xStack DES-3500 Series Layer 2 Stackable Fast Ethernet Managed Switch User Manual
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NOTE:
If no VLANs are configured on the Switch, then all packets will be forwarded to any desti-
nation port. Packets with unknown source addresses will be flooded to all ports. Broadcast and
multicast packets will also be flooded to all ports.
An example is presented below:
VLAN Name
VID
Switch Ports
System (default)
1
5, 6, 7, 8, 21, 22, 23, 24
Engineering
2
9, 10, 11, 12
Marketing
3
13, 14, 15, 16
Finance
4
17, 18, 19, 20
Sales
5
1, 2, 3, 4
Table 6- 2. VLAN Example - Assigned Ports
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.