DELL N3000 User Manual

Configuring Port-Based Traffic Control

The Priority Flow Control (PFC) feature, which is available on the N4000 

switches only, provides a way to distinguish which traffic on a physical link is 

paused when congestion occurs based on the priority of the traffic. For more 

IEEE 802.3 Annex 31B flow control allows nodes that transmit at slower 

speeds to communicate with higher speed switches by requesting that the 

higher speed switch refrain from sending packets. Transmissions are 

temporarily halted to prevent buffer overflows. Enabling the flow control 

feature allows Dell Networking series switches to process pause frames 

received from connected devices. Dell Networking switches do not transmit 

pause frames.
Flow control is supported only on ports that are configured for full-duplex 

mode of operation.

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