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| 3.1 |  | Spanning-Tree Protocol Operation |
| 3.1.4 |  | Bridge priority |

An algorithm is a formula or set of steps for solving a particular problem. Algorithms rely on a set of rules. They have a clear beginning and end. The spanning-tree algorithm is no exception.

The spanning-tree algorithm is defined in the IEEE 802.1D standard. The parameters used by the algorithm, including the Bridge ID, are explored here. The remaining parameters, Path Cost and Port ID, will be covered in the following two topics.

The spanning-tree algorithm characterizes STP. The spanning-tree Algorithm relies on a set of parameters to make decisions. The Bridge ID (BID) is the first parameter used by the spanning-tree algorithm. The Bridge ID (BID) is used by STP to determine the center of the bridged network, known as the Root Bridge. The Bridge ID (BID) parameter is an 8-byte field consisting of an ordered pair of numbers. The first is a 2-byte decimal number called the Bridge Priority, and the second is a 6-byte (hexadecimal) MAC address. The Bridge Priority is a decimal number used to measure the preference of a bridge in the spanning-tree Algorithm. The possible values range between 0 and 65,535. The default setting is 32,768.

The MAC address in the BID is one of the MAC addresses of the switch. Each switch has a pool of MAC addresses, one for each instance of STP, used as BIDs for the VLAN spanning-tree instances (one per VLAN). For example, Catalyst 6000 switches each have a pool of 1024 MAC addresses assigned to the supervisor module or backplane for this purpose.

Let (s,t) and (u,v) represent two BIDs, where the first coordinate is the Bridge Priority and the second coordinate is the MAC address, (Priority, MAC address). Then (s,t)<(u,v) if and only if:

- s<u or
- s=u and t<v

Two BIDs cannot be equal, because Catalyst switches are assigned unique MAC addresses. In terms of the spanning-tree algorithm, when a comparison is made between two values of a given STP parameter, the lower value is always preferred.