D-Link DGS-3208TG User Guide - Page 35

Gigabit Ethernet Switch User's Guide 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 portbased 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 a VLAN ID number, 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 DGS-3208TG, have broadcast sensors and filters built into each port to further control broadcast storms. Segmenting Broadcast Domains The Switch allows you to segment broadcast domains. It does this by forwarding packets only to ports in the same broadcast domain or VLAN. Thus, broadcast packets will only be forwarded to ports that are members of the same broadcast domain or VLAN. Other parts of the network are effectively shielded. As a result, the smaller the broadcast domain, the less effect a broadcast storm will have. Since VLANs and broadcast 23