nning tree sends out frames called bridge protocol data units (BPDUs) every two seconds. The format of the BPDU frame is shown in Figure 8-4. Protocol ID Version Message type Flags Root priority 12123456 Root ID 123456 Root MAC address Path cost Bridge priority 12123456 Bridge ID 123456 Bridge MAC address Port priority Port ID Message age Max age Hello timer Fwd delay Figure 8-4. BPDU format These frames contain the information necessary for switches in the network to perform the following functions: Elect a root bridge When a switch boots, it assumes that it is the root bridge, and sets the root ID to the local bridge ID in all outgoing BPDUs. If it receives a BPDU that has a lower root ID, the switch considers the switch identified by the root ID in the BPDU to be the root switch. The local switch then begins using that root ID in the BPDUs it sends. Every bridge has a bridge ID. The bridge ID is a combination of the bridge prior- ity and the bridge's MAC address. The bridge priority is a configurable two-byte field with a default value of 32,768. The lower the bridge ID value is, the more likely it is that the bridge will become the root bridge (the bridge with the lowest bridge ID becomes the root bridge). Chapter 8: Spanning Tree The root ID is similarly composed of two fields: the root priority and the root MAC address. The root priority is also configured with a value of 32768 (0x8000) by default. Should there be a tie between root priorities, the lower root MAC address is used to break the tie. Determine the best path to the root bridge If BPDUs from the root bridge are received on more than one port, there is more than one path to the root bridge. The best path is considered to be via the port on which the BPDU with the lowest path cost was received. Path costs are determined by adding each bridge's port priority to the initial path cost as BPDUs are forwarded from bridge to bridge. Determine the root port on each bridge The root port is the port on the switch that has the shortest path to the root bridg