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DES-3326S Layer 3 Fast Ethernet Switch User’s Guide
Switch Management and Operating Concepts 79
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.
VLANs in Layer 2 Only Mode
The switch initially configures one VLAN, VID = 1, called the
DEFAULT_VLAN. The factory default setting assigns all ports
on the switch to the DEFAULT_VLAN.
Packets cannot cross VLANs if the switch is in Layer 2 Only
mode. If a member of one VLAN wants to connect to another
VLAN, the link must be through an external router.
When the switch is in Layer 2 Only mode, 802.1Q VLANs are
supported.
If no VLANs are configured on the switch and the switch is in
Layer 2 Only mode, then all packets will be forwarded to any
destination port. Packets with unknown source addresses will
be flooded to all ports. Broadcast and multicast packets will
also be flooded to all ports.