In a 3750 stack configuration, what is a "stack port" and what does it mean if it's flapping?

Unanswered Question
May 5th, 2010

Hi all,

In one of our LAN closets we have four 3750s connected in a stack configuration.  Initially the stack was only three switches, then we added a fourth switch and the stack has been acting a bit funny ever since.  When I checked the logs this morning I see the following message:


May  5 08:00:06.247: %STACKMGR-4-STACK_LINK_CHANGE: Stack Port 2 Switch 4 has changed to state UP
May  5 08:00:07.253: %STACKMGR-4-STACK_LINK_CHANGE: Stack Port 2 Switch 4 has changed to state DOWN
May  5 08:00:07.773: %STACKMGR-4-STACK_LINK_CHANGE: Stack Port 1 Switch 3 has changed to state DOWN
May  5 08:00:08.780: %STACKMGR-4-STACK_LINK_CHANGE: Stack Port 1 Switch 3 has changed to state UP

This is just a sample output, there were actually many more messages.  They all referenced Switches 3 and 4, so I suspect it's something to do with that problematic fourth switch that we added.

Clearly something is going up and down, but what is a Stack Port?  Everything else on the switch looks okay - traffic appears to be doing okay, all the interfaces look okay, etc.

Any info is appreciated.  Thanks.

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francisco_1 Wed, 05/05/2010 - 06:01

Direct from Cisco site

Problem: StackWise Port is Flapping

These conditions might cause StackWise port flapping:

An unreliable or intermittent (bad) connection between a StackWise cable connector and the switch stack port connector

A defective StackWise cable

A defective StackWise interface on a switch

When one or more StackWise ports are flapping, the source of the problem is probably an unreliable connection between a StackWise cable and a StackWise switch port.

If you suspect StackWise port flapping, first verify that the retainer screws are adequately tightened and fully seated against the connector shell. Make sure both retainer screws are tightened the same amount. In some cases you need to disconnect and reconnect the cable while you are troubleshooting, but do not test the connection unless the retainer screws are firmly holding the connector.

An unreliable StackWise connection might produce a similar error message:

#

STACKMGR-4-STACK_LINK_CHANGE: Stack Port 1 Switch 1 has changed to state UP

STACKMGR-4-STACK_LINK_CHANGE: Stack Port 1 Switch 1 has changed to state DOWN

STACKMGR-4-STACK_LINK_CHANGE: Stack Port 1 Switch 1 has changed to state UP

STACKMGR-4-STACK_LINK_CHANGE: Stack Port 1 Switch 1 has changed to state DOWN

STACKMGR-4-STACK_LINK_CHANGE: Stack Port 1 Switch 1 has changed to state UP

STACKMGR-4-STACK_LINK_CHANGE: Stack Port 1 Switch 1 has changed to state DOWN

-- or --

#

STACKMGR-4-STACK_LINK_CHANGE: Stack Port 1 Switch 4 has changed to state UP

STACKMGR-4-STACK_LINK_CHANGE: Stack Port 2 Switch 5 has changed to state UP

STACKMGR-4-STACK_LINK_CHANGE: Stack Port 1 Switch 4 has changed to state DOWN

STACKMGR-4-STACK_LINK_CHANGE: Stack Port 2 Switch 5 has changed to state DOWN

STACKMGR-4-STACK_LINK_CHANGE: Stack Port 1 Switch 4 has changed to state UP

STACKMGR-4-STACK_LINK_CHANGE: Stack Port 1 Switch 4 has changed to state DOWN

SUPQ-4-CPUHB_RECV_STARVE: Still seeing receive queue stuck after throttling 

STACKMGR-4-STACK_LINK_CHANGE: Stack Port 2 Switch 5 has changed to state UP

STACKMGR-4-STACK_LINK_CHANGE: Stack Port 2 Switch 5 has changed to state DOWN

An unreliable cable connection caused both of these patterns. Neither the StackWise cable nor the StackWise port was defective. A bad connection between the StackWise cable and the stack connector on the switch caused the problem.

A problem at either end of a StackWise link might cause port flapping at the other end of the link

See http://www.ciscosystems.com/en/US/docs/switches/lan/catalyst3750/software/troubleshooting/switch_stacks.html

pweinhold Wed, 05/05/2010 - 07:08

Thanks for the reply, I guess that makes sense.

So here's the follow-up - we changed the stack cable to see if it would fix the issue.  It did not.  Here's the output showing the bad ports:

Switch_stack#show switch stack-ports
  Switch #    Port 1       Port 2
  --------    ------       ------
    1           Ok           Ok
    2           Ok           Ok
    3          Down          Ok
    4           Ok          Down

Also, when we swapped out the stack cable, the following message appeared the log:

May  5 13:28:33.649: %STACKMGR-5-MAJOR_VERSION_MISMATCH: Major Version Mismatch (Local 1 - Received 4) with neighbor-0

Does anyone recognize that message?  We've confirmed that all four switches are running the same IOS.

I should point out that one of these switches is a 3750-12S, while the rest are 3750-24FS.  I know there are potentially some issues when a 3750-12S is stacked with different model 3750s.

Thanks again.

francisco_1 Wed, 05/05/2010 - 07:52

so all switches using same IOS?

Another reason for the mismatch could be SDM Mismatch mode.  The 3750-12S support both desktop & aggregator SDM while the 3750 switch support only desktop. If the 3750-12S is the master, make sure it setup to use desktop template.

Can you post  "show switch" & "show sdm prefer" to see what the current state is.

francisco_1 Wed, 05/05/2010 - 07:57

also can you check if the "archive-copy-sw" or "archive download-sw" is part of running config!

glen.grant Wed, 05/05/2010 - 10:35

  That is specifically showing that you do not have a connection between switch 3  stackport 1  and switch 4 stackport 2 .   So the cable is either bad , its been replaced or the stackport on one end or the other is bad .  Sometimes the cables can look like they are all the way but they are not  , you can gently use a screwdriver to tighten both ends , do not overtighten and stripp the threads. Can you post a sh version  to for us to verify the versions on all switches.  You may have initially got that message before the stack upgraded the switch you inserted.

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