STP - BPDU generation and port roles


When deploying [Spanning Tree Protocol (STP)](/spanning-tree-protocol-stp) of any type, switches will exchange Bridge Protocol Data Units ([BPDUs](/stp-bpdu-types)) of various types to allow STP to function correctly.  Beyond simply [reading the contents of a BPDU](/stp-contents-of-a-bpdu), a switch will recognize that a particular is a BPDU by the information found within the [Ethernet header](/ethernet-header).

Take a look at this Wireshark capture of a BPDU frame.

![stp-BPDU-wireshark-capture.png](/attachments/stp-BPDU-wireshark-capture.png)Note that this BPDU is encapsulated within a Logical Link Control (LLC) layer PDU, which contains the information in the Destination Service Access Point (DSAP) field as a **Spanning Tree BPDU**, with a code of **0x42**. This tells the switch that the encapsulated information is an STP BPDU.

But even that LLC layer is encapsulated within an [IEEE](/ieee) 802.3 Ethernet frame, which in turn uses a destination [MAC](/switching-media-access-control-mac-addresses) address of **01:80:c2:00:00:00**. This is a well-known destination MAC address defined by IEEE 802.3D as the Spanning Tree for Bridges destination address.

All of the above information tells the switch that this is a BPDU, and it is interpreted as such.

Links

https://forum.networklessons.com/t/introduction-to-spanning-tree/853/180?u=lagapides

STP how a switch identifies a BPDU

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 - Topology Change Acknowledgement (TCA)

STP - UDLD and Fiber Links

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

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 Max Age

STP Message Age

STP PortFast

STP Portfast Options

STP Root Bridge selection

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 contents of a BPDU

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

STP topology change process

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

Serial Interface - Clock Rate

Serial Interface default broadcast address

Session Border Controller

Session Initiation Protocol

Show open TCP-UDP ports on Cisco router

Spanning Tree Protocol (STP)

STP how a switch identifies a BPDU

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