Set duplex on linux network card

Statically/manually define/set duplex on linux network card
Use mii-tool or ethtool


A Note About Duplex Settings

By default, Linux NICs negotiate their speed and duplex settings
with the switch. This is done by exchanging electronic signals
called Fast Link Pulses (FLP). When the speed and duplex are forced
to a particular setting the FLPs are not sent. When a NIC is in
auto-negotiation mode and detects a healthy, viable link but receives
no FLPs, it errs on the side of caution and sets its duplex to
half-duplex and sometimes it will also set its speed to the lowest
configurable value. It is therefore possible to force a switch port to
100 Mbps full duplex, but have the auto-negotiating server NIC set
itself to 100Mbps half-duplex which will result in errors. The same is
true for the switch if the switch port is set to auto-negotiate and
server NIC is set to 100 Mbps full duplex. It is best to either force
both the switch port and server NIC to either auto-negotiate or
the same forced speed and duplex values.


//////////////////////////////// mii-tool


[root@bigboy tmp]# mii-tool
SIOCGMIIPHY on ‘eth0’ failed: Operation not supported
eth1: 100 Mbit, half duplex, link ok
[root@bigboy tmp]#

[root@bigboy tmp]# mii-tool -v
eth1: negotiated 100baseTx-FD, link ok
product info: vendor 00:10:18, model 33 rev 2
basic mode:   autonegotiation enabled
basic status: autonegotiation complete, link ok
capabilities: 100baseTx-FD 100baseTx-HD 10baseT-FD 10baseT-HD
advertising:  100baseTx-FD 100baseTx-HD 10baseT-FD 10baseT-HD
link partner: 100baseTx-FD 100baseTx-HD 10baseT-FD 10baseT-HD flow-control
[root@bigboy tmp]#

mii-tool -F 100baseTx-FD eth0

//////////////////////////////// Ethtool


[root@bigboy tmp]# ethtool eth0
Settings for eth0:
Supported ports: [ TP MII ]
Supported link modes:   10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
Supports auto-negotiation: Yes
Advertised link modes:  10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
Advertised auto-negotiation: No
Speed: 100Mb/s
Duplex: Full
Port: MII
Transceiver: internal
Auto-negotiation: off
Supports Wake-on: g
Wake-on: g
Current message level: 0x00000007 (7)
Link detected: yes
[root@bigboy tmp]#

# File: /etc/sysconfig/network-scripts/ifcfg-eth0
ETHTOOL_OPTS=”speed 100 duplex full autoneg off”

////////////////////// or
ethtool -s eth1 speed 100 duplex full autoneg off