Duplex/Interface error on Cisco 3750

[petterjoh]

The problem is that the switchports goes into Half Duplex mode even though they are forced to full duplex and the other side runs full duplex

This problems occurs on random ports and against random devices, here is and example:

Config:

interface GigabitEthernet1/0/1
switchport access vlan 101
switchport mode access
service-policy input qos-tag
duplex full
speed 100
no mdix auto

interface GigabitEthernet1/0/2
switchport access vlan 101
switchport mode access
service-policy input qos-tag
duplex full
speed 100
no mdix auto

interface GigabitEthernet1/0/3
switchport access vlan 101
switchport mode access
service-policy input qos-tag
duplex full
speed 100
no mdix auto

Name : Gi1/0/1
Administrative Speed: 100
Administrative Duplex: full
Administrative Auto-MDIX: off
Administrative Power Inline: N/A
Operational Speed: 100
Operational Duplex: half
Operational Auto-MDIX: off

Name : Gi1/0/2
Administrative Speed: 100
Administrative Duplex: full
Administrative Auto-MDIX: off
Administrative Power Inline: N/A
Operational Speed: 100
Operational Duplex: half
Operational Auto-MDIX: off

Name : Gi1/0/3
Administrative Speed: 100
Administrative Duplex: full
Administrative Auto-MDIX: off
Administrative Power Inline: N/A
Operational Speed: 100
Operational Duplex: half
Operational Auto-MDIX: off

If i go into int sub config mode and shutdown the port and then bring it up again operational Duplex goes over to full.

This error is reproducable by powering off the switch and then powering it on again.

The ports will stay stable until a port is reset, then its random what happenes

Anyone have any idea of what it could be ?
im really stuck here..

Checked all cisco bug databases and so on..

Cisco IOS Software, C3750 Software (C3750-IPBASE-M), Version 12.2(25)SEB4, RELEASE SOFTWARE (fc1)
Copyright (c) 1986-2005 by Cisco Systems, Inc.
Compiled Tue 30-Aug-05 15:47 by yenanh

 

[ADB100]

I would start with upgrading the IOS on the 3750 to either 12.2(25)SEE2 or the most recent 12.2(35)SE2 and then re-test. If you still get the same issue raise a TAC case.

HTH

Andy

 

[jneiberger]

This is expected behavior depending on the specific devices you're connecting. The short answer to this is that you should *always* use AUTO. Do not hard-set speed and duplex. I'd be willing to be that if you set them to auto, all of them will go to 100/Full and stay there.

 

[petterjoh]

Hey, thx for your input.
Regards to the last post,
I have had alot of problems running AUTO towards equipment that is always in the network, like other cisco switches (and other brands esp.) and servers.
Therefore i always hardset the duplex towards servers and other networkdevices on both sides.

This has been working very well through many years except this one case.

Im going to try and upgrade the firmware of the switch, but since its critical switch it will be hard to take it out of the network, going to try and get a hold of a similar switch, downgrade it to this firmware and see if i can reproduce the errpr

 

[jneiberger]

It is always better to set both sides of a Fast Ethernet connection to auto. Autonegotiation is the only method mentioned in the Fast Ethernet standard. There is nothing in the specification that says how a device should behave when manually configured. This creates a problem because there are two possible ways to behave:

1. Turn off autonegotiation entirely and use the manually configured settings

2. Use the manually configured settings but still "advertise" them via autonegotiation

All of Cisco's switches made in the last several years use behavior #1. Older switches, like the XL line, use behavior #2.

The problem happens when you connect a NIC using #2 to a switch using #1. If they are both manually set, the NIC still expects an autonegotiating link partner. Since the switch is not participating in negotiation any longer, the NIC assumes that it is attached to a hub and falls back to half duplex. That is the correct behavior according to specification.

If you want a connection to run at 100/Full, the safest and most reliable way to do that is to leave both sides at auto. If you manually set them, you will have problems when you use NICs that use behavior #1.

I used to work at a fairly big company with a large deployment of newer Cisco switches. We used to be in the "Auto is evil" camp, but then I ended up spending a great deal of time troubleshooting and fixing duplex mismatches. For a couple of years, not a week went by where I didn't fix a duplex mismatch somewhere in the network that was caused by manually configured settings.