Selective Forwarding Delay for Inter-Chassis Link L2 Loops

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Solution Overview

Problem

Existing multi-chassis trunking (MCT) networks face issues with inter-chassis-link (ICL) loops when the link goes down and comes back up, leading to potential L2 loops due to immediate data packet forwarding, which can cause data traffic storms and service disruptions.

Innovation Solution

Implementing a selective forwarding delay mechanism that blocks additional data packets for a predefined interval when the ICL link goes down and comes back up, ensuring ports are in a configured state before resuming data forwarding to prevent L2 loops.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Speed

If the ICL link immediately forwards data packets after coming back up, then network connectivity is restored quickly, but L2 loops occur causing data traffic storms

Engineering Contradiction:

Improvelink recovery speedVSAvoidnetwork stability

Core Design Contradiction:

SpeedVSReliability

Solution Approach 1:

The patent applies preliminary action by implementing a forwarding delay mechanism that activates before data packets are forwarded after link recovery. When the ICL transitions from down to up state, the system imposes a predefined delay period during which data packet forwarding is blocked, allowing the network to stabilize and ports to reach configured states before resuming normal forwarding operations.

Inventive Principle:

Principle #10Preliminary action

2Reliability

If BPDU forwarding is enabled to prevent loops, then loop prevention is achieved when cluster behaves like hub, but loops still occur when ICL is down and comes back up

Engineering Contradiction:

Improveloop preventionVSAvoidhandling link state changes

Core Design Contradiction:

ReliabilityVSAdaptability or versatility

Solution Approach 1:

The patent applies preliminary anti-action by implementing a forwarding delay mechanism that preemptively blocks data packet forwarding when the ICL transitions from down to up state. This delay prevents the formation of L2 loops before they can occur, addressing the specific scenario where BPDU forwarding alone is insufficient to prevent loops during link recovery.

Inventive Principle:

Principle #9Preliminary anti-action

3Reliability

If a forwarding delay is implemented to prevent loops, then L2 loops are prevented, but data forwarding is delayed

Engineering Contradiction:

Improveloop preventionVSAvoiddata forwarding delay

Core Design Contradiction:

ReliabilityVSLoss of time

Solution Approach 1:

The patent applies partial action by implementing a forwarding delay that applies selectively only to data packets, while allowing control plane traffic and BPDUs to continue forwarding normally. This partial application of the delay mechanism prevents L2 loops while minimizing the impact on overall network performance and control plane operations.

Inventive Principle:

Principle #16Partial or excessive action

Data Source

PatentUS12452166B2Preventing an inter-chassis-link loop using a selective forwarding delay

Publication Date: 2025.10.21 RUCKUS IP HOLDINGS LLC

AI summary

A computer network device (such as a switch or a router) that prevents loop occurrence is described. During operation, the computer network device communicates, via one or more ports, data packets with a corresponding one or more second ports in a second computer network device, where a given data packet includes a source address and a destination address, and where the one or more ports and the second one or more ports specify a link between the computer network device and the second computer network device. Moreover, when the link goes down and then comes back up, the computer network device may block forwarding, via the one or more ports, of additional data packets based at least in part on a predefined delay.