Một quá trình bầu cử trong số tất cả các thiết bị chuyển mạch được kết nối lựa chọn Root Bridge. Chuyển đổi từng có một ID cầu duy nhất mà xác định nó để chuyển mạch khác. ID Bridge là một giá trị 8-byte bao gồm các lĩnh vực sau: | IEEE Overview 219 An election process among all connected switches chooses the Root Bridge. Each switch has a unique Bridge ID that identifies it to other switches. The Bridge ID is an 8-byte value consisting of the following fields Bridge Priority 2 bytes The priority or weight of a switch in relation to all other switches. The priority field can have a value of 0 to 65 535 and defaults to 32 768 or 0x8000 on every Catalyst switch. MAC Address 6 bytes The MAC address used by a switch can come from the Supervisor module the backplane or a pool of 1024 addresses that are assigned to every Supervisor or backplane depending on the switch model. In any event this address is hardcoded and unique and the user cannot change it. When a switch first powers up it has a narrow view of its surroundings and assumes that it is the Root Bridge itself. This notion will probably change as other switches check in and enter the election process. The election process then proceeds as follows Every switch begins by sending out BPDUs with a Root Bridge ID equal to its own Bridge ID and a Sender Bridge ID of its own Bridge ID. The Sender Bridge ID simply tells other switches who is the actual sender of the BPDU message. After a Root Bridge is decided upon configuration BPDUs are only sent by the Root Bridge. All other bridges must forward or relay the BPDUs adding their own Sender Bridge IDs to the message. Received BPDU messages are analyzed to see if a better Root Bridge is being announced. A Root Bridge is considered better if the Root Bridge ID value is lower than another. Again think of the Root Bridge ID as being broken up into Bridge Priority and MAC address fields. If two Bridge Priority values are equal the lower MAC address makes the Bridge ID better. When a switch hears of a better Root Bridge it replaces its own Root Bridge ID with the Root Bridge ID announced in the BPDU. The switch is then required to recommend or advertise the new Root Bridge ID in its own BPDU .
