Sometimes 1000mbps and sometimes only 100mbps?

FireRaven · 10-11-2010, 03:37 PM

This morning I knocked around some cables near the gigabit switch and one of the servers turned orange instead of the usual green on the switch.
I SSH'd into that server and saw under /var/log/syslog this message:

Code:

Oct 12 07:07:01 nas kernel: [3507982.947307] atl1 0000:03:00.0: eth1 link is down
Oct 12 07:07:03 nas kernel: [3507985.468783] atl1 0000:03:00.0: eth1 link is up 1000 Mbps full duplex
Oct 12 07:07:16 nas kernel: [3507998.342862] atl1 0000:03:00.0: eth1 link is down
Oct 12 07:07:27 nas kernel: [3508009.412212] atl1 0000:03:00.0: eth1 link is up 100 Mbps full duplex
Oct 12 07:16:00 nas postfix/qmgr[1220]: 5E325D342: from=<>, size=8131, nrcpt=1 (queue active)

So bumping around cables had this machine take it's eth1 down then up again but only at 100mbps.

Does anyone know why this might have happened?
I am positive I did not unplug the switch at all.

I think it's a 5m Cat5e or Cat6 link (too hard to determine which one) about 1 year old. What determines if the server brings up as 100mbps or 1000mbps?

jefro · 10-11-2010, 03:46 PM

Quality of service.

eSelix · 10-11-2010, 04:19 PM

If your link has changed after touching cables it is likely that the cables (and connectors) are poor quality. For 1Gb they are more "sensitive" for imperfection. Try replacing them.

Quote:

What determines if the server brings up as 100mbps or 1000mbps?

It is automatic by default, but you can force it:

Code:

ethtool -s eth1 autoneg off speed 1000

and check current settings by

Code:

ethtool eth1

FireRaven · 10-11-2010, 04:39 PM

Quote:

Originally Posted by eSelix

If your link has changed after touching cables it is likely that the cables (and connectors) are poor quality. For 1Gb they are more "sensitive" for imperfection. Try replacing them.

Thanks I will do this.
Is there a way to force linux to connect at 1000mbps though?
I take it somehow know when to decide to connect to 100 vs 1000 as in the logs...

marauderx · 10-11-2010, 10:39 PM

Yes with miitools.

FireRaven · 10-11-2010, 10:58 PM

Quote:

Originally Posted by marauderx

Yes with miitools.

Thanks will try that if it happens again. I fixed it by turning the server completely off at the wall (to disable the light at the switch) then restart it.

Just doing soft reboot didn't do it, had to be hard reboot.

Code:

#mii-tool --force=1000baseT-FD

H_TeXMeX_H · 10-12-2010, 04:51 AM

I agree with eSelix, the cable may be low quality or damaged, I would get a new one if it keeps happening.

onebuck · 10-12-2010, 08:32 AM

Hi,

One more thought: You could have a poor connector at the switch.

FireRaven · 10-12-2010, 03:49 PM

Quote:

Originally Posted by H_TeXMeX_H

I agree with eSelix, the cable may be low quality or damaged, I would get a new one if it keeps happening.

It is just cat5e cable. I might try upgrading it to cat6. I believe this should make a difference for long distances such as this.

Electro · 10-14-2010, 03:00 AM

CAT-5e is rated for 1000 megabit. Also it should be able to handle 5 meters or about 16 feet. If it can not and you made the cable, you did not do it correctly. Make sure the jacket of the cable is in the connector and make sure the teeth is making a connection. Also make sure the connector is designed for the cable you are using. If you are using stranded cable, use a connector designed for stranded. If the cable uses solid, use the connector designed for solid.

The network will always go to the lowest denominator, so check your network to find any 100 megabit devices. If there are 100 megabit devices, then your connection will be 100 megabit and it does not matter what category of Ethernet cable that you use.

michaelk · 10-14-2010, 07:01 AM

FYI 1000Base-T uses all 4 wire pairs but 100Base-T only uses 2. As stated if the cable/connector is marginal then the link will fall back to 100. Auto negotiation is strictly a hardware function but you can force it as previously stated.

In addition use ethtool, mii-tool is obsolete and does not support 1000Base-T network adapters.