User`s guide

Typical Configurations 2-7

Excessive Flow Control

During times of peak network usage, you may occasionally see the flow control

indicator (Fc) flash. This is normal. However, either of the following conditions could

indicate a problem with your network configuration: if the indicator stays lit for more

than a few seconds at a time, or there is an excessive number of flow controls reported

by the NMS. A port’s flow control indicator flashes whenever a packet is received that

needs to be forwarded to a port that already has too many packets queued for it. This

indicates a temporary over-bandwidth situation on one port; that is, the total traffic

attempting to be forwarded to the port was in excess of 100 Mb/s, and the switch’s

buffers were full. This typically occurs when there are several fast machines on

different ports trying to access a machine across the switch. If this occurrence was due

to an unusual event, then no further action is necessary. If this is part of the normal

usage pattern for the network, then the station(s) causing the flow control to activate

should be identified and moved to the same segment as the machine it is

communicating with.

When a situation arises where Ethernet bandwidth is insufficient for the traffic, there

are only two possible actions: drop packets or use flow control. Buffering packets

works for a time period, but an extended over-bandwidth situation will eventually

overflow buffers and cause dropped packets. Flow control is an alternative solution,

since it relies on Ethernet’s inherent collision detection mechanism to relieve

temporary over-bandwidth situations (half-duplex mode only).