Is there any formula to calculate the number of ports that will go into blocking state by looking at the topology.

[chandra_rc16]

Is there any formula to calculate the number of ports that will go into blocking state by looking at the topology.

Regards,

Chandu

[Lei Tian · Accepted solution]

Hi,

Assume you are talking about spanning tree. If you have all the switch and link cost information, yes, you can tell the ports will be in blocking state by looking the topology. The formula is SPT path selection process.

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_configuration_example09186a008009467c.shtml

HTH,

Lei Tian

[Joseph W. Doherty]

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All multiple paths in the same L2 domain, but one should be blocked.  However, the later versions of STP are often per VLAN so on trunk ports, blockage will be per VLAN on that port not the port itself.  (Note: Similar for MST.)