2.1.f Implement and troubleshoot spanning-tree

2.1.f [ii] Switch priority, port priority, path cost, STP timers

All Bridges (Switches) are assigned a numerical value called bridge priority. The Bridge (Switch) priority value is used to find the Bridge (Switch) ID.

The Switch ID is made from two values:

The Switch Priority, which is a numerical value defined by IEEE 802.1D, which is equal to 32,768 by default.

The MAC Address of the Switch. If all the Switches in your Local Area Network (LAN) are configured with the default Switch Priority (32,768), the Switch MAC address will become the decisive factor in electing the Root Bridge (Switch). The Bridge (Switch) with the lowest MAC Address is then elected as Root Bridge (Switch).

If you want one particular switch to be the Root Bridge (Switch), change the priority to a lower value than 32,768.

If you want to affect how to the local switch elects the root port change the cost on the links. Cost is cumulative throughout the STP domain. The higher cost is the less preferred. If you want to affect how downstream switch elects its root port change the priority. This is only local significance between the two directly connected switches. Highest priority is less preferred. Going away from the root of the tree use priority whereas, when going towards the root of the tree use cost.

There are several STP timers, as this list shows:

● hello—The hello time is the time between each bridge protocol data unit (BPDU) that is sent on a port. This time is equal to 2 seconds (sec) by default, but you can tune the time to be between 1 and 10 sec.

● forward delay—The forward delay is the time that is spent in the listening and learning state. This time is equal to 15 sec by default, but you can tune the time to be between 4 and 30 sec.

● max age—The max age timer controls the maximum length of time that passes before a bridge port saves its configuration BPDU information. This time is 20 sec by default, but you can tune the time to be between 6 and 40 sec. Among all these parameters, the only ones which you can tune are:

● hello

● max age

● forward delay

● diameter

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 1794-1800).  . Kindle Edition.

http://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/19120-122.html