The spanning tree algorithm depends on the exchange of BPDUs to determine a root bridge. A BPDU frame contains 12 distinct fields that convey path and priority information used to determine the root bridge and paths to the root bridge.
Click the BPDU fields in Figure 1 to see more detail.
- The first four fields identify the protocol, version, message type, and status flags.
- The next four fields are used to identify the root bridge and the cost of the path to the root bridge.
- The last four fields are all timer fields that determine how frequently BPDU messages are sent and how long the information received through the BPDU process (next topic) is retained.
Figure 2 shows a BPDU frame that was captured using Wireshark. In the example, the BPDU frame contains more fields than previously described. The BPDU message is encapsulated in an Ethernet frame when it is transmitted across the network. The 802.3 header indicates the source and destination addresses of the BPDU frame. This frame has a destination MAC address of 01:80:C2:00:00:00, which is a multicast address for the spanning tree group. When a frame is addressed with this MAC address, each switch that is configured for spanning tree accepts and reads the information from the frame; all other devices on the network disregard the frame.
In the example, the root ID and the BID are the same in the captured BPDU frame. This indicates that the frame was captured from a root bridge. The timers are all set to the default values.