C H A P T E R 1 Hardware Installation and Network Topology
NOTE
By default, flow control is enabled on all ports.
If you configure a port for full duplex, flow control is auto- matically disabled.
Excessive flow control problems
During times of peak network usage, you may occasionally see the FC/FD LED blink (for a description of the LED, see page 6). This is normal. However, if it stays lit for more than a few seconds at a time, or if there’s an excessive number of flow controls reported by the network management software, it could indicate a problem with your network configuration.
A port’s FC/FD LED indicator flashes whenever a packet is received that 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 the buffer can hold. This typically occurs when there are several fast machines on different ports trying to access a machine across the switch. If this is infrequent, you don’t need to do anything. However, if this occurs often on the network, then the devices causing flow control to activate should be identified and moved to the same segment as the device that they are talking to.
When Ethernet bandwidth is temporarily insufficient for the traffic, there are only three possible actions: 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.
Repeater count limitations
The switch doesn’t count as a repeater. Each port on the switch can support a full Fast Ethernet network. There can be one repeater/hub between the switch and any workstations or servers (a stack of Intel Express
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