Spanning Tree Protocol Quiescent Period for Hitless Software Upgrades
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Solution Overview
Problem
Software upgrades in network devices using Spanning Tree Protocol (STP) cause disruption due to inconsistent bridge protocol data units (BPDUs) during control plane reboot, leading to recomputation of the spanning tree and network traffic disruption.
Innovation Solution
Implement a quiescent period during control plane software upgrades where the network device receives and processes BPDUs to learn port roles and states without transmitting BPDUs, ensuring consistency with the existing spanning tree topology.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the control plane reboots during software upgrade, then the network device can be upgraded to new software version, but inconsistent BPDUs are transmitted causing spanning tree recomputation and network traffic disruption
Solution Approach 1:
The patent applies preliminary action by implementing a quiescent period before the control plane fully reboots. During this period, the device prevents transmission of BPDUs and maintains port states in a non-forwarding condition, thereby preventing inconsistent BPDUs from being transmitted that would trigger spanning tree recomputation. This preparatory measure ensures that when the software upgrade completes, network traffic can resume without disruption.
2Measurement precision
If BPDUs are transmitted during control plane reboot, then the spanning tree protocol can detect topology changes, but it causes recomputation of the spanning tree and network disruption
Solution Approach 1:
The patent applies the taking out principle by extracting the BPDU transmission function during the quiescent period. The device selectively disables BPDU transmission only when the control plane is in a reboot state, while maintaining the ability to receive BPDUs and detect topology changes. This selective removal of the transmission function prevents harmful spanning tree recomputation while preserving necessary topology awareness capabilities.
Solution Approach 2:
The patent applies preliminary anti-action by implementing preventive measures before the harmful effect occurs. The quiescent period acts as a protective barrier that prevents inconsistent BPDUs from being transmitted during control plane reboot. By maintaining ports in a non-forwarding state and blocking BPDU transmission in advance, the system prevents the chain reaction that would lead to spanning tree recomputation and network disruption.
3Reliability
If ports are placed in non-forwarding state during quiescent period, then inconsistent BPDUs are prevented from causing spanning tree recomputation, but network traffic is blocked during the upgrade
Solution Approach 1:
The patent minimizes traffic interruption by implementing the quiescent period as a brief preliminary action rather than a prolonged state. The ports are placed in non-forwarding state only for the duration necessary to prevent inconsistent BPDU transmission during control plane reboot. Once the reboot completes and consistent BPDUs are transmitted again, ports quickly transition back to forwarding state, minimizing the overall traffic interruption time.
Data Source
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AI summary
Starting a spanning tree protocol (STP) process in a network device excludes performing a root bridge election sequence. The network device receives and learns the STP roles and STP states of its ports based on the received bridge protocol data units (BPDUs) during a quiescent period of time subsequent to starting STP processing. Port role and port state learning occur absent the generation and transmission of BPDUs during the quiescent period of time. Subsequent to the end of the quiescent period, the network device commences generating and transmitting BPDUs.