666 Configuring Port-Based Traffic ControlFor information about Priority Flow Control (PFC), which provides a way todistinguish which traffic on a physical link is paused when congestion occursbased on the priority of the traffic, see "Configuring Data Center BridgingFeatures" on page 799What is Flow Control?IEEE 802.3 Annex 31B flow control allows nodes that transmit at slowerspeeds to communicate with higher speed switches by requesting that thehigher speed switch refrain from sending packets. Transmissions aretemporarily halted to prevent buffer overflows. Enabling the flow controlfeature allows PowerConnect 8000-series and 8100-series switches to processpause frames received from connected devices. PowerConnect switches donot transmit pause frames.Flow control is supported only on ports that are configured for full-duplexmode of operation. Since ports set to auto negotiate may not be added asLAG members, LAG member ports cannot have flow control configured toauto.What is Storm Control?A LAN storm is the result of an excessive number of broadcast, multicast, orunknown unicast messages simultaneously transmitted across a network by asingle port. Forwarded message responses can overload network resources andcause network congestion.The storm control feature allows the switch to measure the incomingbroadcast, multicast, and/or unknown unicast packet rate per port and discardpackets when the rate exceeds the defined threshold. Storm control is enabledper interface, by defining the packet type and the rate at which the packetsare transmitted. For each type of traffic (broadcast, multicast, or unknownunicast) you can configure a threshold level, which is expressed as apercentage of the total available bandwidth on the port. If the ingress rate ofthat type of packet is greater than the configured threshold level the portdrops the excess traffic until the ingress rate for the packet type falls belowthe threshold.The actual rate of ingress traffic required to activate storm-control is based onthe size of incoming packets and the hard-coded average packet size of 512bytes - used to calculate a packet-per-second (pps) rate - as the forwarding-plane requires PPS versus an absolute rate Kbps. For example, if the
