3750G: connect only to 10/100 devices

[alessandro.corbelli]

Hi,

i have a strange issue with a 3750G.

some ports will go up only if connected to 10/100 devices.

Conntecting those ports to a gigabit device will result in no led activity and L1 down.

cables are ok because the same cable connected to the same remote device on a different port will go up.

any idea?

[glen.grant]

Look at the port configs , you can restrict the autonegotiate to 10/100 if you want. Verify  there are no restrictions on the ports, it would say something like speed auto 10/100 .

[Latchum Naidu]

Hi,

Please set the interfaces config like...

duplex auto
speed auto

If still facing the same issue, please post your interfaces config.

Please rate the helpful posts.
Regards,
Naidu.

[alessandro.corbelli]

How can I ensure that there isn't any speed auto 10/100? I'm able to force autoneg off , but there isn't any autoneg off and speed is set to auto.

By the way, is possibile to set speed auto but only at 10/100?

[Latchum Naidu]

Please post your interface config. If it set speed and duplex auto then you can see like below under your interface cofnig...

Check the below link...

http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps5528/prod_bulletin0900aecd80122f85.html

speed auto

duplex auto

Please rate the helpful posts.

Regards,

Naidu.

[Reza Sharifi]

By default the speed is auto and since it is auto, it will do 10/100/1000 not noly 10/100.

are you sure the device you are connecting to this ports can support Gig?

if yes, what is the different between the ports that are working at 10/100/1000 and the once that are 10/100?

[alessandro.corbelli]

This doesn't work:

interface GigabitEthernet1/0/17

switchport trunk encapsulation dot1q

switchport trunk allowed vlan 501

switchport mode trunk

no cdp enable

channel-protocol lacp

channel-group 7 mode active

end

This work:

interface GigabitEthernet1/0/13

switchport trunk encapsulation dot1q

switchport trunk allowed vlan 501

switchport mode trunk

no cdp enable

channel-protocol lacp

channel-group 7 mode active

end

It's the same configuration, on two different ports on the same switch.

The other end works, because that device (with the same cables) connected to port 1/0/13 will go up.

[alessandro.corbelli]

by forcing speed 100 i'm able to connect at 100mbit with gigabit device.

So I think that there is something wrong with gigabit capability of this port.

[IcebergTitanic]

Can you confirm that it is in fact a port problem?

Swap the connection that isn't working with a port that has a working gigabit connection, and see if the problem follows the device, or if it stays on the port.

If it follows the device, it's most likely a driver or firmware issue on the end device. Gigabit autonegotation is iffy on a lot of the earlier implementations of gigabit ethernet.

[alessandro.corbelli]

The problem stays on the port, as wrote above.

I think that there is a short in the port, because a "cable-test tdm" show me all "open" pairs except one that is marked as "normal", even with no cable attach.

All working ports, with no cable attached, show me "open" on every pairs.

[Leo Laohoo]

I've seen this before.  It's most likely caused by a faulty cabling.

Since you have a 3750G then you can test all four pairs of the TDR feature.  Here's how you do it:

1.  Command:  test cable tdr interface ;

2.  Wait for about 5 to 7 seconds;

3.  Command:  test cable tdr interface ; and

4.  Post the complete output.

What you should be looking for:

1.  For GigabitEthernet to work, you need Pairs A, B and D to be "Normal".  If you get either one to be "Open" then it means you have an OPEN circuit.  Another analogy:  OPEN means that a "pin" or copper cabling is not "touching" anything.

2.  PoE is using Pair C.

3.  If you get something like "impedimence mismatch" then it means that you've got a very, very bad cabling (not the termination).

Hope this helps.

[alessandro.corbelli]

Are wrote many times before, this isn't a cabling issue. I've changed ALL CABLES and ALL DEVICES and the issue is still here. I've also tried the TDR feature with no cable plugged in. I have all pairs OPEN and one pair in NORMAL.

I don't this this is good, with no cable attached, all pairs should be in OPEN state. So, i think that there is a short circuit inside the switch port

[Leo Laohoo]

First time I've seen this before.  What IOS are you running?