Linux LAN card: Find out full duplex / half speed or mode
Q. How do I find out if my Lan (NIC) card working at full or halt duplex mode / speed under Linux?
A. LAN card or NIC is use to send and receive data. Technically, we use word Duplex for this functionality. Full duplex means you are able to send and receive data (files) simultaneously. In half duplex, you can either send or receive data at a time (i.e. you cannot send receive data (files) simultaneously). Obviously, full duplex gives you best user experience. However, how can I find out whether I am using full duplex/half duplex speed/mode?
Task: Find full or half duplex speed
You can use dmesg command to find out your duplex mode:
# dmesg | grep -i duplex
Output:
eth0: link up, 100Mbps, full-duplex, lpa 0x45E1
ethtool command
Uss ethtool to display or change ethernet card settings. To display duplex speed, enter:
# ethtool eth1
Output:
Settings for eth1:
Supported ports: [ TP ]
Supported link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Full
Supports auto-negotiation: Yes
Advertised link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Full
Advertised auto-negotiation: Yes
Speed: 10Mb/s
Duplex: Full
Port: Twisted Pair
PHYAD: 0
Transceiver: internal
Auto-negotiation: on
Supports Wake-on: umbg
Wake-on: g
Current message level: 0x00000007 (7)
Link detected: yes
mii-tool command
You can also use mii-tool to find out your duplex mode. Type following command at shell prompt:
# mii-tool
Output:
eth0: negotiated 100baseTx-FD flow-control, link ok
Remember,
- 100baseTx-FD: 100Mbps full duplex (FD)
- 100baseTx-HD: 100Mbps half duplex (HD)
- 10baseT-FD: 10Mbps full duplex (FD)
- 10baseT-HD: 10Mbps half duplex (HD)
mii-tool utility checks or sets the status of a network interface’s Media Independent Interface (MII) unit. Most fast ethernet adapters use an MII to autonegotiate link speed and duplex setting. If you are using old card then this utility may not work (use dmesg command).
This utility is useful for forcing specific Ethernet speed and duplex settings too, setup 100Mbps full duplex speed under Linux:
# mii-tool -F 100baseTx-FD
Setup 10Mbps half duplex:
# mii-tool -F 10baseT-HD
You can find more information about setting duplex speed here using ethtool command.
Updated for accuracy!
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- FreeBSD Set Speed Duplex For My Network Card ( NIC )
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- Determine / Find ethernet connection speed
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Discussion on This FAQ
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Tags: 100baset, 100basetx, 10baset, auto negotiation, dmesg_command, duplex mode, duplex output, ethtool_command, full duplex, half duplex, link modes, link speed, mii-tool_command, mode type, simultaneously, speed mode, utility checks
September 22nd, 2006 at 8:57 am
Very informative. Need some Real Time Datacenter , Linux Info..
July 17th, 2007 at 6:06 pm
Thank you for that information, especially the name of that tool. Very useful. Again, Thanks.
October 25th, 2007 at 10:39 pm
ethtool, mii-tool and dmesg output show very different results
first off mii-tool -v eth0
Output
eth0: negotiated 10baseT-HD, link ok
product info: vendor 00:50:ef, model 21 rev 1
basic mode: autonegotiation enabled
basic status: autonegotiation complete, link ok
capabilities:
advertising: 100baseTx-HD 10baseT-HD
link partner: 100baseTx-FD 10baseT-HD
2)
ethtool eth0
Settings for eth0:
Supported ports: [ FIBRE ]
Supported link modes: 1000baseT/Full
Supports auto-negotiation: Yes
Advertised link modes: 1000baseT/Full
Advertised auto-negotiation: Yes
Speed: 1000Mb/s
Duplex: Full
Port: FIBRE
PHYAD: 2
Transceiver: internal
Auto-negotiation: on
Supports Wake-on: d
Wake-on: d
Link detected: yes
3) dmesg | grep -i duplex
bnx2: eth0 NIC Link is Up, 1000 Mbps full duplex
bnx2: eth1 NIC Link is Up, 1000 Mbps full duplex
Which one should be believed? mii-tool shows the nic is advertising 100baseT/HD but negotiated 10BaseT/HD (which is really slow for what we want to do); ethtool shows nic is advertising 1000Mb/s FD which, confirms with dmesg output.
Conclusion? mii-tool may not be compatible with the fibre channel nics?
How to interpret this output?
October 26th, 2007 at 6:37 am
Only use ethtool as mii-tool is not compatible and it will be not included in future version