> For the complete documentation index, see [llms.txt](https://mudassirs46.gitbook.io/network-fundamentals/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://mudassirs46.gitbook.io/network-fundamentals/stp-messages-bpdu.md).

# STP Messages (BPDU)

[STP ](https://app.gitbook.com/@mudassirs46/s/network-fundamentals/~/drafts/-MRZ8l67L5MHnaQIEh9W/stp-spanning-tree-protocol)messages are called BPDU (Bridge Protocol Data Unit). BPDU contains BID (Bridge Priority + MAC Address)

* BID is 8 Byte in size. MAC Address is 6 byte, hence the remaining 2 bytes are Priority.

### Hello BPDU contains the following fields:

1. **RB ID**
   * Bridge ID of the Root Bridge
2. **Sender's BID**
   * BID of the switch sending this BPDU
3. **Sender's Root Cost**
   * STP cost between this Switch & the current Root
4. **Timer Values on the RB**
   * Hello Timer, Max Age Timer, Forward Delay Timer

## Reference:

* CCNA 200-301 OCG, Volume 1, Pg. 218- Wendell Odom.