DES-3226L Layer 2
Fast Ethernet Switch User’s Guide
Port-based VLANs
Port-based VLANs limit traffic that flows into and out of switch ports. Thus, all devices
connected to a port are members of the VLAN(s) the port belongs to, whether there is a single
computer directly connected to a switch, or an entire department.
On port-based VLANs, NICs do not need to be able to identify 802.1Q tags in packet headers.
NICs send and receive normal Ethernet packets. If the packet’s destination lies on the same
segment, communications take place using normal Ethernet protocols. Even though this is
always the case, when the destination for a packet lies on another switch port, VLAN
considerations come into play to decide if the packet gets dropped by the switch or delivered.
VLAN Segmentation
Take for example a packet that is transmitted by a machine on Port 1 that is a member of
VLAN 2. If the destination lies on another port (found through a normal forwarding table
lookup), the switch then looks to see if the other port (Port 10) is a member of VLAN 2 (and
can therefore receive VLAN 2 packets). If Port 10 is not a member of VLAN 2, then the
packet will be dropped by the switch and will not reach its destination. If Port 10 is a member
of VLAN 2, the packet will go through. This selective forwarding feature based on VLAN
criteria is how VLANs segment networks. The key point being that Port 1 will only transmit
on VLAN 2.
Network resources such as printers and servers however, can be shared across VLANs. This is
achieved by setting up overlapping VLANs. That is ports can belong to more than one VLAN
group. For example, setting VLAN 1 members to ports 1, 2, 3, and 4 and VLAN 2 members
to ports 1, 5, 6, and 7. Port 1 belongs to two VLAN groups. Ports 8, 9, and 10 are not
configured to any VLAN group. This means ports 8, 9, and 10 are in the same VLAN group.
VLAN and Trunk Groups
The members of a trunk group have the same VLAN setting. Any VLAN setting on the
members of a trunk group will apply to the other member ports.
Note
: In order to use VLAN segmentation in conjunction with port trunk
groups, you can first set the port trunk group(s), and then you may
configure VLAN settings. If you wish to change the port trunk grouping
with VLANs already in place, you will not need to reconfigure the VLAN
settings after changing the port trunk group settings. VLAN settings will
automatically change in conjunction with the change of the port trunk
group settings.
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