Inquiry on Inter-region Routing Interference and Failover Performance during vHub Updates in Azure Virtual WAN.

Hi 이재옥,

For production Active-Active vWAN deployments across regions like Korea Central and Korea South, here’s what you can expect when performing write operations (such as adding VNet connections) on one vHub, based on how Azure Virtual WAN behaves in large enterprise deployments and Microsoft’s architecture guidance:

1. Inter-region Propagation Interference:
   • Yes, there can be minor ripple effects on the peer vHub even though its gateway stays physically stable. When you update vHub A, its gateway restarts locally, but vHub A withdraws routes from the global vWAN fabric.
   • This triggers the hub-to-hub mesh to update, causing vHub B to receive route withdrawals and new advertisements. ECMP paths recalculate across the fabric. In production, this typically means a few seconds of micro packet loss or slight latency increase on vHub B not a full outbreak.
   • The gateway on vHub B does not reset and its BGP session stays up, but route churn does occur because vWAN is a single global SD-WAN fabric, not independent regional hubs.
2. Real-world Failover Convergence Time:
   • When vHub A’s VPN or ExpressRoute tunnels reset, your on-premises edge devices typically failover to vHub B’s tunnels in 15 to 60 seconds for planned write operations. For hard unplanned tunnel failures, expect 30 to 90 seconds.
   • The worst-case scenario (only BGP hold timer expiry) is around 180 seconds, but that’s uncommon because Azure vWAN uses internal tunnel health probes, IPsec/IKE liveness checks, and gateway health monitoring in addition to BGP.
   • Azure vWAN BGP timers are fixed: Keepalive is 60 seconds and Hold Time is 180 seconds. Importantly, Azure vWAN does not support BFD, so enabling BFD on your on-premises edge does not give sub-second failover toward Azure the CPE cannot accelerate Azure’s internal failure detection.
3. Routing Intent Synchronization and Asymmetric Routing
   • Yes, asymmetric routing and session drops can occur if routing intent isn’t perfectly synchronized between hubs. When you modify Routing Intent on one hub in a Secure Hub environment (with firewall or NVA), the routing tables between hubs may temporarily diverge.
   • Traffic from on-premises may egress one hub while return traffic takes the other, breaking stateful sessions.
   • To avoid this, always configure identical Routing Intent on both hubs before making changes, enable Inter-hub routing in the routing intent if branches need to use either hub, and avoid changing routing intent on both hubs simultaneously. The default route (0.0.0.0/0) can propagate hub-to-hub, but whether it does depends on your Secure Hub policy configuration (such as Internet security enabled and route table associations), not a hard architectural limit.
4. Operational Strategy for Production Yes, you should perform updates sequentially (rolling updates) across regions to minimize impact. Here’s the recommended approach:
   • Update vHub A (Korea Central) by adding VNet connections or modifying routing
   • Wait for the gateway to stabilize (about 1–2 minutes)
   • Wait for BGP convergence and confirm your on-prem edge has failovered to vHub B if needed (typical convergence: 15–60 seconds)
   • Verify traffic flows and check for asymmetric routing or session drops
   • Then update vHub B (Korea South) and repeat the same steps

Never update both hubs simultaneously, because you risk total connectivity loss during overlapping gateway restart windows. Sequential updates ensure at least one hub is always stable, which matches how large enterprises operate multi-region Active-Active vWAN in production.

Key Gotchas to Prepare For

• Point-to-Site clients may experience TCP 443 drops during vHub restart, so schedule during maintenance windows and inform users
• Site-to-Site tunnels will reconnect and BGP sessions reset, so ensure you have active-active tunnels to both hubs
• Route propagation delay of 2–5 minutes for full peering sync, so wait before performing subsequent operations
• Asymmetric routing after Routing Intent changes can cause session drops, so pre-sync routing intent on both hubs
• Save configuration snapshots before making routing intent changes so you can rollback if needed
• Prefer GCMAES256 encryption if both ends support it for optimal performance, though AES256/SHA256 won’t inherently cause packet drops if your CPE supports it

Final Mental Model

Think of vWAN as one global SD-WAN fabric with regional gateway edges. Gateway reset is local to the updated vHub, but route convergence is global across the mesh.

Expect 15–60 seconds of failover time during planned write operations, plan for short route churn where the peer vHub may see micro packet loss, never rely on BFD to speed up Azure-side detection, keep routing intent identical on both hubs before any change, and always use rolling updates in production.

Official Microsoft Documentation:

• Azure Virtual WAN Overview
• About Virtual Hub Routing
• Azure Virtual WAN FAQ
• How to Configure Virtual WAN Hub Routing Policies
• Virtual WAN Routing Deep Dive
• Azure Virtual WAN Configuration Best Practices
• Azure Well-Architected Framework: Virtual WAN

Kindly let us know if the above helps or you need further assistance on this issue.

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