Blocking Vs Forwarding

hz36t5

Trying to figure out what path a switch will take when it has multiple patch to a neighbor? Which switch ports will forward? Which will block?
Example: 10 vs 100 vs 1000

kplab

The switches run the Spanning Tree Protocol (STP) to calculate a loop-free topology for the network. STP ensures that there is only one active path between any two network segments by blocking the redundant paths.

Details about STP can be found in the following papers:

Fundamentals of Switching (by KPLab):
http://www.kp-lab.com/free_study_guide_download.htm

Configuring STP (by Cisco):
http://www.cisco.com/en/US/products/hw/switches/ps628/products_configuration_guide_chapter09186a00802c3019.html

In summary, only the following ports are put in the forwarding state. All other ports are put in the blocking state:
1. Ports of the root bridge (the bridge with the lowest Bridge ID).
2. The root port (the port that is the closest to the root bridge) of each non-root bridge.
3. The designated port of each LAN segment. For a LAN segment, The bridge with the lowest cost to the root bridge is elected as the designated bridge. The port of the designated bridge connected to the LAN segment is the designated port.

darkuser

and my favorite ////

http://www.cisco.com/univercd/cc/td/doc/product/lan/c3550/12113ea1/3550scg/swstp.htm