D-Link DES-3225G - Manual (Page 35)

24-port NWay Ethernet Switch User’s Guide
Switch Management Concepts 25
In the above example, step 4 is the key element. Because the packet has 802.1Q VLAN data encoded in its header, the
ingress port can make VLAN-based decisions about its delivery: whether server #2 is attached to a port that is a member
of VLAN 2 and thus, should the packet be delivered; the queuing priority to give to the packet, etc. It can also perform
these functions for VLAN 1 packets as well, and, in fact, for any tagged packet it receives regardless of the VLAN number.
If the ingress port in step 4 were connected to a non-802.1Q-compliant device and was thus receiving untagged packets, it
would tag its own PVID onto the packet and use this information to make forwarding decisions. As a result, the packets
coming from the non-compliant device would automatically be placed on the ingress ports VLAN and could only
communicate with other ports that are members of this VLAN.
Port-based VLANs
Port-based VLANs are a simplified version of the 802.1Q VLANs described in the previous section. In port-based VLANs,
all the 802.1Q settings are pre-configured allowing you to quickly and easily setup and maintain port-based VLANs on
your network.
In port-based VLANs, broadcast, multicast and unknown packets will be limited to within the VLAN. Thus, port-based
VLANs effectively segment your network into broadcast domains. Furthermore, ports can only belong to a single VLAN.
Because port-based VLANs are uncomplicated and fairly rigid in their implementation, they are best used for network
administrators who wish to quickly and easily setup VLANs in order to isolate limit the effect of broadcast packets on
their network.
For the most secure implementation, make sure that end stations are directly connected to the switch. Attaching a hub,
switch or other repeater to the port causes all stations attached to the repeater to become members of the Port-based
VLAN.
To setup port-based VLANs, simply select one of 24 VLAN ID numbers, name the VLAN and specify which ports will be
members. All other ports will automatically be forbidden membership, even dynamically as a port can belong to only one
VLAN.
Broadcast Storms
Broadcast storms are a common problem on today’s networks. Basically, they consist of broadcast packets that flood
and/or are looped on a network causing noticeable performance degradation and, in extreme cases, network failure.
Broadcast storms can be caused by network loops, malfunctioning NICs, bad cable connections, and applications or
protocols that generate broadcast traffic, among others.
In effect, broadcast storms can originate from any number of sources, and once they are started, they can be self-
perpetuating, and can even multiply the number of broadcast packets on the network over time. In the best case, network
utilization will be high and bandwidth limited until the hop counts for all broadcast packets have expired, whereupon the
packets will be discarded and the network will return to normal. In the worst case, they will multiply, eventually using up
all the network bandwidth (although network applications will usually crash long before this happens), and cause a
network meltdown.
Broadcast storms have long been a concern for network administrators with routers traditionally being used to prevent
their occurrence, and if that failed, to at least limit their scope. However, with the advent of VLANs, switches are now able
to limit broadcast domains better and cheaper than routers. Also, many switches, including the DES-3225G series, have
broadcast sensors and filters built into each port to further control broadcast storms.