Product Manual

DES-3526 / DES-3526DC Fast Ethernet Layer 2 Switch
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.
Default VLANs
The Switch initially configures one VLAN, VID = 1, called "default." The factory default setting
assigns all ports on the Switch to the "default." As new VLANs are configured in Port-based mode,
their respective member ports are removed from the "default."
Packets cannot cross VLANs. If a member of one VLAN wants to connect to another VLAN, the link
must be through an external router.
NOTE: If no VLANs are configured on the Switch, 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.
An example is presented below:
Port-based VLANs
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 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.
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