the link must be through an external
router.
