Amer Networks SS3GR24i - Manual (Page 37)

SS3GR24i/SS3GR48i Layer3 Stackable Gigabit Switch User’s Guide
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 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-
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