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3750 Error disabled ports from loopback??

Hi all,

Was are seeing certain FastEthernet ports on our 3750 switches going into error disabled mode. According to Cisco Network Assistant the cause of the error disable is "Loopback". We have spanning tree portfast enabled on all FastEthernet ports as all links are direct to PCs. We have left the POE port settings at their default of automatcic although we are not using any POE devise yet. Could this be the problem? Has anyone else seen FastEthernet ports on the 3750 range going into error disabled mode for no obvious reason?

Thanks,

Paul

Reply to
thefunnel
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Paul,

What's your cabling like? Every time I've seen this, it's been a cabling issue.

Steve

Reply to
StivH

Thanks Steve,

We are using CAT5 cabling. This was never a problem with the old 3548 switches we just removed.

Paul

Reply to
thefunnel

Paul,

3548s don't support loopback detection, whereas by default it's enabled on 3750s. Try "no errdisable detect loopback" from global config mode on the 3750.

Cheers Steve

Reply to
StivH

Many thanks for that Steve. One last question though... what does it actually mean by loopback?

Reply to
thefunnel

It means that the switch has detected that it can "see" itself over the cable. I run into this at one of my sites where they use IBM Type I cabling. With the Type I plug, if it's not plugged into something, it loops back the Tx pair onto the Rx pair (it's a 2-pair cable). So, if you plug a RJ45 - Type I lead into your switch, but there's nothing on the end of the Type I plug, the switch errdisables the port.

Cheers Steve

Reply to
StivH

Thanks Steve,

I have now set the detection status for loopbacks to disabled using the command you suggested. Is this setting to be confused with spanning tree? Obviously I still want loops to be detected and blocked on the uplink Gigabit ports using STP.

Thanks again,

Paul

Reply to
thefunnel

Hi Paul,

What do you have connected to that port? Is it PC or another switch/hub?

If it is another switch, make sure this port is only way to this switch. It's the purpose of Spanningtree protocol - detect loops in the network and disable one link to break the loop.

If you have PC, then you may have a duplicate MAC addresses in your network. It's one of the ways how switch detects loops - if it sees one MAC address on two ports, it thinks that it's possible loop.

Mike

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CiscoHeadsetAdapter.com

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