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Configuring VLANs
The switch initially configures one VLAN, VID = 1, called the default VLAN. The factory default setting
assigns all ports on the switch to the default VLAN. As new VLANs are configured, there respective
member ports are removed from the default VLAN.
Packets cannot be transmitted accross VLANs. If a member of one VLAN wants to connect to another
VLAN, the link must be through an external router.
If no VLANs are configured on the switch 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.
Traffic Control
Broadcast and multicast storms 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 malfunctioning NICs, bad cable connections and applications or protocols that
generate broadcast traffic, among others.
Broadcast and multicast storms have long been a concern for network administrators with routers
traditionally being used to prevent their occurrence, and if that failed, limit their scope. However, with
the advent of VLANs, switches are now able to limit broadcast domains better and cheaper than
routers. Also, many managed and unmanaged switches have broadcast sensors and filters built into
each port to further control broadcast storms.
Segmenting Broadcast Domains
VLANs can be used to segment broadcast domains. They do this by forwarding packets only to ports
that are members of the same VLAN. Other parts of the network are effectively shielded. Thus, the
smaller the broadcast domain, the smaller effect a broadcast storm will have. Because VLANs are
implemented at each switch port, they can be quite effective in limiting the scope of broadcast storms.
Eliminating Broadcast Storms
SNMP agents can be programmed to monitor the number of broadcast packets on switch ports and
act on the data. When the number of broadcast packets on a given port rises past an assigned
threshold, an action can be triggered. When enabled, the rate of broadcast packets coming in through
the affected port will be limited. Any traffic above the threshold limit will be discarded. The Switch also
supports multicast storm control.
In the Switch, the default trigger threshold is set to 128,000 broadcast packets per second (128 Kbps)
for both 100 Mbps Fast Ethernet ports and the 1000 Mbps GBIC ports.
Multicasting
Multicasting enables a single network source to send packets to multiple interested recipients with
persistent connections. An interested recipient is defined as a host that has requested beforehand to
be associated with a multicast group. This distinguishes it from traditional unicast (one-to-one) and
broadcast (one-to-all) methods of delivery. The main advantage to multicasting is that when it is
correctly configured it can decrease network load for communications that would otherwise use
broadcasting.
Typically multicasting is implemented for specific applications and functions such as video and other
multimedia streaming across campus or extended networks, distribution of operating system images to
staff workstations using tools like Ghost, and certain VoIP features such as conference calling. When
properly implemented, the Switch can support multicasting applications by forwarding multicast traffic
only to participating hosts and only for the amount of time required for the application.
For multicasting to function successfully on a network, it is necessary to have the participating systems
set up with the proper configuration. At a minimum, the following requirements should be met:
•
The application running on the multicasting source device must determine the multicast
address (address/port combination) on which to send multicast data packets.
•
The application on the receiving hosts must listen for the required multicast address (or
multiple addresses).
Switch Management
DES-7000/DES-7100 Layer 2 Switch User's Guide
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