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Introduction

In a previous blog post, we described how Azure Virtual Network Manager (AVNM) enables central teams to enforce security admin rules across hundreds of virtual networks—bring consistency and governance to complex enterprise environments.

But enforcement at scale introduces a new challenge: deployment confidence. Security admin rules take priority over NSG rules and can span subscriptions and management groups. That makes them powerful—but a single misconfigured rule can disrupt critical traffic across your entire network. Governance teams need a way to understand the real-world impact of a rule before it reaches production—not after.

This is exactly the problem Azure Virtual Network Manager now solves with the Rule Impact Analyzer—a capability that simulates proposed security admin rules against your real network traffic, so you can see exactly what will change, what won’t, and deploy with confidence instead of guesswork.

The Challenge: Understanding Rule Impact Before Deployment

As enterprises scale up their use of security admin rules, a visibility gap emerges. Consider a common scenario: a central governance team needs to block high-risk ports across all production virtual networks. The rules are well-intentioned, but the team has no visibility into which existing traffic flows would be affected. Without a way to preview the impact, teams face an uncomfortable tradeoff—move quickly and risk disruption, or slow down manual review across every affected network.

The Rule Impact Analyzer is designed to close this gap—giving teams with a clear, data-driven view of what a rule of change will do before it reaches production.

What Is the Rule Impact Analyzer?

The Rule Impact Analyzer is a joint capability of Azure Virtual Network Manager and Azure Network Watcher. It lets you simulate proposed security admin rules against traffic data derived from virtual network (VNet) flow logs and Traffic Analytics in your environment.

Instead of relying on manual review, the analyzer evaluates proposed rules against observed traffic and classifies each flow:

• Affected — The proposed rule would change the current evaluation outcome for this flow (e.g., traffic that is currently allowed would be blocked).


• Not Affected — The flow would continue as-is; the rule does not apply.


• Indeterminate — The flow cannot be conclusively evaluated (e.g., insufficient traffic data).

This gives governance teams and network administrators a clear, data-driven view of what a rule of change will do—before it reaches production.

Note: The analysis is based on traffic data available through flow logs and Traffic Analytics. Results reflect recorded traffic patterns; traffic that has not yet been observed will not appear in results.

The Customer Journey: From Rule Authoring to Validated Deployment

The Rule Impact Analyzer fits naturally into the lifecycle of security admin rule management:

This workflow lets teams author rules, simulate impact, review results, and refine policies before committing a single change to production. Teams can cycle through simulation as many times as needed.

Key Capabilities

1. Predicted Impact Visibility

See briefly how your proposed security admin rules would affect existing traffic flows. Results are based on Traffic Analytics data, helping teams make informed deployment decisions.

2. Flow-Level Drill-Down

Go beyond summary counts. Inspect specific source and destination paths, see which rule affects each flow, and identify legitimate traffic that would be unintentionally blocked. This makes it easy to pinpoint issues and refine your rules.

3. Configurable Scope

You don’t have to analyze everything at once. Target your analysis to specific:

• Rule collections or individual security admin rules


• Network groups or specific virtual networks

This lets you focus on the areas that matter most, whether you’re validating a single rule change or assessing a broad policy rollout.

4. Controlled Iteration

Modify your security admin rules, re-run the analysis, and repeat—as many times as you need. Deploy only when the simulated impact matches your intended connectivity outcome.

5. Inbound and Outbound Evaluation

The analyzer evaluates both inbound and outbound traffic directions, giving you full visibility into the rule’s impact across your network.

Real-World Scenario: Locking Down Internet-Exposed Management Ports at Scale

Let’s look at a real-world scenario as an example. Your organization runs hundreds of VNets across multiple subscriptions. Over time, different teams have created NSG rules that allow inbound SSH (port 22) and RDP (port 3389) from broad source ranges — some even from 0.0.0.0/0. Your security team mandates: block all inbound management-port access except from trusted bastion subnets.

The challenge? You can’t just flip a switch. Blocking the wrong traffic could be risky, and you want to know the impact of applying the security rules.

With Rule Impact Analyzer, you can:

• Define the proposed security admin rule — deny inbound TCP 22/3389 from all sources except your bastion subnet prefix


• Simulate before you commit — see exactly which VNets, subnets, and NICs currently have traffic matching the rule, and which existing NSG rules would be overridden


• Identify conflicts — spot cases where a team’s NSG “Allow” rule would be superseded by your new admin-level “Deny,” so you can coordinate before deployment


• Deploy with confidence — roll out the rule knowing the blast radius is fully understood, not guessed

Before Rule Impact Analyzer, this required manually auditing NSG rules across every subscription, cross-referencing with resource inventories, and hoping nothing was missed. Now, a single simulation gives you a complete picture in minutes — turning a week-long audit into a self-service workflow.

How It Works: Architecture and Design

Rule Impact Analyzer uses existing Azure networking telemetry and analytics components. It does not require a separate data collection pipeline.

The following diagram provides an interactive version of the architecture:

Step 1: Traffic Analytics as Ground Truth. The analyzer queries your existing VNet flow logs through Traffic Analytics. No new agents, log pipelines, or storage accounts are required.

Step 2: Log Analytics as the Query Engine. Traffic Analytics data resides in your Log Analytics workspace. The Rule Impact Analyzer runs Kusto Query Language (KQL) queries to retrieve the observed flows relevant to your analysis scope.

Step 3: AVNM Rule Evaluation Engine. The retrieved flows are evaluated using AVNM’s own enforcement logic—the same priority ordering, allow/deny behavior, and scope resolution used in production. This ensures that what you see in the analyzer matches what would happen when rules are enforced.

Step 4: Results Correlation and Surfacing. Each flow is classified and surfaced in the Azure Portal with drill-down capabilities—from summary impact counts down to individual flow paths and the specific rules affecting them.

What Means for You

• Uses existing infrastructure. If you already have Traffic Analytics enabled, there is nothing new to deploy.


• No data duplication. Queries run in place within your existing Log Analytics workspace, under your existing RBAC and data retention policies.


• Transparent costs. Only standard Log Analytics query costs apply—no hidden charges or separate billing.

Getting Started

You can access Rule Impact Analyzer from two entry points in the Azure Portal:

1. From Azure Virtual Network Manager: Navigate to your security admin configuration → select a rule collection → launch the Rule Impact Analyzer.


2. From Azure Network Watcher: Navigate to Monitoring → Traffic Analytics → Rule Analyzer.

Both paths lead to the same analysis experience, so you can start with whichever tool fits your workflow.

Prerequisites

Before using the Rule Impact Analyzer, ensure the following are in place:

• VNet flow logs are enabled on the virtual networks you want to analyze.


• Traffic Analytics is configured and sends data to a Log Analytics workspace.


• You have the necessary RBAC permissions to access the AVNM security admin configuration and the Log Analytics workspace.

Steps

1. Enable VNet flow logs and Traffic Analytics on your target virtual networks. Learn more about Traffic Analytics.


2. Author or update your security admin rules in Azure Virtual Network Manager. Learn more about AVNM security admin rules.


3. Launch the Rule Impact Analyzer from either portal entry point, configure your scope (rule collections, network groups, or specific VNets), and run the analysis.


4. Review, refine, and deploy. Iterate your rules until the simulated impact matches your intended outcome, then deploy with confidence.

The screenshot below shows the Rule Impact Analyzer in the Azure Portal. After running a simulation, you can see a summary of predicted traffic impact—total paths analyzed, how many are affected or not affected—along with a detailed results table to drill into individual flows and identify which rule impacts each one.

Why It Matters

Outage Prevention

For organizations rolling out network isolation policies at scale, Rule Impact Analyzer acts as a safety net. By simulating rule impact against recorded traffic patterns, teams can catch misconfigurations before they reach production.

Faster Rule Adoption

Without the analyzer, deploying new admin rules often requires lengthy manual review cycles. With self-service impact analysis, governance teams can validate and deploy rules faster—without waiting for manual approval.

Aligning with Behavior

Security policies express intent—what traffic should or shouldn’t be allowed. Rule Impact Analyzer validates whether a proposed rule achieves that intent against your observed traffic, closing the loop between policy design and operational behavior.

Conclusion

The AVNM Rule Impact Analyzer closes the gap between policy intent and deployment confidence. Simulating rules against observed traffic—with no additional infrastructure required—governance teams can validate impact before enforcement.

Enforcement without visibility is a risk. Visibility without enforcement is incomplete. This capability brings both together.

We welcome your feedback as you start using this capability. Share your experience through the Azure Portal feedback button or your Microsoft account team.

Learn more:

• Azure Virtual Network Manager


• Azure Network Watcher


• Traffic Analytics


• AVNM Security Admin Rules


• Using Azure Virtual Network Manager to Enhance Network Security

Authors: 

Deepak Bansal, Corporate Vice President and Technical Fellow, Microsoft Azure, Xinyan Zan, Vice President, Ashish Bhargava, Principal Software Development Manager, and Jay Li, Senior Product Manager