Amer Networks SS3GR24i - Manual (Page 38)

SS3GR24i/SS3GR48i Layer3 Stackable Gigabit Switch User’s Guide
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. The VLAN information in the tag can then be used by other
802.1Q compliant devices on the network to make packet
forwarding decisions.
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
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