DES-1218R/DES-1226R 10/100/1000 Mbps Ethernet Switch User ’s Guide
20
Another benefit of VLANs is that you can change the network topology without physically moving
stations or changing cable connections. Stations can be ‘moved’ simply by changing VLAN settings
from one VLAN (the sales VLAN, for example) to another VLAN (the marketing VLAN). This allows
VLANs to accommodate network moves, changes and additions with the utmost flexibility.
VLANs can also provide a level of security to your network. Port-based VLANs allow you to
configure ports to not accept packets from outside of the VLAN.
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 it’s 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 groups.
Setting VLAN 1 members are port 1,2,3,4 and VLAN 2 members are 1,5,6,7. The Port 1 is belonging
to two VLAN groups. The port 8, 9, 10 are not configured to any VLAN group. This is means port
8,9,10 are 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 must 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 need to reconfigure
the VLAN settings after changing the port trunk group settings.
