Dell PowerConnect 7048P - What Is Flow Control; What Is Storm Control

688 Configuring Port-Based Traffic Control
What is Flow Control?
IEEE 802.3x flow control allows nodes that transmit at slower speeds to
communicate with higher speed switches by requesting that the higher speed
switch refrains from sending packets. Transmissions are temporarily halted to
prevent buffer overflows. Enabling the flow control feature allows
PowerConnect 7000 Series switches to receive pause frames from connected
devices.
Flow control is available for ports that are configured for full-duplex mode of
operation. Since ports set to auto negotiate may not be added as LAG
members, LAG member ports cannot have flow control configured to auto.
Flow control is incompatible with head of line (HOL) blocking prevention
mode. The switch can operate in either mode, but not at the same time. If
flow control is enabled, HOL blocking is disabled.
What is Storm Control?
A LAN storm is the result of an excessive number of broadcast, multicast, or
unknown unicast messages simultaneously transmitted across a network by a
single port. Forwarded message responses can overload network resources and
cause network congestion.
The storm control feature allows the switch to measure the incoming
broadcast, multicast, and/or unknown unicast packet rate per port and discard
packets when the rate exceeds the defined threshold. Storm control is enabled
per interface, by defining the packet type and the rate at which the packets
are transmitted. For each type of traffic (broadcast, multicast, or unknown
unicast) you can configure a threshold level, which is expressed as a
percentage of the total available bandwidth on the port. If the ingress rate of
that type of packet is greater than the configured threshold level the port
drops the excess traffic until the ingress rate for the packet type falls below
the threshold.
The actual rate of ingress traffic required to activate storm-control is based on
the size of incoming packets and the hard-coded average packet size of 512
bytes - used to calculate a packet-per-second (pps) rate - as the forwarding-
plane requires PPS versus an absolute rate Kbps. For example, if the
configured limit is 10%, this is converted to ~25000 PPS, and this PPS limit
is set in the hardware. You get the approximate desired output when 512 bytes
packets are used.