Spanning Tree Issuse - I Think...

Unanswered Question
Aug 26th, 2009
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Hi All,

I hope you can help.

I have a Cisco 3750 switch stack connected to an Extreme Networks switch via 2 fast ethernet ports.

On both switches the speed and duplex have been set to 100/Full.

The Cisco 3750 switch is acting as the root bridge, and I see both ports are in 'FORWARDING' mode. The Extreme network switch has one port in FORWARDING and the other in BLOCKING, as I would expect. However when I run a constant ping to the Extreme Networks switch I lose approx 3/4 packets at 1 minute intervals. When I remove the 'BLOCKING' connection between the switches there is no packet loss.

Any help would be greatly appreciated!


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Peter Paluch Wed, 08/26/2009 - 01:36
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An interesting problem indeed. In the period when the packets are being lost, can you have a look into the MAC table of both switches and check if the source and destination MAC corresponding to the source and destination IP address do not incidentally refer to the link that is BLOCKING on one side?

Also, what kind of STP are you using?

Best regards,


AxiomConsulting Wed, 08/26/2009 - 02:32
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Hi Peter,

Thanks for your reply...It seems the plot thickens.

When the ping to the switch IP address is successful, the following entry is in the MAC address table

10 0004.961f.0418 DYNAMIC Gi1/0/25

However when the ping to the switch IP address is unsuccessful I get...

10 0004.961f.0418 DYNAMIC Gi2/0/25

Which would lead me to beleive that the FORWARDING port connection (at either end) is flapping.

However, I have a host on the Network Extreme switch and pinging to here is not an issue, regardless whether the switch IP address is pingable the following entry remains in the MAC address table...

10 000f.1ffd.b90f DYNAMIC Gi1/0/25

We are running in pvst mode.


Thanks for any help/suggestions you may have


Peter Paluch Wed, 08/26/2009 - 03:15
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This is strange indeed. Is it possible to run some debug on the Network Extreme switch to see if it changing its STP root port for whatever reason?

Also, would it be possible for you to sniff the traffic on the links between your 3750 stack and the Network Extreme switch and look for the MAC sender 0004.961f.0418? I would be most interested in seeing what kind of frame is it and why is it sent from a port that should be completely silent because of its STP blocking status.

Assuming that the MAC 0004.961f.0418 belongs to the Network Extreme switch, can you also crawl over all its interfaces and check whether any other interface has this MAC as well?

Best regards,



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