Fast Ethernet switches: Autosensing can be catastrophic
 
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In an earlier newsletter, we discussed problems that can arise when devices fail to autonegotiate the highest speed or duplex setting common to both devices. For instance, two devices that support full-duplex 100M bit/sec Ethernet could erroneously select half duplex or 10M bit/sec instead. This would mean that devices would not take advantage of the highest speed they both supported.
Unfortunately, the result with duplex can be even worse. Specifically, devices may fail to complete the autonegotiation process properly and configure themselves for different settings; for instance, one device is set to full duplex and the other is set to half. In this condition, of course, the full-duplex device can send data constantly; consequently, the half-duplex device, sensing a carrier on the wire, is unable to transmit.
During preliminary testing at The Tolly Group, one leading Fast Ethernet switch suffered considerably when set to autonegotiate duplex mode. When the switch autonegotiated ports with its neighbors, the negotiation resulted in one device selecting half duplex and the other selecting full duplex. (In fact, the switch reported that the negotiation had succeeded at full duplex.)
As a result of the collision problem described above, FTP server download performance hovered around 15M to 20M bit/sec of aggregate network utilization. However, after engineers manually reconfigured the switch and the attached devices to full-duplex Fast Ethernet, server download performance approached 100M bit/sec.
Needless to say, hard-coding every switch and endstation for speed and duplex can add significantly to network administration and management and is, therefore, inappropriate for many customers.
In the end, customers considering using any autonegotiation in their Fast Ethernet switches should be prepared to devote considerable resources to determine just how well the network will perform with autonegotiation enabled; even a switch that delivers wire-speed performance with autonegotiation disabled may still deliver significantly lower throughput with autonegotiation enabled. That leaves users with the unfortunate choice between poor network performance and additional network management overhead.
RELATED LINKS
Auto-negotiation. The newsletter referred to above. Describes the need for care to verify that switches with this feature properly negotiate the right setting.
Network World High Speed LANs, 3/2/98.
An Introduction to Auto-Negotiation
Switching's dark side: Why packet collisions can wreak havoc with LAN and ATM switch performance,
Network World, 2/10/97.
