C H A P T E R 4 | Intel Express 10/100 Fast Ethernet Switch |
A port’s Flow LED flashes whenever a received packet needs to be forwarded to a port that already has too many packets queued. This indicates a temporary overload situation on one port; the total traffic to the port exceeds the amount its buffer can hold. This usually occurs when there are several fast devices on different ports trying to access a device across the switch (for example, 100 Mbps workstations accessing a 10 Mbps server). If this occurs rarely, don’t do anything. However, if it occurs often, identify the devices causing flow control and move them to the same segment as the device they’re talking to.
When Ethernet bandwidth is temporarily insufficient for the traffic, three actions are possible: drop packets, use flow control, or segment the network. Buffering packets only works for a very short while. An extended overload will eventually overflow buffers and cause dropped packets. Flow control stops transmission on a port and forces devices to resend packets, ensuring that packets aren’t lost. This is the most reasonable solution, since it relies on Ethernet’s inherent collision detection mechanism to relieve temporary overload.
Additionally, because it relies on collision detection, full duplex isn’t possible when flow control is enabled. The exception is between two Express 10/100 switches. A special piece of information added to packets traveling between switches accomplishes this.
74