STP contents of a BPDU


In the Rapid Spanning Tree Protocol (RSTP) version of [STP](/spanning-tree-protocol-stp), the [flag fields](/stp-contents-of-a-bpdu) found within the [BPDUs](/stp-bpdu-types) have specific roles. Unlike the conventional 802.1D BPDUs, which use only bits 0 and 7 of the 1-byte flags field (with bits 2 to 6 unused), RSTP redefines these bits. In 802.1D, bit 0 is the [Topology Change (TC)](/stp-topology-change-tc-flag) flag, and bit 7 is the [Topology Change Acknowledgment (TCA)](/stp-topology-change-acknowledgement-tca) flag. However, in RSTP, bit 0 is used for TCA, and bit 7 is used for TC.

The TCA flag in RSTP acknowledges the receipt of a BPDU with the TC flag set. This mechanism helps prevent unnecessary flooding of BPDUs across the network. Although the TCA flag might not frequently be set to 1 when using RSTP, it still plays a crucial role in maintaining network stability by acknowledging topology changes.

## Links

https://networklessons.com/spanning-tree/rapid-spanning-tree-rstp

STP - RSTP BPDU Flags and their usage

SD-WAN - vEdge

SD-WAN - vManage

SD-WAN - vSmart

SD-WAN OMP VPN Route Status Codes

SD-WAN Template Attach Error on vManage

SD-WAN set TLOC color

SD-WAN

SDN - what is orchestration

SG300-10 - InterVLAN routing defaults

SNMP - Engine ID

SNMP - Index shuffling

SNMP - Simple Network Management Protocol

SNMP - Version 3 security levels

SNMP - snmp-server community and host commands

SNMP monitoring routing on a Nexus device

SNMP trap vs inform

SNMP walk

SPAN capturing control packets

SPAN multiple destination ports on Cisco IOS device

SPAN using VLANs as a source on a Nexus device

SPAN

SSH - domain-name prerequisite

SSH connectivity - troubleshooting

SSM Multicast Group Verification

STP - Aging time reduction with a TCN

STP - Aging time return to default

STP - BPDU Filter Use Cases

STP - BPDU generation and port roles

STP - BPDUGuard, BPDUFilter, and Portfast

STP - BPDUs on a switch where STP is disabled

STP - BPDUs, VLAN IDs, and PVST+

STP - Cost of last link added by receiving switch

STP - Disabling spanning-tree

STP - Dispute

STP - Factors to Consider When Choosing a Root Bridge

STP - Learning State

STP - MST CIST Root

STP - MST CST and CIST

STP - MST Virtual Bridge and its role in interfacing with other STP topologies

STP - MST and PVST+ selecting root port

STP - MST maps groups of VLANs

STP - MST region attributes

STP - MST viewed as a single switch by PVST+

STP - Max Age vs Message Age

STP - Multiple Spanning Tree (MST)

STP - Multiple types of STP on the same switch

STP - PVST+ and Rapid PVST+ interaction

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 - 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 Best Practice

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 cost long mode

STP cost short mode

STP determining blocked port using cost

STP - RSTP BPDU Flags and their usage

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