D-Link DES-3028, User Manual

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DES-3028 DES-3028P DES-3028G DES-3052 DES-3052P Layer 2 Fast Ethernet Managed Switch
If the packet is not tagged with VLAN information, the ingress port will tag the packet with its own PVID as a VID. 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.
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."
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 destination
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 7- 1. VLAN Example - Assigned Ports
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.
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