L2GP Pseudo Root Identity Configuration for Loop Prevention

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Solution Overview

Problem

In switched Ethernet networks, the 'count-to-infinity' problem occurs when a Layer 2 Gateway Port (L2GP) fails, leading to temporary loops due to outdated root switch information, which can cause network instability and loop creation during the time it takes to update the new root switch.

Innovation Solution

Assigning the same pseudo root identity to all L2GPs and setting different root path costs to ensure only one L2GP is open at a time, preventing simultaneous activation and thus avoiding count-to-infinity loops by treating a L2GP failure as a link failure and maintaining consistent pseudo root identity.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If L2GP is used to separate STP domains, then domain separation and redundancy are achieved, but count-to-infinity loops occur when L2GP fails due to outdated root switch information

Engineering Contradiction:

Improvedomain separationVSAvoidcount-to-infinity loops

Core Design Contradiction:

ReliabilityVSObject-affected harmful factors

Solution Approach 1:

The patent applies preliminary action by pre-configuring a root path cost on the L2GP port before failure occurs. This pre-configuration ensures that when the L2GP fails, the neighboring switch already has the correct root path cost information and can immediately update its routing tables without relying on outdated BPDUs, thus preventing the count-to-infinity loop problem

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The patent implements feedback by having the neighboring switch detect the L2GP failure and automatically update the root path cost based on the pre-configured value. This feedback mechanism ensures that the network quickly adapts to the failure condition and prevents temporary loops by maintaining accurate routing information

Inventive Principle:

Principle #23Feedback

2Loss of time

If multiple L2GPs are configured with same pseudo root identity, then convergence time is decreased, but network stability may be compromised without proper root path cost differentiation

Engineering Contradiction:

Improveconvergence timeVSAvoidnetwork stability

Core Design Contradiction:

Loss of timeVSStability of the object's composition

Solution Approach 1:

The patent applies asymmetry by configuring different root path costs on different L2GP ports even though they share the same pseudo root identity. This asymmetric configuration ensures that only one L2GP port is active at a time (the one with the lowest root path cost), preventing simultaneous activation that could cause loops, while still allowing fast convergence when failures occur

Inventive Principle:

Principle #4Asymmetry

Data Source

PatentUS8787211B2Methods and arrangements for configuring the L2GPs in a first STP domain connected towards a second STP domain

Publication Date: 2014.07.22 TELEFONAKTIEBOLAGET LM ERICSSON (PUBL)

AI summary

Embodiments of the present invention relate to methods and arrangements for configuring the L2GPs in a first STP domain connected towards a second STP domain such that L2GPs in the first STP domain are assigned with the same Pseudo Root identity. Thus the Pseudo Root identity does not have to be changed if one of the L2GPs goes down, which implies that count-to-infinity loops may be avoided. It should be noted that only one L2GP is allowed to be open towards the second STP domain. The root path costs are assigned to the L2GPs towards the pseudo root and the value of the root path cost controls which of the L2GPs that should be open. The most preferred L2GP will be given a root path cost=0, the second best L2GP will have a path cost which is high enough so the two ports never will be open at the same time if there is connectivity between the Ethernet switches having the L2GPs.