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Spanning Tree loop after shut down of Destination monitor port


I have to core 3550 switches (12.1(22)EA1a) doing layer2/layer3 duties with HSRP and STP running for ~30 VLANs. Down stream from them are 3 access layer swithes (cisco 3548s) acting at layer 2 only each with a link to both "core1" and "core2". On the core switches we have a monitor session set up to send traffic from one port to another for monitoring various traffic, etc.

monitor session 1 source interface Gi0/7

monitor session 1 destination interface Gi0/11

Last week, after we were done with our monitor session, we decided to shut the destination port down (otherwise our SNMP polling picks it up as a down interface and alarms). Upon shutting down, connectivity was lost to the switch, and upon further troubleshooting (logged in via console), clear signs of a STP loop were seen (logs showed HSRP interface constantly flapping, HSRP couldn't see its neighbors, CDP couldn't see their neighbors, huge amounts of traffic on the links to between switches).

My recourse at the time was to try to kill the loop by shutting down the redundant links to the access layer. Once I did that, the switch freaked out and reloaded itself with this in the logs:

May 31 19:45:20.473: %PLATFORM_CAT3550-1-CRASHED: System previously crashed with the following message:

May 31 19:45:20.473: %PLATFORM_CAT3550-1-CRASHED: Cisco Internetwork Operating System Software

May 31 19:45:20.473: %PLATFORM_CAT3550-1-CRASHED: IOS (tm) C3550 Software (C3550-I5Q3L2-M), Version 12.1(22)EA1a, RELEASE SOFTWARE (fc1)

May 31 19:45:20.473: %PLATFORM_CAT3550-1-CRASHED: Copyright (c) 1986-2004 by cisco Systems, Inc.

May 31 19:45:20.473: %PLATFORM_CAT3550-1-CRASHED: Compiled Fri 20-Aug-04 00:44 by yenanh

May 31 19:45:20.473: %PLATFORM_CAT3550-1-CRASHED:

May 31 19:45:20.477: %PLATFORM_CAT3550-1-CRASHED: Unassigned Exception (0xD0D0D0D)!

May 31 19:45:20.477: %PLATFORM_CAT3550-1-CRASHED: CPU Register Context:

May 31 19:45:20.477: %PLATFORM_CAT3550-1-CRASHED: Vector = 0x0D0D0D0D PC = 0x008299A8 MSR = 0x4000B030 CR = 0x44000044


Does anyone know of a bug that would cause this behavior. I have replicated it once again with the same behavior (monitor port gets shut down, STP loop strarts, I attempt to shut redundant link to access layer, switch crashes).

I appreciate any info you may have.

j. flaherty

New Member

Re: Spanning Tree loop after shut down of Destination monitor po

BTW, as far as I can tell accordiing to the Cisco docs, this should not have happened:


When SPAN is enabled, configuration changes have these results:


If you disable all source ports or the destination port, the SPAN function stops until both a source and the destination port are enabled.


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