On a flapping vlan, interface does an up/down sequence with no cable connected.

Answered Question
Feb 10th, 2010

Hi Everyone out there.

I have a situation that I need someone to explain what could have happened.  I have a 2960 switch which was having a vlan flapping problem in one vlan - lets call it vlan 200.  I was preparing to connect some more switches into the same vlan using and etherchannel.  I created the etherchannel and placed the ports into the etherchannel and did a "no shut".  All this without connecting any cables, the switch logged an up/down sequence event.  The two switches are connected via a different port which is passing traffic with no issues.  Can any try and help explain to me what could have happened here?

Thanks in Advance

Sydney.

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Correct Answer by normanjeter about 6 years 10 months ago

I just recreated the same situation on my 2950 as you did on your 2960 for testing purposes. Here are my results:

I have currently 2 sessions open on the switch 1 console and 1 telnet ssh (this is not a live switch at this time so I use it for testing things regularly. no harm is actually being done lol). There are 2 cables curently hooked up to the switch, Console and 1 network plugged into fa0/1 in vlan1. I created 2 other vlans (2 and 3) and then creaed an eterchannel using fa 0/16 - 18. Everytime I issued a no shutdown command on one of the ports in the etherchannel I got an Up/down message on the console side. If I issued the commands on the console session I got them there as well. This is what I thought was going on. Everytime you issue a no shut command on a port it brings the status of said port to an active state (i.e. it powers it up and brings it online) whether anything is connected to it or not, causing the link up message, followed a second later with a link down message since it has no physical connection. This Should not have caused any convergance issues, and it shouldn't cause any routes to fail or affect stp at all as it is a normal operation of the switch. If you did lose paths in this process or cause a delay in any convergances then there may be other issues you need to investigate. But otherwise this is a normal message given by the switch when you bring any port online and active, cable or not.

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kandasydie Wed, 02/10/2010 - 13:47

Sorry thought I could add on something. The etherchannel had been configured to trunk this vlan - and the same vlan configured as the native vlan on the trunk.

Cheers.  Please help me out..

Sydney

Reza Sharifi Wed, 02/10/2010 - 14:06

Hello Sydney,

Maybe I do not understand the question, but If there is no cable connected to the port, it will not come up.

Here is test I did on my 2960 with no cable connected to port 0/48

Management-Switch(config)#interface port-channel 3
Management-Switch(config)#interface fastEthernet 0/42
Management-Switch(config-if)#channel-group 3 mode desirable
Management-Switch(config-if)#no sh
Management-Switch(config-if)#
1w0d: %LINK-3-UPDOWN: Interface FastEthernet0/42, changed state to down    
Management-Switch(config-if)#exit                          
Management-Switch(config)#interface port-channel 3      
Management-Switch(config-if)#no sh

Management-Switch(config-if)#do sh ip int brief fa0/42
Interface                  IP-Address      OK? Method Status                Protocol
FastEthernet0/42           unassigned      YES unset  down                  down   
Management-Switch(config-if)#do sh ip int brief po3  
Interface                  IP-Address      OK? Method Status                Protocol
Port-channel3              unassigned      YES unset  down                  down   
Management-Switch(config-if)#

HTH

Reza

kandasydie Wed, 02/10/2010 - 14:20

Hi Reza

Thanks for your reply, it is not supposed to happen.

But it really happened to me.  All configured (I was only preparing for a later change) but the moment I did a no shut after placing the interfaces into the etherchannel, they did that sequence and I am buffled.  I am looking for any information on what could have happened or what could have caused it.  Some people spoke about a fault in the hardware of the interface wrongly interpreting some voltage levels as a cable connected.  Once this was not sustainable they went back to normal.

I wish someone can help.

Sydney

Reza Sharifi Wed, 02/10/2010 - 14:57

Hi Sydney,

It the port down now?

Did you see below message, if yes that is normal

1w0d: %LINK-3-UPDOWN: Interface FastEthernet0/42, changed state to down   

Reza

kandasydie Wed, 02/10/2010 - 18:56

Yes it is down now.  Only problem is we are trying to establish what caused it to come up with no cable connected.  Because during the same time, looks like STP re-converged and we lost a few services in the affected vlan.

normanjeter Thu, 02/11/2010 - 09:44

So, what I am interpreting in simple terms from reading your post, is that you did all the configuring to get the etherchannel ready, but did not have any cables plugged into etherchannel, and then issued the no shutdown command on the etherchannel you just configured, and are confused why it gave a link up/link down message? Its not randomly coming up and down now, and its just remaining down as it would normally with no cable attached to a port?

kandasydie Thu, 02/11/2010 - 16:36

Hi Norman

This might help explain:  After creating an etherchannel and placed ports in it and performing a "no shut"  the switch produced a response as below WITHOUT any cables connected to the port:

21w3d: %LINK-3-UPDOWN: Interface GigabitEthernet0/14, changed state to down

21w3d: %LINK-3-UPDOWN: Interface GigabitEthernet0/14, changed state to up

21w3d: %LINK-3-UPDOWN: Interface GigabitEthernet0/14, changed state to down

First line is ok but not line 2 and 3.  It would appear like a cable was connected but it was not.  So what I am asking is how would that happen.  I have heard a few responses in other discussions ranging from jitter to power issues causing the software to think there is a cable on the port etc.  What are yo thoughts?

Cheers

Sydney

Correct Answer
normanjeter Thu, 02/11/2010 - 17:55

I just recreated the same situation on my 2950 as you did on your 2960 for testing purposes. Here are my results:

I have currently 2 sessions open on the switch 1 console and 1 telnet ssh (this is not a live switch at this time so I use it for testing things regularly. no harm is actually being done lol). There are 2 cables curently hooked up to the switch, Console and 1 network plugged into fa0/1 in vlan1. I created 2 other vlans (2 and 3) and then creaed an eterchannel using fa 0/16 - 18. Everytime I issued a no shutdown command on one of the ports in the etherchannel I got an Up/down message on the console side. If I issued the commands on the console session I got them there as well. This is what I thought was going on. Everytime you issue a no shut command on a port it brings the status of said port to an active state (i.e. it powers it up and brings it online) whether anything is connected to it or not, causing the link up message, followed a second later with a link down message since it has no physical connection. This Should not have caused any convergance issues, and it shouldn't cause any routes to fail or affect stp at all as it is a normal operation of the switch. If you did lose paths in this process or cause a delay in any convergances then there may be other issues you need to investigate. But otherwise this is a normal message given by the switch when you bring any port online and active, cable or not.

kandasydie Mon, 02/22/2010 - 19:31

Hi Norman

I think you are right.  Thanks for that.  The response came late, but I think it is because we already having some vlan flapping issues on that switch.

Fantastic - thanks again.

Sydney

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