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Important VLAN Definitions:
Ingress
The point at which a frame is received on a switch and the switching decisions must be made.
The switch examines the VID (if present) in the received frames header and decides whether or
not and where to forward the frame. If the received frame is untagged, the switch will tag the frame
with the PVID for the port on which it was received. It will then use traditional Ethernet bridging
algorithms to determine the port to which the packet should be forwarded.
Next, it checks to see if each destination port is on the same VLAN as the PVID and thus can
transmit the frame. If the destination port is a member of the VLAN used by the ingress port, the
frame will be forwarded. If the received frame is tagged with VLAN information, the switch checks
its address table to see whether the destination port is a member of the same VLAN. Assuming
both ports are members of the tagged VLAN, the frame will be forwarded.
Tagging
Every port on an 802.1Q compliant switch can be configured as tagging or un-tagging
.
Ports with a tagging 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 been tagged previously, the port will not
alter the packet and keep 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.
Un-tagging
Ports without a tagging will strip the 802.1Q tag from all packets that flow into and out of those
ports. If the packet does not have an 802.1Q VLAN tag, the port will not alter the packet. Thus, all
packets received by and forwarded by an un-tagging port will have no 802.1Q VLAN information.
(Remember that the PVID is only used internally within the switch). Un-tagging is used to send
packets from an 802.1Q-compliant network device to a non-compliant network device. Simply put,
un-
tagging means that once you set up the port as “
U
” (untagged), all egress packets (in the same
VLAN group) from the port will have no tags.
Summary of Contents for FOS-3126-PLUS SERIES
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