SPAN capturing control packets

STP - RSTP BPDU Flags and their usage

STP - Topology Change Acknowledgement (TCA)

STP - Understanding MST regional root identifier and proposal-agreement process

STP Max Age

STP Message Age

STP Root Bridge selection

STP how a switch identifies a BPDU

STP topology change process


A [Bridge Protocol Data Unit (BPDU)](/stp-bpdu-types) has a number of fields that contain information for [STP](/spanning-tree-protocol-stp) to operate correctly:

![bpdu_fields.excalidraw.png](/attachments/bpdu_fields.excalidraw.png)

| Field No. | Bytes | Field Description                                                                                                                                           |
| --------- | ----- | ----------------------------------------------------------------------------------------------------------------------------------------------------------- |
| 1         | 2     | Protocol ID - set to zero for [IEEE](/ieee) 802.1D                                                                                                               |
| 2         | 1     | Protocol version ID - set to 0 for a config and TCN BPDU, set to 2 for RST, 3 for MST, and 4 for SPT BPDUs                                                  |
| 3         | 1     | BPDU Type - set to 0 for config BPDU, 128 for TCN, 2 for RST/MST Config BPDU                                                                                |
| 4         | 1     | Flags - indicate various things such as topology changes and acknowledgments among others.  The format of the flags depends on the type of STP. (See below) |
| 5         | 8     | Root ID - indicates the root bridge by listing a 2-byte priority, followed by the MAC address ID                                                            |
| 6         | 4     | Root path cost - indicates the cost of the path from the bridge sending the BPDU to the root bridge                                                         |
| 7         | 8     | Bridge ID - indicates the priority and MAC address ID of the bridge sending the BPDU                                                                        |
| 8         | 2     | Port ID - indicates the port number from which the BPDU was sent                                                                                            |
| 9         | 2     | Message age - the amount of time in seconds since the root bridge sent the current configuration BPDU                                                       |
| 10        | 2     | Max age - indicates the age in seconds at which time the current BPDU should be discarded                                                                   |
| 11        | 2     | Hello time - the interval between BPDU messages                                                                                                             |
| 12        | 2     | Forward delay - the length of time in seconds that a bridge should wait before transitioning to a new state after a topology change                         |
Note that a version ID of 4 corresponds to a Shortest Path Tree (SPT) BPDU which is part of the development and implementation of [Shortest Path Bridging (SPB)](/stp-shortest-path-bridging-spb) as specified in the IEEE 802.1aq standard.

## Flags format

The format of the flags field depends upon the type of STP being run.

### IEEE 802.1D
![bpdu_802.1d_flags.png](/attachments/bpdu_802.1d_flags.png)

Where each of the above fields is one bit:
* TC - Topology Change flag
* U - Unused flag
* TCA - Topology Change Acknowledgement flag

### IEEE 802.1w

![bpdu_802.1w_flags.png](/attachments/bpdu_802.1w_flags.png)

Where each of the above fields is one bit except for Port Role:
* TC - Topology Change flag
* Proposal flag
* Port role (two bits):
	* 00 - Unknown
	* 01 - Alternate port
	* 10 - Root port
	* 11 - Designated port
* Learning flag
* Forwarding flag
* Agreement flag
* Topology Change Acknowledgement flag


For more information on the topology change process and how BPDUs are involved, take a look at:

* [STP - RSTP BPDU Flags and their usage](/stp-rstp-bpdu-flags-and-their-usage)
* [STP topology change process](/stp-topology-change-process)
* [STP Topology Change (TC) flag](/stp-topology-change-tc-flag)
* [STP - Topology Change Acknowledgement (TCA)](/stp-topology-change-acknowledgement-tca)

Links:

https://networklessons.com/cisco/ccie-routing-switching-written/spanning-tree-topology-change-notification-tcn

https://networklessons.com/cisco/ccie-routing-switching-written/spanning-tree-reconvergence

https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/24062-146.html#anc7

https://en.wikipedia.org/wiki/Spanning_Tree_Protocol#Bridge_Protocol_Data_Unit_fields

STP contents of a BPDU

STP - Per VLAN Spanning Tree plus (PVST+)

STP - RSTP EdgePort

STP - RSTP Proposal Timer Operation

STP - RSTP convergence process

STP - RSTP port roles

STP - RSTP prerequisites for immediate move to forward state

STP - RSTP proposal-agreement process

STP - RSTP while timer cannot be observed

STP - RSTP while timer

STP - Root Inconsistency State

STP - Root Link Query (RLQ) BPDU

STP - Shortest Path Bridging (SPB)

STP - Time to select root bridge

STP - UDLD and Fiber Links

STP - UplinkFast and reconvergence

STP - UplinkFast and root port delay timer

STP - UplinkFast

STP - bridge priority

STP - connecting one port to another port on the same switch

STP - determining the blocked port using port ID

STP - port identification

STP - port roles

STP - port states

STP - spanning-tree root primary command

STP - what bridge ID does the MST region use

STP - what is a topology change

STP BPDU Filter

STP BPDU Guard

STP BPDU Types

STP BPDU destination MAC address

STP Cost Calculation Methods

STP Loop Guard

STP PortFast

STP Portfast Options

STP Root Guard

STP Topology Change (TC) flag

STP Topology Change Timer

STP UDLD and Autonegotiation

STP UDLD

STP bridge priority values

STP changing link cost

STP configuring on EtherChannel

STP configuring portfast

STP cost long mode

STP cost short mode

STP determining blocked port using cost

STP direct vs indirect link failure

STP fine tuning the spanning tree protocol

STP how to find the root bridge

STP interface type

STP prio nbr value

Security - 802.1x with WLC and AAA server

Security - AAA for RESTCONF

Security - Best Practices for Repurposing Network Devices

Security - Cisco AnyConnect Secure Mobility Client

Security - Cisco Secure Client

Security - Cisco VPN Client

Security - Cisco VPN client software

Security - Diffie-Hellman groups

Security - Extensible Authentication Protocol over LAN (EAPOL)

Security - GETVPN

Security - IEEE 802.1X

Security - IOS Usernames and Passwords

Security - IPsec nonce

Security - Kerberos

Security - MAC Authentication Bypass

Security - Network Access Control (NAC)

Security - Next Generation Firewall

Security - Secure Unique Device Identifier (SUDI)

Security - WPA 4-way handshake

Security - WebVPN

Security - accessing VTY from another VRF

Security - assigning privilege levels, syntax and behavior

Security - bogons

Security - broadcast-multicast storm

Security - configuring TACACS from another VRF

Security - privilege levels and command output

Security - privilege levels and enable keyword

Security - public key exchange

Security - root CAs and private keys

Security - spoofing

Security - storm control algorithm

Security IKE policy hash command

Security Trusted Platform Module (TPM)

Security aaa new-model

Security authentication on VTY lines

Security keyrings

Segment Routing

Serial - Data Communication Equipment (DCE)

Serial - Data Terminal Equipment(DTE)

Serial - Link between two routers

Field No.	Bytes	Field Description
1	2	Protocol ID - set to zero for IEEE 802.1D
2	1	Protocol version ID - set to 0 for a config and TCN BPDU, set to 2 for RST, 3 for MST, and 4 for SPT BPDUs
3	1	BPDU Type - set to 0 for config BPDU, 128 for TCN, 2 for RST/MST Config BPDU
4	1	Flags - indicate various things such as topology changes and acknowledgments among others. The format of the flags depends on the type of STP. (See below)
5	8	Root ID - indicates the root bridge by listing a 2-byte priority, followed by the MAC address ID
6	4	Root path cost - indicates the cost of the path from the bridge sending the BPDU to the root bridge
7	8	Bridge ID - indicates the priority and MAC address ID of the bridge sending the BPDU
8	2	Port ID - indicates the port number from which the BPDU was sent
9	2	Message age - the amount of time in seconds since the root bridge sent the current configuration BPDU
10	2	Max age - indicates the age in seconds at which time the current BPDU should be discarded
11	2	Hello time - the interval between BPDU messages
12	2	Forward delay - the length of time in seconds that a bridge should wait before transitioning to a new state after a topology change
Note that a version ID of 4 corresponds to a Shortest Path Tree (SPT) BPDU which is part of the development and implementation of Shortest Path Bridging (SPB) as specified in the IEEE 802.1aq standard.

STP contents of a BPDU

Flags format

IEEE 802.1D

IEEE 802.1w

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