General User Release Notes
3.22 Duplex-Mode Mismatch Errors
3.22 Duplex-Mode Mismatch Errors
V8.3
A duplex-mode mismatch condition occurs when a LAN device is operating in full-
duplex mode and the other end of the cable, typically a switch port, is operating
in half-duplex mode. The reverse is also true. A common network configuration
error that results in a duplex mode mismatch condition occurs when the switch
port is set to autonegotiate the speed and duplex settings, and the LAN device is
set to a fixed setting of full duplex. In this configuration, autonegotiation by the
switch results in the selection of half-duplex mode and the LAN device is set to
full-duplex mode, and a duplex-mode mismatch occurs.
The consequence of a duplex-mode mismatch is typically a performance
degradation. In addition, the IEEE 802.3 specification that describes the
autonegotiation process suggests that a duplex-mode mismatch can result in
data corruption. For most LAN devices, the only consequence of a duplex mode
mismatch is the performance degradation. For some LAN devices, packet data is
corrupted with good CRC, resulting in packet corruption undetected by the LAN
subsystem. These devices include all Broadcom-based NICs and embedded LOM
chips. On Alpha systems, these include the DEGPA, DEGXA, BCM5703 LOM on
the AlphaServer DS25, and any implementations using the dual-port BCM5704
chip. On Integrity systems, these include the A6847A, A6725A, A9782A, A9784A,
AB465A, and BCM5701 LOM on the rx2600; BCM5703 LOM on other systems;
and the A6794A.
In prior versions of OpenVMS, the LAN drivers attempt to detect the duplex-
mode mismatch condition. Once an hour while the condition exists, they issue a
console message and error log message warning of the condition.
In OpenVMS Version 8.3, the frequency of the messages is increased from once
per hour to once every 36 seconds for any Broadcom-based LAN devices. The
frequency remains at once per hour for non-Broadcom-based LAN devices. In
addition, to increase the visibility of these messages, the console messages are
sent to OPCOM and to the LANACP log file (SYS$MANAGER:LAN$ACP.LOG).
The purpose of this note is to underscore the importance of avoiding duplex-
mode mismatches, particularly when this condition results in undetected data
corruption for Broadcom-based devices.
Note that the LAN drivers detect a duplex mode mismatch condition by
monitoring device errors. The detection is not perfect, so the LAN drivers
refer to the condition as a "potential duplex-mode mismatch." Upon noticing these
messages, a system or network manager should inspect the LAN counters and
LAN device settings to ensure a duplex-mode mismatch condition does not exist.
3–22 General User Release Notes
