Transient Loop Prevention in Hybrid Layer-2 Networks
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Solution Overview
Problem
In hybrid Layer-2 networks, transient loops occur when Data Center Ethernet (DCE) networks merge faster than Classical Ethernet (CE) networks, causing temporary loops until the Spanning Tree Protocol (STP) can break them, as existing protocols like STP and IS-IS are not coordinated effectively to prevent these loops.
Innovation Solution
Implementing a handshake mechanism at boundary switches between CE and DCE networks to block data packets while allowing protocol messages to flow, enabling CE networks to discover each other and preventing loops, and then unblocking data packets after STP re-computation is complete, coordinating the programming of IS-IS and STP hardware tables to prevent transient loops.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If DCE networks use IS-IS protocol for faster convergence, then convergence speed is improved, but transient loops occur during network merge
Solution Approach 1:
The boundary switch performs preliminary actions by detecting the DCE network merge event and proactively blocking CE network boundary ports before transient loops can form. This preemptive blocking prevents data packets from traversing potential loop paths while allowing STP protocol messages to continue flowing, ensuring loop-free operation during the convergence transition period.
Solution Approach 2:
The boundary switch acts as an intermediary between the DCE and CE networks, mediating the interaction between the two different protocol domains. It monitors DCE network events and coordinates the blocking/unblocking of CE network ports, serving as a bridge that reconciles the different convergence behaviors of IS-IS and STP protocols to prevent transient loops.
2Reliability
If STP protocol blocks all traffic during convergence, then loop prevention is ensured, but data packet transmission is delayed
Solution Approach 1:
The boundary switch applies different quality treatments to different types of traffic: protocol messages (STP BPDUs) are allowed to flow freely while data packets are blocked. This selective approach maintains loop prevention reliability for control traffic while minimizing impact on data transmission, as data packets are only blocked temporarily during the convergence window.
Solution Approach 2:
Instead of completely blocking all traffic, the solution applies partial blocking only to data packets while allowing protocol messages to pass. This partial action is sufficient to prevent transient loops caused by data traffic, while the continued flow of protocol messages enables STP to complete its convergence process and eventually unblock the ports for normal data transmission.
Data Source
AI summary
In one embodiment, a first switch is located at a first border between first networks that utilize a STP to detect and break loops and second networks. The first switch is interconnected via the first networks with a second switch located at a second border between the first networks and the second networks. The first switch detects, via a protocol other than STP, addition of a link in the second networks that provides a new path across the second networks between the first switch and the second switch. The first switch blocks data packets from traversing through one or more network boundary ports of the first switch coupled to the first networks, while allowing STP BPDUs to traverse through the one or more network boundary ports, for a period of time sufficient for the first networks to discover the new path using STP.