A demonstration of Virtual Network TAP
Azure Virtual Network Terminal Access Point (VTAP), at the time of writing in April 2026 in public preview in select regions, copies network traffic from source Virtual Machines to a collector or traffic analytics tool, running as a Network Virtual Appliance (NVA). VTAP creates a full copy of all traffic sent and received by Virtual Machine Network Interface Card(s) (NICs) designated as VTAP source(s). This includes packet payload content - in contrast to VNET Flow Logs, which only collect traffic meta data. Traffic collectors and analytics tools are 3rd party partner products, available from the Azure Marketplace, amongst which are the major Network Detection and Response solutions. VTAP is an agentless, cloud-native traffic tap at the Azure network infrastructure level. It is entirely out-of-band; it has no impact on the source VM's network performance and the source VM is unaware of the tap. Tapped traffic is VXLAN-encapsulated and delivered to the collector NVA, in the same VNET as the source VMs, or in a peered VNET. This post demonstrates the basic functionality of VTAP: copying traffic into and out of a source VM, to a destination VM. The demo consists of 3 three Windows VMs in one VNET, each running a basic web server that responds with the VM's name. Another VNET contains the target - a Windows VM on which Wireshark is installed, to inspect traffic forwarded by VTAP. This demo does not use 3rd party VTAP partner solutions from the Marketplace. The lab for this demonstration is available on Github: Virtual Network TAP. The VTAP resource is configured with the target VM's NIC as the destination. All traffic captured from sources is VXLAN-encapsulated and sent to the destination on UDP port 4789 (this cannot be changed). We use a single source to easier inspect the traffic flows in Wireshark; we will see that communication from the other VMs to our source VM is captured and copied to the destination. In a real world scenario, multiple or all of the VMs in an environment could be set up as TAP sources. The source VM, vm1, generates traffic through a script that continuously polls vm2 and vm3 on http://10.0.2.5 and http://10.0.2.6, and https://ipconfig.io. On the destination VM, we use Wireshark to observe captured traffic. The filter on UDP port 4789 causes Wireshark to only capture the VXLAN encapsulated traffic forwarded by VTAP. Wireshark automatically decodes VXLAN and displays the actual traffic to and from vm1, which is set up as the (only) VTAP source. Wireshark's capture panel shows the decapsulated TCP and HTTP exchanges, including the TCP handshake, between vm1 and the other VMs, and https://ipconfig.io. Expanding the lines in the detail panel below the capture panel shows the details of the VXLAN encapsulation. The outer IP packets, encapsulating the VXLAN frames in UDP, originate from the source VM's IP address, 10.0.2.4, and have the target VM's address, 10.1.1.4, as the destination. The VXLAN frames contain all the details of the original Ethernet frames sent from and received by the source VM, and the IP packets within those. The Wireshark trace shows the full exchange between vm1 and the destinations it speaks with. This brief demonstration uses Wireshark to simply visualize the operation of VTAP. The partner solutions available from the Azure Marketplace operate on the captured traffic to implement their specific functionality.
Consistent DNS resolution in a hybrid hub spoke network topology
DNS is one of the most essential networking services, next to IP routing. A modern hybrid cloud network may have various sources of DNS: Azure Private DNS Zones, public DNS, domain controllers, etc. Some organizations may also prefer to route their public Internet DNS queries through a specific DNS provider. Therefore, it is crucial to ensure consistent DNS resolution across the whole (hybrid) network. This article describes how DNS Private Resolver can be leveraged to build such architecture.Custom DHCP support in Azure
Discover the intricacies of Dynamic Host Configuration Protocol (DHCP), a network protocol used for assigning IP addresses and network parameters. Learn about the DORA process, lease renewals, and the role of DHCP Relay in large enterprises. Gain insight into how DHCP operates in Azure natively and how its support in Azure has evolved over time, including the removal of rate-limiting for relayed traffic. This comprehensive guide also covers the limitations and potential workarounds of DHCP in Azure. Ideal for network administrators and IT professionals.Network Detection and Response (NDR) in Financial Services
New PCI DSS v4.0.1 requirements heighten the need for automated monitoring and analysis of security logs. Network Detection and Response solutions fulfill these mandates by providing 24/7 network traffic inspection and real-time alerting on suspicious activities. Azure’s native tools (Azure vTAP for full packets, VNET Flow Logs for all flows) capture rich network data and integrate with advanced NDR analytics from partners. This combination detects intrusions (satisfying IDS requirements under Requirement 11), validates network segmentation (for scope reduction under Req. 1), and feeds alerts into Microsoft Sentinel for rapid response (fulfilling incident response obligations in Req. 12). The result is a cloud architecture that not only meets PCI DSS controls but actively strengthens security.
Announcing Azure DNS security policy with Threat Intelligence feed general availability
Azure DNS security policy with Threat Intelligence feed allows early detection and prevention of security incidents on customer Virtual Networks where known malicious domains sourced by Microsoft’s Security Response Center (MSRC) can be blocked from name resolution. Azure DNS security policy with Threat Intelligence feed is being announced to all customers and will have regional availability in all public regions.
Announcing the public preview of StandardV2 NAT Gateway and StandardV2 public IPs
In today’s rapidly changing digital landscape, organizations are innovating faster and delivering cloud native experiences at global scale. With this acceleration comes higher expectations: applications must remain always available. An outage in a single availability zone can have a ripple effect on application performance, user experience, and business continuity. To safeguard against zone outages, making your cloud architecture zone resilient isn't an option—it’s a necessity. A key part of any resilient design is ensuring reliable outbound connectivity. Azure NAT Gateway is a fully managed network address translation service that provides highly scalable and secure internet connectivity for resources inside your virtual networks. We’re excited to announce the public preview of the StandardV2 SKU NAT Gateway, an evolution of Azure NAT Gateway built for the next generation of scale, performance, and resiliency. StandardV2 NAT Gateway delivers zone redundancy, enhanced data processing limits, IPv6 support, and flow logs all at the same price as the Standard SKU NAT Gateway. This release also marks the public preview of StandardV2 SKU public IP addresses and prefixes. A new SKU of public IPs that must be used with StandardV2 NAT Gateway. In combination, StandardV2 IPs provide high-throughput connectivity to support demanding workloads. What’s new in StandardV2 NAT Gateway Zone redundancy StandardV2 NAT Gateway is zone-redundant by default in regions with availability zones. Deployed as a single resource operating across multiple zones, StandardV2 NAT Gateway ensures outbound connectivity even if one zone becomes unavailable. For example, a virtual machine in zone 2 connects outbound from zones 1, 2, or 3 through a StandardV2 NAT Gateway (as shown in the figure). If zone 1 experiences an outage, existing connections in that zone may fail, but new connections will seamlessly flow through zones 2 and 3—keeping your applications online and resilient. All existing connections through zones 2 and 3 will persist. To learn more, see StandardV2 NAT Gateway zone-redundancy. remaining healthy zones with StandardV2 NAT Gateway. This kind of resiliency is essential for any critical workload to ensure high availability. Whether you’re a global SaaS provider that needs to maintain service continuity for your customers during zonal outages or an e-commerce platform that needs to ensure high availability during peak shopping seasons, StandardV2 NAT Gateway can help you achieve greater protection against zonal outages. Higher performance StandardV2 doubles the performance of the Standard SKU, supporting up to 100 Gbps throughput and 10 million packets per second. These enhanced data processing limits are ideal for data-intensive and latency-sensitive applications requiring consistent, high-throughput outbound access to the internet. For more information, see StandardV2 NAT Gateway performance. StandardV2 public IPs Alongside NAT Gateway, StandardV2 public IP Addresses and Prefixes are now available in public preview. StandardV2 SKU public IPs are a new offering of public IPs that must be used with StandardV2 NAT Gateway to provide outbound connectivity. Standard SKU public IPs are not compatible with StandardV2 NAT Gateway. See how to deploy StandardV2 public IPs. IPv6 (dual-stack) support StandardV2 NAT Gateway now supports dual-stack (IPv4 + IPv6) connectivity, enabling organizations to meet regulatory requirements, optimize performance for modern architectures, and future-proof workloads at internet scale. Each NAT Gateway supports up to 16 IPv4 and 16 IPv6 StandardV2 public IP addresses or prefixes. See IPv6 support for StandardV2 NAT gateway for more information. Flow logs With StandardV2, you can now enable flow logs to gain deeper visibility into outbound traffic patterns. Flow logs capture detailed IP-level traffic information, helping you: Troubleshoot connectivity issues more efficiently. Identify top talkers behind the NAT Gateway (which virtual machines initiate the most connections outbound). Analyze traffic for compliance and security auditing for your organization Learn more at Enable flow logs on StandardV2 NAT Gateway. Deploying StandardV2 NAT Gateway and public IPs You can deploy StandardV2 NAT Gateway and StandardV2 public IPs using ARM templates, Bicep, PowerShell, or CLI. Portal and Terraform support is coming soon. For more information on client support, see StandardV2 NAT Gateway SKU. Learn More StandardV2 NAT Gateway StandardV2 public IPs Create and validate StandardV2 NAT Gateway NAT Gateway pricingAzure Virtual Network Manager + Azure Virtual WAN
Azure continues to expand its networking capabilities, with Azure Virtual Network Manager and Azure Virtual WAN (vWAN) standing out as two of the most transformative services. When deployed together, they offer the best of both worlds: the operational simplicity of a managed hub architecture combined with the ability for spoke VNets to communicate directly, avoiding additional hub hops and minimizing latency Revisiting the classic hub-and-spoke pattern Element Traditional hub-and-spoke role Hub VNet Centralized network that hosts shared services including firewalls (e.g., Azure Firewall, NVAs), VPN/ExpressRoute gateways, DNS servers, domain controllers, and central route tables for traffic management. Acts as the connectivity and security anchor for all spoke networks. Spoke VNets Host individual application workloads and peer directly to the hub VNet. Traffic flows through the hub for north-south connectivity (to/from on-premises or internet) and cross-spoke communication (east-west traffic between spokes). Benefits • Single enforcement point for security policies and network controls • No duplication of shared services across environments • Simplified routing logic and traffic flow management • Clear network segmentation and isolation between workloads • Cost optimization through centralized resources However, this architecture comes with a trade-off: every spoke-to-spoke packet must route through the hub, introducing additional network hops, increased latency, and potential throughput constraints. How Virtual WAN modernizes that design Virtual WAN replaces a do-it-yourself hub VNet with a fully managed hub service: Managed hubs – Azure owns and operates the hub infrastructure. Automatic route propagation – routes learned once are usable everywhere. Integrated add-ons – Firewalls, VPN, and ExpressRoute ports are first-class citizens. By default, Virtual WAN enables any-to-any routing between spokes. Traffic transits the hub fabric automatically—no configuration required. Why direct spoke mesh? Certain patterns require single-hop connectivity Micro-service meshes that sit in different spokes and exchange chatty RPC calls. Database replication / backups where throughput counts, and hub bandwidth is precious. Dev / Test / Prod spokes that need to sync artifacts quickly yet stay isolated from hub services. Segmentation mandates where a workload must bypass hub inspection for compliance yet still talk to a partner VNet. Benefits Lower latency – the hub detour disappears. Better bandwidth – no hub congestion or firewall throughput cap. Higher resilience – spoke pairs can keep talking even if the hub is under maintenance. The peering explosion problem With pure VNet peering, the math escalates fast: For n spokes you need n × (n-1)/2 links. Ten spokes? 45 peerings. Add four more? Now 91. Each extra peering forces you to: Touch multiple route tables. Update NSG rules to cover the new paths. Repeat every time you add or retire a spoke. Troubleshoot an ever-growing spider web. Where Azure Virtual Network Manager Steps In? Azure Virtual Network Manager introduces Network Groups plus a Mesh connectivity policy: Azure Virtual Network Manager Concept What it gives you Network group A logical container that groups multiple VNets together, allowing you to apply configurations and policies to all members simultaneously Mesh connectivity Automated peering between all VNets in the group, ensuring every member can communicate directly with every other member without manual configuration Declarative config Intent-based approach where you define the desired network state, and Azure Virtual Network Manager handles the implementation and ongoing maintenance Dynamic updates Automatic topology management—when VNets are added to or removed from a group, Azure Virtual Network Manager reconfigures all necessary connections without manual intervention Operational complexity collapses from O(n²) to O(1)—you manage a group, not 100+ individual peerings. A complementary model: Azure Virtual Network Manager mesh inside vWAN Since Azure Virtual Network Manager works on any Azure VNet—including the VNets you already attach to a vWAN hub—you can apply mesh policies on top of your existing managed hub architecture: Spoke VNets join a vWAN hub for branch connectivity, centralized firewalling, or multi-region reach. The same spokes are added to an Azure Virtual Network Manager Network Group with a mesh policy. Azure Virtual Network Manager builds direct peering links between the spokes, while vWAN continues to advertise and learn routes. Result: All VNets still benefit from vWAN’s global routing and on-premises integration. Latency-critical east-west flows now travel the shortest path—one hop—as if the VNets were traditionally peered. Rather than choosing one over the other, organizations can leverage both vWAN and Azure Virtual Network Manager together, as the combination enhances the strengths of each service. Performance illustration Spoke-to-Spoke Communication with Virtual WAN without Azure Virtual Network Manager mesh: Spoke-to-Spoke Communication with Virtual WAN with Azure Virtual Network Manager mesh: Observability & protection NSG flow logs – granular packet logs on every peered VNet. Azure Virtual Network Manager admin rules – org-wide guardrails that trump local NSGs. Azure Monitor + SIEM – route flow logs to Log Analytics, Sentinel, or third-party SIEM for threat detection. Layered design – hub firewalls inspect north-south traffic; NSGs plus admin rules secure east-west flows. Putting it all together Virtual WAN offers fully managed global connectivity, simplifying the integration of branch offices and on-premises infrastructure into your Azure environment. Azure Virtual Network Manager mesh establishes direct communication paths between spoke VNets, making it ideal for workloads requiring high throughput or minimal latency in east-west traffic patterns. When combined, these services provide architects with granular control over traffic routing. Each flow can be directed through hub services when needed or routed directly between spokes for optimal performance—all without re-architecting your network or creating additional management complexity. By pairing Azure Virtual Network Manager’s group-based mesh with VWAN’s managed hubs, you get the best of both worlds: worldwide reach, centralized security, and single-hop performance where it counts.Deploying Third-Party Firewalls in Azure Landing Zones: Design, Configuration, and Best Practices
As enterprises adopt Microsoft Azure for large-scale workloads, securing network traffic becomes a critical part of the platform foundation. Azure’s Well-Architected Framework provides the blueprint for enterprise-scale Landing Zone design and deployments, and while Azure Firewall is a built-in PaaS option, some organizations prefer third-party firewall appliances for familiarity, feature depth, and vendor alignment. This blog explains the basic design patterns, key configurations, and best practices when deploying third-party firewalls (Palo Alto, Fortinet, Check Point, etc.) as part of an Azure Landing Zone. 1. Landing Zone Architecture and Firewall Role The Azure Landing Zone is Microsoft’s recommended enterprise-scale architecture for adopting cloud at scale. It provides a standardized, modular design that organizations can use to deploy and govern workloads consistently across subscriptions and regions. At its core, the Landing Zone follows a hub-and-spoke topology: Hub (Connectivity Subscription): Central place for shared services like DNS, private endpoints, VPN/ExpressRoute gateways, Azure Firewall (or third-party firewall appliances), Bastion, and monitoring agents. Provides consistent security controls and connectivity for all workloads. Firewalls are deployed here to act as the traffic inspection and enforcement point. Spokes (Workload Subscriptions): Application workloads (e.g., SAP, web apps, data platforms) are placed in spoke VNets. Additional specialized spokes may exist for Identity, Shared Services, Security, or Management. These are isolated for governance and compliance, but all connectivity back to other workloads or on-premises routes through the hub. Traffic Flows Through Firewalls North-South Traffic: Inbound connections from the Internet (e.g., customer access to applications). Outbound connections from Azure workloads to Internet services. Hybrid connectivity to on-premises datacenters or other clouds. Routed through the external firewall set for inspection and policy enforcement. East-West Traffic: Lateral traffic between spokes (e.g., Application VNet to Database VNet). Communication across environments like Dev → Test → Prod (if allowed). Routed through an internal firewall set to apply segmentation, zero-trust principles, and prevent lateral movement of threats. Why Firewalls Matter in the Landing Zone While Azure provides NSGs (Network Security Groups) and Route Tables for basic packet filtering and routing, these are not sufficient for advanced security scenarios. Firewalls add: Deep packet inspection (DPI) and application-level filtering. Intrusion Detection/Prevention (IDS/IPS) capabilities. Centralized policy management across multiple spokes. Segmentation of workloads to reduce blast radius of potential attacks. Consistent enforcement of enterprise security baselines across hybrid and multi-cloud. Organizations May Choose Depending on security needs, cost tolerance, and operational complexity, organizations typically adopt one of two models for third party firewalls: Two sets of firewalls One set dedicated for north-south traffic (external to Azure). Another set for east-west traffic (between VNets and spokes). Provides the highest security granularity, but comes with higher cost and management overhead. Single set of firewalls A consolidated deployment where the same firewall cluster handles both east-west and north-south traffic. Simpler and more cost-effective, but may introduce complexity in routing and policy segregation. This design choice is usually made during Landing Zone design, balancing security requirements, budget, and operational maturity. 2. Why Choose Third-Party Firewalls Over Azure Firewall? While Azure Firewall provides simplicity as a managed service, customers often choose third-party solutions due to: Advanced features – Deep packet inspection, IDS/IPS, SSL decryption, threat feeds. Vendor familiarity – Network teams trained on Palo Alto, Fortinet, or Check Point. Existing contracts – Enterprise license agreements and support channels. Hybrid alignment – Same vendor firewalls across on-premises and Azure. Azure Firewall is a fully managed PaaS service, ideal for customers who want a simple, cloud-native solution without worrying about underlying infrastructure. However, many enterprises continue to choose third-party firewall appliances (Palo Alto, Fortinet, Check Point, etc.) when implementing their Landing Zones. The decision usually depends on capabilities, familiarity, and enterprise strategy. Key Reasons to Choose Third-Party Firewalls Feature Depth and Advanced Security Third-party vendors offer advanced capabilities such as: Deep Packet Inspection (DPI) for application-aware filtering. Intrusion Detection and Prevention (IDS/IPS). SSL/TLS decryption and inspection. Advanced threat feeds, malware protection, sandboxing, and botnet detection. While Azure Firewall continues to evolve, these vendors have a longer track record in advanced threat protection. Operational Familiarity and Skills Network and security teams often have years of experience managing Palo Alto, Fortinet, or Check Point appliances on-premises. Adopting the same technology in Azure reduces the learning curve and ensures faster troubleshooting, smoother operations, and reuse of existing playbooks. Integration with Existing Security Ecosystem Many organizations already use vendor-specific management platforms (e.g., Panorama for Palo Alto, FortiManager for Fortinet, or SmartConsole for Check Point). Extending the same tools into Azure allows centralized management of policies across on-premises and cloud, ensuring consistent enforcement. Compliance and Regulatory Requirements Certain industries (finance, healthcare, government) require proven, certified firewall vendors for security compliance. Customers may already have third-party solutions validated by auditors and prefer extending those to Azure for consistency. Hybrid and Multi-Cloud Alignment Many enterprises run a hybrid model, with workloads split across on-premises, Azure, AWS, or GCP. Third-party firewalls provide a common security layer across environments, simplifying multi-cloud operations and governance. Customization and Flexibility Unlike Azure Firewall, which is a managed service with limited backend visibility, third-party firewalls give admins full control over operating systems, patching, advanced routing, and custom integrations. This flexibility can be essential when supporting complex or non-standard workloads. Licensing Leverage (BYOL) Enterprises with existing enterprise agreements or volume discounts can bring their own firewall licenses (BYOL) to Azure. This often reduces cost compared to pay-as-you-go Azure Firewall pricing. When Azure Firewall Might Still Be Enough Organizations with simple security needs (basic north-south inspection, FQDN filtering). Cloud-first teams that prefer managed services with minimal infrastructure overhead. Customers who want to avoid manual scaling and VM patching that comes with IaaS appliances. In practice, many large organizations use a hybrid approach: Azure Firewall for lightweight scenarios or specific environments, and third-party firewalls for enterprise workloads that require advanced inspection, vendor alignment, and compliance certifications. 3. Deployment Models in Azure Third-party firewalls in Azure are primarily IaaS-based appliances deployed as virtual machines (VMs). Leading vendors publish Azure Marketplace images and ARM/Bicep templates, enabling rapid, repeatable deployments across multiple environments. These firewalls allow organizations to enforce advanced network security policies, perform deep packet inspection, and integrate with Azure-native services such as Virtual Network (VNet) peering, Azure Monitor, and Azure Sentinel. Note: Some vendors now also release PaaS versions of their firewalls, offering managed firewall services with simplified operations. However, for the purposes of this blog, we will focus mainly on IaaS-based firewall deployments. Common Deployment Modes Active-Active Description: In this mode, multiple firewall VMs operate simultaneously, sharing the traffic load. An Azure Load Balancer distributes inbound and outbound traffic across all active firewall instances. Use Cases: Ideal for environments requiring high throughput, resilience, and near-zero downtime, such as enterprise data centers, multi-region deployments, or mission-critical applications. Considerations: Requires careful route and policy synchronization between firewall instances to ensure consistent traffic handling. Typically involves BGP or user-defined routes (UDRs) for optimal traffic steering. Scaling is easier: additional firewall VMs can be added behind the load balancer to handle traffic spikes. Active-Passive Description: One firewall VM handles all traffic (active), while the secondary VM (passive) stands by for failover. When the active node fails, Azure service principals manage IP reassignment and traffic rerouting. Use Cases: Suitable for environments where simpler management and lower operational complexity are preferred over continuous load balancing. Considerations: Failover may result in a brief downtime, typically measured in seconds to a few minutes. Synchronization between the active and passive nodes ensures firewall policies, sessions, and configurations are mirrored. Recommended for smaller deployments or those with predictable traffic patterns. Network Interfaces (NICs) Third-party firewall VMs often include multiple NICs, each dedicated to a specific type of traffic: Untrust/Public NIC: Connects to the Internet or external networks. Handles inbound/outbound public traffic and enforces perimeter security policies. Trust/Internal NIC: Connects to private VNets or subnets. Manages internal traffic between application tiers and enforces internal segmentation. Management NIC: Dedicated to firewall management traffic. Keeps administration separate from data plane traffic, improving security and reducing performance interference. HA NIC (Active-Passive setups): Facilitates synchronization between active and passive firewall nodes, ensuring session and configuration state is maintained across failovers. This design choice is usually made during Landing Zone design, balancing security requirements, budget, and operational maturity. : NICs of Palo Alto External Firewalls and FortiGate Internal Firewalls in two sets of firewall scenario 4. Key Configuration Considerations When deploying third-party firewalls in Azure, several design and configuration elements play a critical role in ensuring security, performance, and high availability. These considerations should be carefully aligned with organizational security policies, compliance requirements, and operational practices. Routing User-Defined Routes (UDRs): Define UDRs in spoke Virtual Networks to ensure all outbound traffic flows through the firewall, enforcing inspection and security policies before reaching the Internet or other Virtual Networks. Centralized routing helps standardize controls across multiple application Virtual Networks. Depending on the architecture traffic flow design, use appropriate Load Balancer IP as the Next Hop on UDRs of spoke Virtual Networks. Symmetric Routing: Ensure traffic follows symmetric paths (i.e., outbound and inbound flows pass through the same firewall instance). Avoid asymmetric routing, which can cause stateful firewalls to drop return traffic. Leverage BGP with Azure Route Server where supported, to simplify route propagation across hub-and-spoke topologies. : Azure UDR directing all traffic from a Spoke VNET to the Firewall IP Address Policies NAT Rules: Configure DNAT (Destination NAT) rules to publish applications securely to the Internet. Use SNAT (Source NAT) to mask private IPs when workloads access external resources. Security Rules: Define granular allow/deny rules for both north-south traffic (Internet to VNet) and east-west traffic (between Virtual Networks or subnets). Ensure least privilege by only allowing required ports, protocols, and destinations. Segmentation: Apply firewall policies to separate workloads, environments, and tenants (e.g., Production vs. Development). Enforce compliance by isolating workloads subject to regulatory standards (PCI-DSS, HIPAA, GDPR). Application-Aware Policies: Many vendors support Layer 7 inspection, enabling controls based on applications, users, and content (not just IP/port). Integrate with identity providers (Azure AD, LDAP, etc.) for user-based firewall rules. : Example Configuration of NAT Rules on a Palo Alto External Firewall Load Balancers Internal Load Balancer (ILB): Use ILBs for east-west traffic inspection between Virtual Networks or subnets. Ensures that traffic between applications always passes through the firewall, regardless of origin. External Load Balancer (ELB): Use ELBs for north-south traffic, handling Internet ingress and egress. Required in Active-Active firewall clusters to distribute traffic evenly across firewall nodes. Other Configurations: Configure health probes for firewall instances to ensure faulty nodes are automatically bypassed. Validate Floating IP configuration on Load Balancing Rules according to the respective vendor recommendations. Identity Integration Azure Service Principals: In Active-Passive deployments, configure service principals to enable automated IP reassignment during failover. This ensures continuous service availability without manual intervention. Role-Based Access Control (RBAC): Integrate firewall management with Azure RBAC to control who can deploy, manage, or modify firewall configurations. SIEM Integration: Stream logs to Azure Monitor, Sentinel, or third-party SIEMs for auditing, monitoring, and incident response. Licensing Pay-As-You-Go (PAYG): Licenses are bundled into the VM cost when deploying from the Azure Marketplace. Best for short-term projects, PoCs, or variable workloads. Bring Your Own License (BYOL): Enterprises can apply existing contracts and licenses with vendors to Azure deployments. Often more cost-effective for large-scale, long-term deployments. Hybrid Licensing Models: Some vendors support license mobility from on-premises to Azure, reducing duplication of costs. 5. Common Challenges Third-party firewalls in Azure provide strong security controls, but organizations often face practical challenges in day-to-day operations: Misconfiguration Incorrect UDRs, route tables, or NAT rules can cause dropped traffic or bypassed inspection. Asymmetric routing is a frequent issue in hub-and-spoke topologies, leading to session drops in stateful firewalls. Performance Bottlenecks Firewall throughput depends on the VM SKU (CPU, memory, NIC limits). Under-sizing causes latency and packet loss, while over-sizing adds unnecessary cost. Continuous monitoring and vendor sizing guides are essential. Failover Downtime Active-Passive models introduce brief service interruptions while IPs and routes are reassigned. Some sessions may be lost even with state sync, making Active-Active more attractive for mission-critical workloads. Backup & Recovery Azure Backup doesn’t support vendor firewall OS. Configurations must be exported and stored externally (e.g., storage accounts, repos, or vendor management tools). Without proper backups, recovery from failures or misconfigurations can be slow. Azure Platform Limits on Connections Azure imposes a per-VM cap of 250,000 active connections, regardless of what the firewall vendor appliance supports. This means even if an appliance is designed for millions of sessions, it will be constrained by Azure’s networking fabric. Hitting this cap can lead to unexplained traffic drops despite available CPU/memory. The workaround is to scale out horizontally (multiple firewall VMs behind a load balancer) and carefully monitor connection distribution. 6. Best Practices for Third-Party Firewall Deployments To maximize security, reliability, and performance of third-party firewalls in Azure, organizations should follow these best practices: Deploy in Availability Zones: Place firewall instances across different Availability Zones to ensure regional resilience and minimize downtime in case of zone-level failures. Prefer Active-Active for Critical Workloads: Where zero downtime is a requirement, use Active-Active clusters behind an Azure Load Balancer. Active-Passive can be simpler but introduces failover delays. Use Dedicated Subnets for Interfaces: Separate trust, untrust, HA, and management NICs into their own subnets. This enforces segmentation, simplifies route management, and reduces misconfiguration risk. Apply Least Privilege Policies: Always start with a deny-all baseline, then allow only necessary applications, ports, and protocols. Regularly review rules to avoid policy sprawl. Standardize Naming & Tagging: Adopt consistent naming conventions and resource tags for firewalls, subnets, route tables, and policies. This aids troubleshooting, automation, and compliance reporting. Validate End-to-End Traffic Flows: Test both north-south (Internet ↔ VNet) and east-west (VNet ↔ VNet/subnet) flows after deployment. Use tools like Azure Network Watcher and vendor traffic logs to confirm inspection. Plan for Scalability: Monitor throughput, CPU, memory, and session counts to anticipate when scale-out or higher VM SKUs are needed. Some vendors support autoscaling clusters for bursty workloads. Maintain Firmware & Threat Signatures: Regularly update the firewall’s software, patches, and threat intelligence feeds to ensure protection against emerging vulnerabilities and attacks. Automate updates where possible. Conclusion Third-party firewalls remain a core building block in many enterprise Azure Landing Zones. They provide the deep security controls and operational familiarity enterprises need, while Azure provides the scalable infrastructure to host them. By following the hub-and-spoke architecture, carefully planning deployment models, and enforcing best practices for routing, redundancy, monitoring, and backup, organizations can ensure a secure and reliable network foundation in Azure.Extending Layer-2 (VXLAN) networks over Layer-3 IP network
Introduction Virtual Extensible LAN (VXLAN) is a network virtualization technology that encapsulates Layer-2 Ethernet frames inside Layer-3 UDP/IP packets. In essence, VXLAN creates a logical Layer-2 overlay network on top of an IP network, allowing Ethernet segments (VLANs) or underlay IP packet to be stretched across routed infrastructures. A key advantage is scale: VXLAN uses a 24-bit segment ID (VNI) instead of the 12-bit VLAN ID, supporting around 16 million isolated networks versus the 4,094 VLAN limit. This makes VXLAN ideal for large cloud data centers and multi-tenant environments that demand many distinct network segments. VXLAN’s Layer-2 overlays bring flexibility and mobility to modern architectures. Because VXLAN tunnels can span multiple Layer-3 domains, organizations can extend VLANs across different sites or subnets – for example, creating a tunnel that extends over two data centers over an IP WAN as long as underlying tunnel IP is reachable. This enables seamless workload mobility and disaster recovery: also helps virtual machines or applications can move between physical locations without changing IP addresses, since they remain in the same virtual L2 network. The overlay approach also decouples the logical network from the physical underlay, meaning you can run your familiar L2 segments over any IP routing infrastructure while leveraging features like equal-cost multi-path (ECMP) load balancing and avoiding large spanning-tree domains. In short, VXLAN combines the best of both worlds – the simplicity of Layer-2 adjacency with the scalability of Layer-3 routing – making it a foundational tool in cloud networking and software-defined data centers. Layer-2 VXLAN overlay on a Layer-3 IP network allows customers or edge networks to stretch Ethernet (VLAN) segments across geographically distributed sites using an IP backbone. This approach preserves VLAN tags end-to-end and enables flexible segmentation across locations without needing an extend or continuous Layer-2 network in the core. It also helps hide or avoid the underlying IP network complexities. However, it’s crucial to account for MTU overhead (VXLAN adds ~50 bytes of header) so that the overlay’s VLAN MTU is set smaller than the underlay IP MTU – otherwise fragmentation or packet loss can occur. Additionally, because VXLAN doesn’t inherently signal link status, implementing Bidirectional Forwarding Detection (BFD) on the VXLAN interfaces provides rapid detection of neighbor failures, ensuring quick rerouting or recovery when a tunnel endpoint goes down. VXLAN overlay use case and benefits VXLAN is a standard protocol (IETF RFC 7348) that can encapsulate Layer-2 Ethernet frames into Layer-3 UDP/IP packets. By doing so, VXLAN creates an L2 overlay network on top of an L3 underlay. The VXLAN tunnel endpoints (VTEPs), which can be routers, switches, or hosts, wrap the original Ethernet frame (including its VLAN tag) with an IP/UDP header plus a VXLAN header, then send it through the IP network. The default UDP port for VXLAN is 4789. This mechanism offers several key benefits: Preserves VLAN Tags and L2 Segmentation: The entire Ethernet frame is carried across, so the original VLAN ID (802.1Q tag) is maintained end-to-end through the tunnel. Even if an extra tag is added at the ingress for local tunneling, the customer’s inner VLAN tag remains intact across the overlay. This means a VLAN defined at one site will be recognized at the other site as the same VLAN, enabling seamless L2 adjacency. In practice, VXLAN can transport multiple VLANs transparently by mapping each VLAN or service to a VXLAN Network Identifier (VNI). Flexible network segmentation at scale: VXLAN uses a 24-bit VNI (VXLAN Network ID), supporting about 16 million distinct segments, far exceeding the 4094 VLAN limit of traditional 802.1Q networks. This gives architects freedom to create many isolated L2 overlay networks (for multi-tenant scenarios, application tiers, etc.) over a shared IP infrastructure. Geographically distributed sites can share the same VLANs and broadcast domain via VXLAN, without the WAN routers needing any VLAN configurations. The IP/MPLS core only sees routed VXLAN packets, not individual VLANs, simplifying the underlay configuration. No need for end-to-end VLANs in underlay: Traditional solutions to extend L2 might rely on methods like MPLS/VPLS or long ethernet trunk lines, which often require configuring VLANs across the WAN and can’t scale well. In a VXLAN overlay, the intermediate L3 network remains unaware of customer VLANs, and you don’t need to trunk VLANs across the WAN. Each site’s VTEP encapsulates and decapsulates traffic, so the core routers/switches just forward IP/UDP packets. This isolation improves scalability and stability—core devices don’t carry massive MAC address tables or STP domains from all sites. It also means the underlay can use robust IP routing (OSPF, BGP, etc.) with ECMP, rather than extending spanning-tree across sites. In short, VXLAN lets you treat the WAN like an IP cloud while still maintaining Layer-2 connectivity between specific endpoints. Multi-path and resilience: Since the overlay runs on IP, it naturally leverages IP routing features. ECMP in the underlay, for example, can load-balance VXLAN traffic across multiple links, something not possible with a single bridged VLAN spanning the WAN. The encapsulated traffic’s UDP header even provides entropy (via source port hashing) to help load-sharing on multiple paths. Furthermore, if one underlay path fails, routing protocols can reroute VXLAN packets via alternate paths without disrupting the logical L2 network. This increases reliability and bandwidth usage compared to a Layer-2 only approach. Diagram: VXLAN Overlay Across a Layer-3 WAN – Below is a simplified illustration of two sites using a VXLAN overlay. “Site A” and “Site B” each have a local VLAN (e.g. VLAN 100) that they want to bridge across an IP WAN. The VTEPs at each site encapsulate the Layer-2 frames into VXLAN/UDP packets and send them over the IP network. Inside the tunnel, the original VLAN tag is preserved. In this example, a BFD session (red dashed line) runs between the VTEPs to monitor the tunnel’s health, as explained later. Figure 1: Two sites (A and B) extend “VLAN 100” across an IP WAN using a VXLAN tunnel. The inner VLAN tag is preserved over the L3 network. A BFD keepalive (every 900ms) runs between the VXLAN endpoints to detect failures. The practical effect of this design is that devices in Site A and Site B can be in the same VLAN and IP subnet, broadcast to each other, etc., even though they are connected by a routed network. For example, if Site A has a machine in VLAN 100 with IP 10.1.100.5/24 and Site B has another in VLAN 100 with IP 10.1.100.10/24, they can communicate as if on one LAN – ARP, switches, and VLAN tagging function normally across the tunnel. MTU and overhead considerations One critical consideration for deploying VXLAN overlays is handling the increased packet size due to encapsulation. A VXLAN packet includes additional headers on top of the original Ethernet frame: an outer IP header, UDP header, and VXLAN header (plus an outer Ethernet header on the WAN interface). This encapsulation adds approximately 50 bytes of overhead to each packet (for IPv4; about 70 bytes for IPv6). In practical terms, if your original Ethernet frame was the typical 1500-byte payload (1518 bytes with Ethernet header and CRC, or 1522 with a VLAN tag), the VXLAN-encapsulated version will be ~1550 bytes. **The underlying IP network *must* accommodate these larger frames**, or you’ll get fragmentation or drops. Many network links by default only support 1500-byte MTUs, so without adjustments, a VXLAN carrying a full-sized VLAN packet would exceed that. Though modern networks runs jumbo frames (~9k), if the underlying encapsulated packet frames exceeds 8950 bytes it can create problems like control-plane failure (ex BGP session tear down) or fragmentation for data packet causing out of order packet. Solution: Either raise the MTU on the underlay network or enforce a lower MTU on the overlay. Network architects generally prefer to increase the IP MTU of the core so the overlay can carry standard 1500-byte Ethernet frames unfragmented. For example, one vendor’s guide recommends configuring at least a 1550-byte MTU on all network segments to account for VXLAN’s ~50B overhead. In enterprise environments, it’s common to use “baby jumbo” frames (e.g. 1600 bytes) or full jumbo (9000 bytes) in the datacenter/WAN to accommodate various tunneling overheads. If increasing the underlay MTU is not possible (say, over an ISP that only supports 1500), then the VLAN MTU on the overlay should be reduced – for instance, set the VLAN interface MTU to 1450 bytes, so that even with the 50B VXLAN overhead the outer packet remains 1500 bytes. This prevents any IP fragmentation. Why Fragmentation is Undesirable: VXLAN itself doesn’t include any fragmentation mechanism; it relies on the underlay IP to fragment if needed. But IP fragmentation can harm performance and some devices/drop policies might simply drop oversized VXLAN packets instead of fragmenting. In fact, certain implementations don’t support VXLAN fragmentation or Path MTU discovery on tunnels. The safe approach is to ensure no encapsulated packet ever exceeds the physical MTU. That means planning your MTUs end-to-end: make the core links slightly larger than the largest expected overlay packet. Diagram: VXLAN Encapsulation and MTU Layering – The figure below illustrates the components of a VXLAN-encapsulated frame and how they contribute to packet size. The original Ethernet frame (yellow) with a VLAN tag is wrapped with a new outer Ethernet, IP, UDP, and VXLAN header . The extra headers add ~50 bytes. If the inner (yellow) frame was, say, 1500 bytes of payload plus 18 bytes Ethernet overhead, the outer packet becomes ~1568 bytes (including new headers and FCS). In practice the old FCS is replaced by a new one, so the net growth is ~50 bytes. The key takeaway: the IP transport must handle the total size. Figure 2: Layered view of a VXLAN-encapsulated packet (not to scale). The original Ethernet frame with VLAN tag (yellow) is encapsulated by outer headers (blue/green/red/gray), resulting in ~50 bytes of overhead for IPv4. The outer packet must fit within the WAN MTU (e.g. 1518B if inner frame is 1468B) to avoid fragmentation. In summary, ensure the IP underlay’s MTU is configured to accommodate the VXLAN overhead. If using standard 1500-byte MTUs on the WAN, set your overlay interfaces (VLAN SVIs or bridge MTUs) to around 1450 bytes. In many cases if possible, raising the WAN MTU to 1600 or using jumbo frames throughout is the best practice to provide ample headroom. Always test your end-to-end path with ping sweeps (e.g. using the DF-bit and varying sizes) to verify that the encapsulated packets aren’t being dropped due to MTU limits. Neighbor failure detection with BFD One challenge with overlays like VXLAN is that the logical link lacks immediate visibility into physical link status. If one end of the VXLAN tunnel goes down or the path fails, the other end’s VXLAN interface may remain “up” (since its own underlay interface is still up), potentially blackholing traffic until higher-level protocols notice. VXLAN itself doesn’t send continuous “link alive” messages to check the remote VTEP’s reachability. To address this, network engineers deploy BFD on VXLAN endpoints. BFD is a lightweight protocol specifically designed for rapid failure detection independent of media or routing protocol. It works by two endpoints periodically sending very fast, small hello packets to each other (often every 50ms or less). If a few consecutive hellos are missed, BFD declares the peer down – often within <1 second, versus several seconds (or tens of seconds) with conventional detection. Applying BFD to VXLAN: Many router and switch vendors support running BFD over a VXLAN tunnel or on the VTEP’s loopback adjacencies. When enabled, the two VTEPs will continuously ping each other at the configured interval. If the VXLAN tunnel fails (e.g. one site loses connectivity), BFD on the surviving side will quickly detect the loss of response. This can then trigger corrective actions: for instance, the BFD can generate logs for the logical interface or notify the routing protocol to withdraw routes via that tunnel. In designs with redundant tunnels or redundant VTEPs, BFD helps achieve sub-second failover – traffic can switch to a backup VXLAN tunnel almost immediately upon a primary failure. Even in a single-tunnel scenario, BFD gives an early alert to the network operator or applications that the link is down, rather than quietly dropping packets. Example: If Site A and Site B have two VXLAN tunnels (primary and backup) connecting them, running BFD on each tunnel interface means that if the primary’s path goes down, BFD at Site A and B will detect it within milliseconds and inform the routing control-plane. The network can then shift traffic to the backup tunnel right away. Without BFD, the network might have to wait for a timeout (e.g. OSPF dead interval or even ARP timeouts) to realize the primary tunnel is dead, causing a noticeable outage. BFD is protocol-agnostic – it can integrate with any routing protocols. For VXLAN, it’s purely a monitoring mechanism: lightweight and with minimal overhead on the tunnel. Its messages are small UDP packets (often on port 3784/3785) that can be sourced from the VTEP’s IP. The frequency is configurable based on how fast you need detection vs. how much overhead you can afford; common timers are 300ms with 3x multiplier (detect in ~1s) for moderate speeds, or even 50ms with 3x (150ms detection) for high-speed failover requirements. Bottom line: Implementing BFD dramatically improves the reliability of a VXLAN-based L2 extension. Since VXLAN tunnels don’t automatically signal if a neighbor is unreachable, BFD acts as the heartbeat. Many platforms even allow BFD to directly influence interface state (for example, the VXLAN interface can be tied to go down when BFD fails) so that any higher-level protocols (like VRRP, dynamic routing, etc.) immediately react to the loss. This prevents lengthy outages and ensures the overlay network remains robust even over a complex WAN. Conclusion Deploying a Layer-2 VXLAN overlay across a Layer-3 WAN unlocks powerful capabilities: you can keep using familiar VLAN-based segmentation across sites while taking advantage of an IP network’s scalability and resilience. It’s a vendor-neutral solution widely supported in modern networking gear. By preserving VLAN tags over the tunnel, VXLAN makes it possible to stretch subnets and broadcast domains to remote locations for workloads that require Layer-2 adjacency. With the huge VNI address space, segmentation can scale for large enterprises or cloud providers well beyond traditional VLAN limits. However, to realize these benefits successfully, careful attention must be paid to MTU and link monitoring. Always accommodate the ~50-byte VXLAN overhead by configuring proper MTUs (or adjusting the overlay’s MTU) – this prevents fragmentation and packet loss that can be very hard to troubleshoot after deployment. And since a VXLAN tunnel’s health isn’t apparent to switches/hosts by default, use tools like BFD to add fast failure detection, thereby avoiding black holes and improving convergence times. In doing so, you ensure that your stretched network is not only functional but also resilient and performant. By following these guidelines – leveraging VXLAN for flexible L2 overlays, minding the MTU, and bolstering with BFD – network engineers can build a robust, wide-area Layer-2 extension that behaves nearly indistinguishably from a local LAN, yet rides on the efficiency and reliability of a Layer-3 IP backbone. Enjoy the best of both worlds: VLANs without borders, and an IP network without unnecessary constraints. References: VXLAN technical overview and best practices from vendor documentation and industry sources have been used to ensure accuracy in the above explanations. This ensures the blog is grounded in real-world proven knowledge while remaining vendor-neutral and applicable to a broad audience of cloud and network professionals.Inter-Hub Connectivity Using Azure Route Server
By Mays_Algebary shruthi_nair As your Azure footprint grows with a hub-and-spoke topology, managing User-Defined Routes (UDRs) for inter-hub connectivity can quickly become complex and error-prone. In this article, we’ll explore how Azure Route Server (ARS) can help streamline inter-hub routing by dynamically learning and advertising routes between hubs, reducing manual overhead and improving scalability. Baseline Architecture The baseline architecture includes two Hub VNets, each peered with their respective local spoke VNets as well as with the other Hub VNet for inter-hub connectivity. Both hubs are connected to local and remote ExpressRoute circuits in a bowtie configuration to ensure high availability and redundancy, with Weight used to prefer the local ExpressRoute circuit over the remote one. To maintain predictable routing behavior, the VNet-to-VNet configuration on the ExpressRoute Gateway should be disabled. Note: Adding ARS to an existing Hub with Virtual Network Gateway will cause downtime that expect to last 10 minutes. Scenario 1: ARS and NVA Coexist in the Hub Option A: Full Traffic Inspection ARS and NVA Coexist in the Hub In this scenario, ARS is deployed in each Hub VNet, alongside the Network Virtual Appliances (NVAs). NVA1 in Region1 establishes BGP peering with both the local ARS (ARS1) and the remote ARS (ARS2). Similarly, NVA2 in Region2 peers with both ARS2 (local) and ARS1 (remote). Let’s break down what each BGP peering relationship accomplishes. For clarity, we’ll focus on Region1, though the same logic applies to Region2: NVA1 Peering with Local ARS1 Through BGP peering with ARS1, NVA1 dynamically learns the prefixes of Spoke1 and Spoke2 at the OS level, eliminating the need to manually configure these routes. The same applies for NVA2 learning Spoke3 and Spoke4 prefixes via its BGP peering with ARS2. NVA1 Peering with Remote ARS2 When NVA1 peers with ARS2, the Spoke1 and Spoke2 prefixes are propagated to ARS2. ARS2 then injects these prefixes into NVA2 at both the NIC level with NVA1 as the next hop, and at the OS level. This mechanism removes the need for UDRs on the NVA subnets to enable inter-hub routing. Additionally, ARS2 advertises the Spoke1 and Spoke2 prefixes to both ExpressRoute circuits (EXR2 and EXR1 due to bowtie configuration) via GW2, making them reachable from on-premises through either EXR1 or EXR2. 👉Important: To ensure that ARS2 accepts and propagates Spoke1/Spoke2 prefixes received via NVA1, AS-Override must be enabled. Without AS-Override, BGP loop prevention will block these routes at ARS2, since both ARS1 and ARS2 use the default ASN 65515, and ARS2 will consider the route as already originated locally. The same principle applies in reverse for Spoke3 and Spoke4 prefixes being advertised from NVA2 to ARS1. Traffic Flow Inter-Hub Traffic: Spoke VNets are configured with UDRs that contain only a default route (0.0.0.0/0) pointing to the local NVA as the next hop. Additionally, the “Propagate Gateway Routes” setting should be set to False to ensure all traffic, whether East-West (intra-hub/inter-hub) or North-South (to/from internet), is forced through the local NVA for inspection. Local NVAs will have the next hop to the other region spokes injected at the NIC level by local ARS, pointing to the other region NVA, for example NVA2 will have next hop to Spoke1 and Spoke2 as NVA1 (10.0.1.4) and vice versa. Why are UDRs still needed on spokes if ARS handles dynamic routing? Even with ARS in place, UDRs are required to maintain control of the next hop for traffic inspection. For instance, if Spoke1 and Spoke2 do not have UDRs, they will learn the remote spoke prefixes (e.g., Spoke3/Spoke4) injected via ARS1, which received them from NVA2. This results in Spoke1/Spoke2 attempting to route traffic directly to NVA2, a path that is invalid, since the spokes don’t have the path to NVA2. The UDR ensures traffic correctly routes through NVA1 instead. On-Premises Traffic: To explain the on-premises traffic flow, we'll break it down into two directions: Azure to on-premises, and on-premises to Azure. Azure to On-Premises Traffic Flow: As previously noted, Spokes send all traffic, including traffic to on-premises, via NVA1 due to the default route in the UDR. NVA1 then routes traffic to the local ExpressRoute circuit, using Weight to prefer the local path over the remote. Note: While NVA1 learns on-premises prefixes from both local and remote ARSs at the OS level, this doesn’t affect routing decisions. The actual NIC-level route injection determines the next hop, ensuring traffic is sent via the correct path—even if the OS selects a different “best” route internally. The screenshot below from NVA1 shows four next hops to the on-premises network 10.2.0.0/16. These include the local ARS (ARS1: 10.0.2.5 and 10.0.2.4) and the remote ARS (ARS2: 10.1.2.5 and 10.1.2.4). On-Premises to Azure Traffic Flow In a bowtie ExpressRoute configuration, Azure VNet prefixes are advertised to on-premises through both local and remote ExpressRoute circuits. Because of this dual advertisement, the on-premises network must ensure optimal path selection when routing traffic to Azure. From Azure side, to maintain traffic symmetry, add UDRs at the GatewaySubnet (GW1 and GW2) with specific routes to the local Spoke VNets, using the local NVA as the next hop. This ensures return traffic flows back through the same path it entered. 👉How Does the ExpressRoute Edge Router Select the Optimal Path? You might ask: If Spoke prefixes are advertised by both GW1 and GW2, how does the ExpressRoute edge router choose the best path? (e.g., diagram below shows EXR1 learns Region1 prefixes from GW1 and GW2) Here’s how: Edge routers (like EXR1) receive the same Spoke prefixes from both gateways. However, these routes have different AS-Path lengths: - Routes from the local gateway (GW1) have a shorter AS-Path. - Routes from the remote gateway (GW2) have a longer AS-Path because NVA1’s ASN (e.g., 65001) is prepended twice as part of the AS-Override mechanism. As a result, the edge router (EXR1) will prefer the local path from GW1, ensuring efficient and predictable routing. For example: EXR1 receives Spoke1, Spoke2, and Hub1-VNet prefixes from both GW1 and GW2. But because the path via GW1 has a shorter AS-Path, EXR1 will select that as the best route. (Refer to the diagram below for a visual of the AS-Path difference). Final Traffic Flow: Option-A Insights: This design simplifies UDR configuration for inter-hub routing, especially useful when dealing with non-contiguous prefixes or operating across multiple hubs. For simplicity, we used a single NVA in each Hub-VNet while explaining the setup and traffic flow throughout this article. However, a high available (HA) NVA deployment is recommended. To maintain traffic symmetry in an HA setup, you’ll need to enable the next-hop IP feature when peering with Azure Route Server (ARS). When on-premises traffic inspection is required, the UDR setup in the GatewaySubnet becomes more complex as the number of Spokes increases. Additionally, each route table is currently limited to 600 UDR entries. As your Azure network scales, keep in mind that Azure Route Server supports a maximum of 8 BGP peers per instance (as of the time writing this article). This limit can impact architectures involving multiple NVAs or hubs. Option B: Bypass On-Premises Inspection If on-premises traffic inspection is not required, NVAs can advertise a supernet prefix summarizing the local Spoke VNets to the remote ARS. This approach provides granular control over which traffic is routed through the NVA and eliminates the need for BGP peering between the local NVA and local ARS. All other aspects of the architecture remain the same as described in Option A. For example, NVA2 can advertise the supernet 192.168.2.0/23 (supernet of Spoke3 and Spoke4) to ARS1. As a result, Spoke1 and Spoke2 will learn this route with NVA2 as the next hop. To ensure proper routing (as discussed earlier) and inter-hub inspection, you need apply a UDR in Spoke1 and Spoke2 that overrides this exact supernet prefix, redirecting traffic to NVA1 as the next hop. At the same time, traffic destined for on-premises will follow the system route through the local ExpressRoute gateway, bypassing NVA1 altogether. In this setup: UDRs on the Spokes should have "Propagate Gateway Routes" set to True. No UDRs are needed in the GatewaySubnet. 👉Can NVA2 Still Advertise Specific Spoke Prefixes? You might wonder: Can NVA2 still advertise specific prefixes (e.g., Spoke3 and Spoke4) learned from ARS2 to ARS1 instead of a supernet? Yes, this is technically possible, but it requires maintaining BGP peering between NVA2 and ARS2. However, this introduces UDR complexity in Spoke1 and Spoke2, as you'd need to manually override each specific prefix. This also defeats the purpose of using ARS for simplified route propagation, undermining the efficiency and scalability of the design. Bypass On-Premises Inspection Final Traffic Flow: Option B: Bypass on-premises inspection traffic flow Option-B Insights: This approach reduces the number of BGP peerings per ARS. Instead of maintaining two BGP sessions (local NVA and remote NVA) per Hub, you can limit it to just one, preserving capacity within ARS’s 8-peer limit for additional inter-hub NVA peerings. Each NVA should advertise a supernet prefix to the remote ARS. This can be challenging if your Spokes don’t use contiguous IP address spaces, as described in Option B. Scenario 2: ARS in the Hub and the NVA in Transit VNet In Scenario 1, we highlighted that when on-premises inspection is required, managing UDRs at the GatewaySubnet becomes increasingly complex as the number of Spoke VNets grows. This is due to the need for UDRs to include specific prefixes for each Spoke VNet. In this scenario, we eliminate the need to apply UDRs at the GatewaySubnet altogether. In this design, the NVA will be deployed in Transit VNet, where: Transit-VNet will be peered with local Spoke VNets and with the local Hub-VNet to enable intra-Hub and on-premises connectivity. Transit-VNet also peered with remote Transit VNets (e.g., Transit-VNet1 peered with Transit-VNet2) to handle inter-Hub connectivity through the NVAs. Additionally, Transit-VNets are peered with remote Hub-VNets, to establish BGP peering with the remote ARS. NVAs OS will need to add static routes for the local Spoke VNets prefixes, it can be specific or it can supernet prefix, which will later be advertised to ARSs over BGP Peering, then ARS will advertise it to on-premises via ExpressRoute. NVAs will BGP peer with local ARS and also with the remote ARS. To understand the reasoning behind this design, let’s take a closer look at the setup in Region1, focusing on how ARS and NVA are configured to connect to Region2. This will help illustrate both inter-hub and on-premises connectivity. The same concept applies in reverse from Region2 to Region1. Inetr-Hub: To enable NVA1 in Region1 to learn prefixes from Region2, NVA2 will configure static routes at the OS level for Spoke3 and Spoke4 (or their supernet prefix) and advertise them to ARS1 via remote BGP peering. As a result, these prefixes will be received by NVA1, both at the NIC level, with NVA2 as the next hop, and at the OS level for proper routing. Spoke1 and Spoke2 will have a UDR with a default route pointing to NVA1 as the next hop. For instance, when Spoke1 needs to communicate with Spoke3, the traffic will first route through NVA1. NVA1 will then forward the traffic to NVA2 using VNet peering between the two Hubs. A similar configuration will be applied in Region2, where NVA1 will configure static routes at the OS level for Spoke1 and Spoke2 (or their supernet prefix) and advertise them to ARS2 via remote BGP peering, as a result, these prefixes will be received by NVA2, both at the NIC level (injected by ARS2), with NVA1 as the next hop, and at the OS level for proper routing. Note: At the OS level, NVA1 learns Spoke3 and Spoke4 prefixes from both local and remote ARSs. However, the NIC-level route injection determines the actual next hop, so even if the OS selects a different best route, it won’t affect forwarding behavior. same applies to NVA2. On-Premises Traffic: To explain the on-premises traffic flow, we'll break it down into two directions: Azure to on-premises, and on-premises to Azure. Azure to On-Premises Traffic Flow: Spokes in Region1 route all traffic through NVA1 via a default route defined in their UDRs. Because of BGP peering between NVA1 and ARS1, ARS1 advertises the Spoke1 and Spoke2 (or their supernet prefix) to on-premises through ExpressRoute (EXR1). The Transit-VNet1 (hosting NVA1) is peered with Hub1-VNet, with “Use Remote Gateway” enabled. This allows NVA1 to learn on-premises prefixes from the local ExpressRoute gateway (GW1), and traffic to on-premises is routed through the local ExpressRoute circuit (EXR1) due to higher BGP Weight configuration. Note: At the OS level, NVA1 learns on-prem prefixes from both local and remote ARSs. However, the NIC-level route injection determines the actual next hop, so even if the OS selects a different best route, it won’t affect forwarding behavior. same applies to NVA2. On-Premises to Azure Traffic Flow: Through BGP peering with ARS1, NVA1 enables ARS1 to advertise Spoke1 and Spoke2 (or their supernet prefix) to both EXR1 and EXR2 circuits (due to the ExpressRoute bowtie setup). Additionally, due to BGP peering between NVA1 and ARS2, ARS2 also advertises Spoke1 and Spoke2 (or their supernet prefix) to EXR2 and EXR1 circuits. As a result, both ExpressRoute edge routers in Region1 and Region2 learn the same Spoke prefixes (or their supernet prefix) from both GW1 and GW2, with identical AS-Path lengths, as shown below. EXR1 learns Region1 Spokes's supernet prefixes from GW1 and GW2 This causes non-optimal inbound routing, where traffic from on-premises destined to Region1 Spokes may first land in Region2’s Hub2-VNet before traversing to NVA1 in Region1. However, return traffic from Spoke1 and Spoke2 will always exit through Hub1-VNet. To prevent suboptimal routing, configure NVA1 to prepend the AS path for Spoke1 and Spoke2 (or their supernet prefix) when advertising them to the remote ARS2. Likewise, ensure NVA2 prepends the AS path for Spoke3 and Spoke4 (or their supernet prefix) when advertising to ARS1. This approach helps maintain optimal routing under normal conditions and during ExpressRoute failover scenarios. Below diagram shows NVA1 is setting AS-Prepend for Spoke1 and Spoke2 supernet prefix when BGP peer with remote ARS (ARS1), same will apply for NVA2 when advertising Spoke3 and Spoke4 prefixes to ARS1. Final Traffic Flow: Full Inspection: Traffic flow when NVA in Transit-VNet Insights: This solution is ideal when full traffic inspection is required. Unlike Scenario 1 - Option A, it eliminates the need for UDRs in the GatewaySubnet. When ARS is deployed in a VNet (typically in Hub VNets), the VNet will be limited to 500 VNet peerings (as of the time writing this article). However, in this design, Spokes peer with the Transit-VNet instead of directly with the ARS VNet, allowing you to scale beyond the 500-peer limit by leveraging Azure Virtual Network Manager (AVNM) or submitting a support request. Some enterprise customers may encounter the 1,000-route advertisement limit on the ExpressRoute circuit from the ExpressRoute gateway. In traditional hub-and-Spoke designs, there's no native control over what is advertised to ExpressRoute. With this architecture, NVAs provide greater control over route advertisement to the circuit. For simplicity, we used a single NVA in each Hub-VNet while explaining the setup and traffic flow throughout this article. However, a high available (HA) NVA deployment is recommended. To maintain traffic symmetry in an HA setup, you’ll need to enable the next-hop IP feature when peering with Azure Route Server (ARS). This design does require additional VNet peerings, including: Between Transit-VNets (inter-region), Between Transit-VNets and local Spokes, and Between Transit-VNets and both local and remote Hub-VNets.
