Very odd problem with Spanning Tree

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Nov 8th, 2007
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Hi! I've got a problem with a 3Com switch.

When I connect it to another switch in my LAN, STP blocks the port I've connected and my switch starts receiving about 100 multicasts per second. If I connect the same port to another switch out of my LAN the port keeps in Forwarding state. Without STP everything works right.

I'm using RSTP with all the default values and I have no idea about why this happens.

All opinions are welcome.

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mchin345 Wed, 11/14/2007 - 17:53
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Verify that the trunking mode and encapsulation are configured correctly. Make certain that the native VLAN is the same on all of the 802.1q trunks that connect the Cisco switches to the non-Cisco 802.1q cloud.

If multiple Cisco switches are connected to a non-Cisco 802.1q cloud, all of the connections must be through 802.1q trunks. Do not connect Cisco switches to a non-Cisco 802.1q cloud through ISL trunks or through access ports. If this is done, it causes the switch to place the ISL trunk port or access port into the spanning tree port inconsistent state and no traffic passes through the port.


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