| A | B |
|---|
| bridge protocol data unit (BPDU) | A network packet that is used to identify a hierarchy and notify of changes in the topology. |
| configuration BPDU | The BPDU that is responsible for switches electing a root bridge and communicating the root path cost so that a hierarchy can be built. |
| hello time | The time interval for which a BPDU is advertised out of a port. |
| designated port (DP) | A network port that receives and forwards BPDUs to other downstream switches. |
| forward delay | The amount of time that a port stays in a listening and learning state. |
| local bridge identifier | A combination of the advertising switch’s bridge system MAC, the system ID extension, and the system priority of the local bridge. |
| max age | The timer that controls the maximum length of time that passes before a bridge port saves its BPDU information. |
| root bridge | the topmost switch in an STP topology. The root bridge is responsible for controlling STP timers and creating configuration BDPUs. All ports on a root bridge are designated ports that are in a forwarding state. |
| root bridge identifier | A combination of the root bridge system MAC address, system ID extension, and system priority of the root bridge. |
| root path cost | The cost for a specific path toward the root switch. |
| root port | The most preferred switch port that connects a switch to the root bridge. Often this is the switch port with the lowest root path cost. |
| system priority | A 4-bit value that indicates the preference for a switch to be root bridge. |
| system ID extension | A 12-bit value that indicates the VLAN that the BPDU correlates to. |
| topology change notification (TCN) | A BPDU that is advertised toward the root bridge to notify the root of a topology change on a downstream switch. |
