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| 3.2 |  | STP Processes |
| 3.2.8 |  | BPDU format |

The format of a Spanning-Tree Protocol BPDU frame varies depending on the type of protocol used. Figure shows the format of an IEEE 802.1D Spanning-Tree Protocol BPDU frame. Figure shows the format of a Cisco Spanning-Tree Protocol BPDU frame.

The Frame Control field is always 01.

- The Destination field indicates the destination address as specified in the Bridge Group Address table. For IEEE Spanning-Tree Protocol BPDU frames, the address is 0x800143000000.
- The Source Address field indicates the base MAC address used by the switch. For Cisco Spanning-Tree Protocol BPDU frames, the multicast bit is set to indicate the presence of a Routing Information Field (RIF) in the header.
- The Routing Information field is only applicable to Cisco Spanning-Tree Protocol BPDU frames, the Routing Information field must be set to 0x0200.
- The Logical Link Control field controls all types of Spanning-Tree Protocol BPDU frames, this field is set to 0x424203.

Figure shows the format of the fields inside a BPDU. All fields in the BPDU are common to all Spanning-Tree Protocols except for the Port ID field. For IEEE and Cisco Spanning-Tree Protocol BPDU frames, the Port ID field specifies the transmitting port number of the originating bridge.

The Protocol Identifier, Version, and Message fields are all set to zero.

The Flags field includes one of the following:

- A Topology change (TC) bit, which signals a topology change, and signifies this BPDU as a Topology Change Notification (TCN) BPDU. Without this bit set, the BPDUs are Configuration BPDUs.
- A Topology change acknowledgment (TCA) bit, which is set to acknowledge receipt of a configuration message with the TC bit set.

The remaining fields contained in a BPDU frame are as follows:

- The Root ID field indicates the root bridge by listing its 2-byte priority followed by its 6-byte ID.
- The Root Path Cost field indicates the cost of the path from the bridge sending the configuration message to the root bridge.
- The Bridge ID field indicates the priority and ID of the bridge sending the message.
- The Port ID field indicates the port number (IEEE or Cisco Spanning-Tree Protocol BPDU) from which the configuration message was sent. This field allows loops created by multiple attached bridges to be detected and corrected.
- The Message Age field indicates the amount of time that has elapsed since the root sent the configuration message on which the current configuration message is based.
- The Maximum Age field indicates when the current configuration message should be deleted.
- The Hello Time field indicates the time between root bridge configuration messages.

The Forward Delay field indicates the length of time that bridges should wait before transitioning to a new state after a topology change. If a bridge transitions too soon, it is possible that not all network links will be ready to change their state and loops can result.