AIX > Administrator > Networks

Network Troubleshooting


Recently, a user opened a problem ticket reporting that copying files back and forth from a server we support was taking an unusually long time. The files weren't all that large, but the throughput was just terrible. After poking around a bit, we found that the Ethernet card wasn't set to the correct speed. When we ran lsattr --El ent0, we found the media_speed set to Auto_Negotiation. I knew what the problem was immediately.


We've seen the Auto_Negotiation setting on Ethernet adapters to be problematic on AIX. Our fast Ethernet port on the switch was always set to be 100/Full. With Auto_Negotiation on, sometimes the card would correctly set itself to 100/Full, but at other times it would go to 100/Half. This causes the slowdown on the network because you now have collisions on the network, which you can see with netstat --v.

Packets with Transmit collisions:


 1 collisions: 204076      6 collisions: 37         11 collisions: 1

 2 collisions: 65375       7 collisions: 6          12 collisions: 0

 3 collisions: 16894       8 collisions: 2          13 collisions: 0

 4 collisions: 2404        9 collisions: 0          14 collisions: 0

 5 collisions: 255        10 collisions: 2          15 collisions: 0


You can also determine if you're having Receive Errors and see what speed your adapter is running at by using netstat --v.  You'll see something similar to the following:


RJ45 Port Link Status : up

Media Speed Selected: Auto negotiation

Media Speed Running: 100 Mbps Full Duplex


Transmit Statistics:                      Receive Statistics:

--------------------                          -------------------

Packets: 33608151                      Packets: 82280769

Bytes: 3364953629                      Bytes: 89992126877

Interrupts: 15105                          Interrupts: 79762362

Transmit Errors: 0                         Receive Errors: 14000

Packets Dropped: 1                      Packets Dropped: 14

                                                     Bad Packets: 0


How did we fix the duplex issue? We detached the interface and ran a chdev to make it 100/Full: chdev  -l 'ent0' -a media_speed='100_Full_Duplex'. Once we made this change, there were no more collisions and the user was a happy camper.



Rob McNelly is a Senior AIX Solutions Architect for Meridian IT Inc. and a technical editor for IBM Systems Magazine. He is a former administrator for IBM. Rob can be reached at

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