ExpressRoute Global Reach

ExpressRoute Global Reach: Mastering Hybrid Network Interconnectivity

Introduction: The Challenge of Global Network Latency

In modern enterprise architecture, the reliance on cloud infrastructure is no longer confined to a single geographic region. Organizations frequently operate across multiple continents, requiring their on-premises data centers to communicate with cloud resources that are physically separated by thousands of miles. Traditionally, connecting these distributed environments involved complex routing through the public internet or expensive, manually managed site-to-site VPN meshes. These methods often introduce unacceptable latency, jitter, and security risks, which can cripple performance-sensitive applications.

ExpressRoute Global Reach is a feature of Azure ExpressRoute designed specifically to solve this problem by leveraging the massive, private Microsoft global network. Instead of forcing traffic to hairpin through a specific region or traverse the public internet, Global Reach allows you to link your ExpressRoute circuits together. This creates a private, high-speed, and low-latency backbone that effectively connects your on-premises sites to each other through the Microsoft edge. By understanding how to design and implement this service, you can simplify your wide-area network (WAN) architecture while significantly improving the performance of your distributed hybrid applications.

Understanding the Architecture of Global Reach

To grasp ExpressRoute Global Reach, you must first understand the standard ExpressRoute model. An ExpressRoute circuit provides a dedicated, private connection between your on-premises infrastructure and the Microsoft cloud. In a standard setup, this connection is "private" only between your local network and the Azure region it connects to. If you have two different data centers connected to two different Azure regions, they cannot "see" each other through the Microsoft network natively.

Global Reach changes this by enabling the exchange of BGP (Border Gateway Protocol) routes between your two circuits. When enabled, your local routers at Site A learn the routes for the network at Site B through the Microsoft global backbone. The traffic flows from your local router into the Microsoft edge, travels across the private fiber-optic network, and exits through the other ExpressRoute circuit into your second on-premises data center. This architecture effectively turns the Microsoft global network into your private WAN provider.

Callout: Global Reach vs. Site-to-Site VPN While a Site-to-Site VPN creates a tunnel over the public internet, Global Reach operates on dedicated, private fiber. VPNs are subject to the congestion and unpredictability of public internet routing, whereas Global Reach benefits from the deterministic performance of Microsoft’s managed infrastructure. Furthermore, Global Reach avoids the encryption overhead associated with IPsec VPNs, resulting in higher throughput and lower latency for your traffic.

Prerequisites and Requirements

Before you can implement Global Reach, there are several foundational requirements that must be met. Ignoring these often leads to configuration failures or unexpected routing behavior during the deployment phase.

• Circuit Ownership: You must have two or more ExpressRoute circuits. These circuits do not necessarily need to be in the same subscription, but they must be managed under the same Azure tenant.
• Provider Support: Your connectivity provider must support Global Reach. While most major providers do, it is essential to verify this with your account representative before starting.
• BGP Configuration: Your on-premises routers must be configured to support BGP peering. Global Reach relies entirely on BGP to exchange routing information between the two circuits.
• ASN Requirements: You must ensure that the Autonomous System Numbers (ASNs) used on your local networks do not conflict. If your sites use private ASNs, ensure they are unique or correctly handled within your routing policy.
• Regional Availability: While Global Reach is available in most regions, you should verify that the specific peering locations you are using support the feature.

Tip: Always check the Azure regional availability documentation before planning your deployment. Some newer or specialized regions may have different support tiers for Global Reach connectivity.

Step-by-Step Implementation Guide

Implementing Global Reach is a process that involves linking two existing, functional ExpressRoute circuits. This guide assumes you have two circuits already established and peered with your on-premises routers.

1. Identifying the Resource IDs

The first step in the process is obtaining the Resource IDs for both ExpressRoute circuits. You will need these to create the authorization link. You can find these IDs in the Azure Portal under the "Properties" blade of each ExpressRoute circuit, or by using the Azure CLI.

Using the Azure CLI, you can list your circuits to identify the IDs:

az network express-route list --query "[].{Name:name, ID:id}" --output table

2. Creating an Authorization

Global Reach requires an authorization to be created on one circuit, which is then used to connect to the second circuit. Think of this as a "key" that allows the two circuits to communicate.

Run the following command to create the authorization on the first circuit:

az network express-route auth create \
    --circuit-name MyCircuit1 \
    --name MyGlobalReachAuth \
    --resource-group MyResourceGroup

3. Linking the Circuits

Once the authorization is created, you must capture the Authorization Key and the Resource ID of the first circuit. You then use these to link the second circuit to the first.

# Capture the Auth Key
auth_key=$(az network express-route auth show \
    --circuit-name MyCircuit1 \
    --name MyGlobalReachAuth \
    --resource-group MyResourceGroup \
    --query "authorizationKey" -o tsv)

# Link the second circuit
az network express-route peering connection create \
    --circuit-name MyCircuit2 \
    --connection-name MyGlobalReachConnection \
    --resource-group MyResourceGroup \
    --peer-circuit MyCircuit1_ResourceID \
    --authorization-key $auth_key

4. Verification and Routing

After executing these commands, the circuits will begin exchanging routes. You should monitor your local edge routers to ensure that the prefixes from Site A appear in the routing table of Site B, and vice versa. Use standard BGP diagnostic commands on your router, such as show ip bgp summary or show ip bgp neighbors, to confirm the peering state is established.

Routing and Traffic Flow Management

A common misconception is that Global Reach automatically routes all traffic between sites. In reality, it only facilitates the exchange of BGP routes. The actual traffic flow is determined by your local BGP configuration and the metrics you set on your routers.

Controlling Traffic Paths

If you have multiple paths to reach a destination (for example, a secondary VPN backup and the primary Global Reach link), you must manipulate BGP attributes like AS-Path Prepending or Local Preference. By increasing the AS-Path length on the less desirable path, you can force traffic to prefer the Global Reach connection.

Asymmetric Routing

Asymmetric routing is a significant risk in hybrid networking. This occurs when traffic from Site A to Site B takes the Global Reach path, but the return traffic from Site B to Site A takes a different route (like a VPN). This often causes stateful firewalls to drop packets, as they only see one half of the conversation. To avoid this, ensure your BGP policies are symmetric across both sites.

Warning: Always test your routing path with traceroute or mtr after enabling Global Reach. If you observe traffic leaving via an unexpected path, stop and re-examine your BGP metrics immediately to prevent connectivity issues.

Security Considerations

Since Global Reach extends your private network across the Microsoft backbone, security must be handled at the edge. The traffic traversing the Microsoft network is not encrypted by default. If your corporate policy requires data-in-transit encryption, you must implement an additional layer, such as MACsec (if supported by your provider) or an IPsec tunnel running over the ExpressRoute connection.

Network Segmentation

Consider using VRFs (Virtual Routing and Forwarding) on your edge routers to segment traffic. By placing your Global Reach prefixes into a specific VRF, you can ensure that only authorized traffic reaches your internal core network, effectively creating a "DMZ" for your hybrid connectivity.

Access Control Lists (ACLs)

Do not rely on the privacy of the ExpressRoute circuit as your only security control. Implement robust ACLs on your on-premises firewalls to restrict traffic between the subnets at Site A and Site B. Only allow the specific ports and protocols necessary for your applications to function.

Comparison: ExpressRoute Connectivity Options

Best Practices for Deployment and Operations

1. Monitor BGP Health: Set up alerts for BGP state changes on your edge routers. If a peering session drops, your traffic may fail over to a more expensive or slower path without you realizing it.
2. Plan for Failover: Always have a secondary, non-Global Reach path (like a VPN) configured as a backup. Use BGP metrics to ensure this path is only used when the primary ExpressRoute is down.
3. Document Routing Policies: Routing tables can become complex quickly. Maintain an updated diagram of your BGP Autonomous Systems and the prefix advertisements for every site.
4. Use Automation: If you manage multiple sites, use Infrastructure-as-Code (IaC) tools like Terraform or Bicep to deploy your Global Reach connections. This ensures consistency and makes it easier to audit your configuration.
5. Bandwidth Planning: Remember that Global Reach traffic consumes the bandwidth of both ExpressRoute circuits. Ensure your circuit capacity is sufficient to handle both the traffic to the Azure region and the traffic destined for the remote on-premises site.

Troubleshooting Common Pitfalls

Even with careful planning, issues can arise. Below are the most common scenarios that administrators encounter when working with Global Reach.

1. BGP Peering Fails to Establish

If the BGP session doesn't come up, the first step is to check the peering configuration. Ensure that the ASN and the IP addresses match the expected values on both ends. Verify that there are no firewalls or ACLs blocking BGP traffic (port 179) between your local routers and the Microsoft edge.

2. Routes Are Not Propagated

If the BGP session is up but you don't see the expected routes, check your route filters. Sometimes, specific route maps on your local routers might be filtering out incoming or outgoing updates. Also, confirm that you have enabled the "Global Reach" setting on the ExpressRoute circuit itself, as this is a toggle that must be explicitly turned on.

3. High Latency on the Path

If you experience performance issues, run a traceroute. If the traffic is hitting public internet nodes, your routing is leaking, and traffic is not staying on the private backbone. If the latency is high but the path is correct, check for packet loss on the physical local loop to the provider's edge.

4. Overlapping Subnets

A classic mistake is having the same IP address range at both on-premises sites. BGP cannot distinguish between these routes if they are identical. You must re-IP one of the sites or use Network Address Translation (NAT) to resolve the conflict, though re-IPing is strongly preferred for long-term stability.

Advanced Configuration: Using Route Filters and Maps

In large-scale environments, you may not want every site to learn every route from every other site. This is where advanced BGP route maps become essential. You can apply route maps to your BGP neighbors to filter prefixes.

For example, if you have a hub-and-spoke network, you might want the hub to see all routes, but the spokes to only see a default route or a specific subset of the network. By applying a distribute-list or route-map on your edge router, you can control exactly what enters your routing table.

! Example of a route map to filter specific prefixes
route-map FILTER_GLOBAL_REACH permit 10
 match ip address prefix-list ALLOWED_NETWORKS

router bgp 65001
 neighbor 10.0.0.1 route-map FILTER_GLOBAL_REACH in

This configuration ensures that only the networks you explicitly allow are accepted into your local routing table, preventing potential routing loops or unwanted traffic exposure.

Scaling Your Global Reach Infrastructure

As your organization grows, you might need to connect more than two sites. Global Reach supports a "mesh" topology, where you can link multiple circuits together. However, keep in mind that the complexity of managing a full mesh grows exponentially.

If you find yourself managing more than 5-10 sites, consider transitioning to a managed SD-WAN solution that integrates with ExpressRoute. Many modern SD-WAN vendors provide virtual appliances that can sit in your Azure VNETs and orchestrate the routing between sites, providing a layer of abstraction over the underlying BGP configurations.

The Role of ExpressRoute Gateways

It is important to clarify that Global Reach does not require an ExpressRoute Gateway to be present in your VNETs to function. The connection is established between the circuits themselves. However, if you also need your VNETs to communicate with both on-premises sites, you will need to ensure your ExpressRoute Gateways are configured to allow "Gateway Transit."

When Gateway Transit is enabled, your VNET acts as a transit point. This is useful for smaller deployments but can lead to congestion if the VNET gateway becomes a bottleneck. Always monitor the throughput of your Virtual Network Gateways if you intend to use them for cross-premises traffic flow.

Callout: The Importance of Deterministic Routing

Callout: Deterministic Routing Unlike the public internet, where paths change dynamically based on ISP peering, ExpressRoute Global Reach provides a deterministic path. Because you control the BGP advertisements, you can guarantee that traffic from Site A to Site B will always follow the same, predictable path through the Microsoft backbone. This is critical for applications that require consistent latency, such as database replication or real-time voice and video traffic.

Key Takeaways

1. Private Backbone Efficiency: ExpressRoute Global Reach allows you to bypass the public internet and use the Microsoft private fiber network to connect on-premises sites, drastically reducing latency and improving reliability.
2. BGP-Centric Architecture: The service is built entirely on BGP. Success in implementation relies on your ability to configure, troubleshoot, and manipulate BGP attributes to control traffic paths and avoid asymmetry.
3. Security is Shared: While the transport is private, it is not encrypted by default. Always augment Global Reach with additional security layers like firewalls, ACLs, or IPsec tunnels if your compliance requirements demand it.
4. Strategic Planning: Avoid IP address overlaps and ensure your circuit capacity is sized to handle both cloud-bound and site-to-site traffic. Proper planning prevents routing loops and performance bottlenecks.
5. Operational Monitoring: Implement proactive monitoring for your BGP peering state. Because Global Reach is a critical part of your WAN, you must be the first to know if a path fails so you can manage the failover to backup connections.
6. Scalability and Management: For large-scale deployments, utilize Infrastructure-as-Code (IaC) and consider SD-WAN integration to manage the growing complexity of routing tables and connectivity policies.
7. Symmetry Matters: Always design your routing to be symmetric. Asymmetric routing is the most common cause of "silent" network failures in hybrid environments, often leading to dropped connections that are notoriously difficult to debug.

Frequently Asked Questions (FAQ)

Q: Does Global Reach cost extra? A: Yes, Global Reach incurs additional costs based on the amount of data transferred and the capacity of the circuits involved. Check the Azure pricing calculator for the most up-to-date regional rates.

Q: Can I use Global Reach with a VPN circuit? A: No, Global Reach specifically requires two ExpressRoute circuits. It cannot be used to link an ExpressRoute circuit to a standard Site-to-Site VPN.

Q: What happens if one of my ExpressRoute circuits goes down? A: If the circuit goes down, the BGP session will drop, and the routes learned via that circuit will be removed from your routing table. If you have a backup path (like a VPN) configured with a higher metric, your routers will automatically shift traffic to that path.

Q: Is there a limit to how many circuits I can link? A: There are limits defined by the specific circuit type and regional policies. Always consult the Azure subscription limits documentation for the current maximums applicable to your subscription.

Q: Do I need to enable Global Reach on both circuits? A: The feature must be enabled on the circuits, and the linking process must be performed. The configuration is effectively a bilateral agreement between the two circuits, which is why the authorization key process is required.

Conclusion

ExpressRoute Global Reach is a powerful tool for any organization looking to optimize its wide-area network performance. By treating the Microsoft global network as a private WAN, you gain the ability to move data between geographically dispersed sites with the speed and reliability of a dedicated fiber connection. While the setup requires a solid understanding of BGP and routing fundamentals, the resulting performance gains and simplified architecture are well worth the effort.

As you implement this in your own environment, remember to focus on the basics: clean BGP configurations, symmetric routing, and proactive monitoring. By following these best practices, you can build a resilient, high-performance hybrid network that serves your business needs today and scales effectively for tomorrow.