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24-port NWay Ethernet Switch User’s Guide
50 Switch Management Concepts
than routers. Also, many switches, including the DES-
3225G, 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 in the same VLAN.
Thus, broadcast packets will only be forwarded 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 rise
past an assigned threshold, an action can be triggered.
When enabled, the usual action is to block the port to
broadcast frames, which discards all broadcast frames
arriving at the port from the attached segment. Not only
does this isolate the broadcast domain, but it actually
starts removing broadcast packets from the affected
segment. When the number of broadcast packets falls to an
acceptable level (below a falling threshold), the SNMP
agent can remove the blocking condition, returning the
port to its normal operational state.
In the DES-3225G, the default rising threshold is set to 500
broadcast packets per second (pps), and the default falling