BPDU Format

Packet format:
————–
-> Protocol ID:
* 2 bytes.
*Always 0. Future enhancements to the protocol might cause the Protocol ID values to increase.

-> Version:
* 1 byte.
* Always 0. Future enhancements to the protocol might cause the Protocol ID values to increase.

-> Message Type:
* 1 byte.
* Determines which of the two BPDU formats this frame contains (Configuration BPDUor TCN BPDU).
* • 0x00 (Binary: 0000 0000) Configuration BPDU
• 0x80 (Binary: 1000 0000) Topology Change Notification (TCN) BPDU

-> Flags:
* 1 byte.
*Used to handle changes in the active topology.

-> Root BID:
* 8 byte.
* Contains the Bridge ID of the Root Bridge.
* After convergence, all Configuration BPDUs in the bridged network should contain the same value for this field (for a single VLAN).

-> Root Path Cost:
* 4 bytes.
* The cumulative cost of all links leading to the Root Bridge.

-> Bridge ID (Sender BID);
* 8 bytes.
* The BID of the bridge that created the current BPDU.

-> Port ID:
* 2 bytes.
* Contains a unique value for every port. Port 1/1 contains the value 0×8001, whereas Port 1/2 contains 0×8002.

-> Message Age:
* 2 bytes.
* Records the time since the Root Bridge originally generated the information that the current BPDU is derived from.
* If a bridge looses connectivity to the Root Bridge (and hence, stops receiving BPDU refreshes), it needs to increment this counter in any BPDUs it sends to signify that the data is old.
* The root bridge sends all its BPDUs with a message age value of 0, and all subsequent switches add 1 to this value. Effectively, this value contains the information on how far you are from the root bridge when you receive a BPDU.

-> Max Age:
* 2 bytes.
* Maximum time that a BPDU is saved. Also influences the bridge table aging timer during the Topology Change Notification process.

-> Hello Time:
* 2 bytes.
* Time between periodic Configuration BPDUs.
* The Root Bridge sends a Configuration BPDU on every active port every Hello Time seconds. This causes the other bridges to propagate BPDUs throughout the bridged network

-> Forward Delay:
* 2 bytes.
* The time spent in the Listening and Learning states. Also influences timers during the Topology Change Notification process.