Ingress Filtering - Enterasys VH-2402-L3 Management Manual

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 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
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