SWITCH 300-115 1.6 Configure and verify spanning tree

1.6.b Switch priority, port priority, path cost, STP timers

STP summary
1.  all bridge (switch ports) stabilize at forwarding or blocking.  Forwarding ports are considered part of the spanning tree.
2.  one switch is elected root, and its ports will all move to forwarding state.
3.  each switch receives hellos from the root, directly or through another switch. The port that receives the least cost BPDU is placed in forwarding and becomes that switch’s root port
4.  for each segment one switch forwards the BPDU with the lowest cost.  That switch becomes that segment’s designated bridge.
5.  the other interfaces are placed in blocking
6.  the root sends BPDU’s every 2 seconds. This time interval can be modified and will be noted in the BPDU.
7.  if max-age elapses (20 seconds, default) and no BPDU, panic ensues and the spanning tree changes.
8.  forward delay (default 15 seconds) is the time it takes for a port to transition through the dumbass states to forwarding. (listening, 15 seconds, learning 15 seconds)
9.  when a switch goes into listening, it sends a TCN BPDU (topology change notification) through the new path to the root. Other switches will refresh their tables with the new entry.

10. spanning tree creates these delays to prevent transitional loops