ExpressRoute Peering Types

ExpressRoute Peering Types: A Comprehensive Guide

Introduction: Why ExpressRoute Peering Matters

In the world of cloud architecture, the connection between your on-premises data center and your cloud provider is the backbone of your operations. Azure ExpressRoute provides a private, dedicated connection that bypasses the public internet, offering higher reliability, faster speeds, and lower latencies. However, simply establishing a physical circuit is only the first step. The true intelligence of an ExpressRoute implementation lies in how you configure "peering."

Peering is the logical configuration that dictates how your traffic flows between your local network and the Microsoft cloud. Without the correct peering setup, your traffic will either fail to reach its destination or, worse, traverse insecure paths. Understanding the different types of peering—Microsoft, Private, and Public—is essential for any network engineer or architect tasked with designing hybrid connectivity. This lesson will dissect each peering type, explain when to use them, and provide the technical guidance needed to implement them correctly in your environment.

Understanding the ExpressRoute Architecture

Before diving into the specific types of peering, it is helpful to visualize the ExpressRoute circuit as a logical pipe. When you provision an ExpressRoute circuit, you are essentially asking your provider (like Equinix, AT&T, or Verizon) to create a private pathway to a Microsoft edge router. Once this physical connection is established, you must then configure the Border Gateway Protocol (BGP) to announce your prefixes to Microsoft and receive Microsoft's prefixes in return.

Peering is the configuration of these BGP sessions. Each circuit supports multiple peering types, and each type serves a distinct purpose in your network topology. By segmenting your traffic using these peering types, you gain granular control over security, routing, and access to services.

Callout: The Logical Separation of Traffic Think of peering types as distinct lanes on a highway. While all traffic travels through the same physical ExpressRoute circuit (the highway), the peering types ensure that your internal database traffic (Private Peering) does not get mixed up with your access to public services or SaaS applications (Microsoft Peering). This separation allows you to apply different security policies, route filters, and bandwidth limitations to each lane, ensuring that your most critical traffic is never bottlenecked by less important data flows.

1. Private Peering: Connecting to your Virtual Networks

Private Peering is the most commonly used configuration for hybrid networking. Its primary purpose is to extend your on-premises network into your Azure Virtual Networks (VNets). When you implement Private Peering, you are creating a "private extension" of your data center.

How it Works

When you configure Private Peering, you assign a set of private IP addresses (typically a /30 subnet) to the connection between your edge router and the Microsoft router. You then advertise your on-premises IP prefixes to Azure using BGP. Conversely, Azure advertises the IP ranges of your VNets back to your on-premises router. This allows your local servers to communicate directly with your cloud-based virtual machines using their private IP addresses, just as if they were sitting in the same physical rack.

Practical Use Cases

• Database Synchronization: Replicating on-premises SQL Server databases to Azure SQL Managed Instances or VMs.
• File Shares: Accessing Azure Files or on-premises file servers over a high-speed, private connection.
• Active Directory Integration: Authenticating users against domain controllers that are split between on-premises and the cloud.
• Application Tiers: Connecting an on-premises web front-end to an Azure-hosted back-end API.

Configuration Step-by-Step

1. Define Subnets: You must reserve two /30 subnets. One for the primary link and one for the secondary link. These subnets must not overlap with any of your existing network address spaces.
2. Assign VLAN IDs: Select a VLAN ID that is not currently in use by your provider.
3. Configure BGP: On your local edge router, configure the BGP peer IP (the Microsoft-side IP) and the Autonomous System Number (ASN).
4. Create the Peering: Use the Azure Portal or Azure CLI to create the peering object under your ExpressRoute circuit resource.

Tip: Avoiding IP Overlap One of the most frequent causes of connectivity failure is IP address overlap. Before configuring Private Peering, ensure that the address space you intend to use for the peering subnets does not exist anywhere else in your global network. If you use overlapping ranges, your routing tables will become corrupted, leading to unpredictable traffic drops.

2. Microsoft Peering: Accessing SaaS and Public Services

Microsoft Peering is designed to provide connectivity to Microsoft’s public-facing services, such as Microsoft 365, Dynamics 365, and the public endpoints of Azure services (like Azure Storage or Azure SQL Database). This peering type is unique because it uses public IP addresses to communicate.

How it Works

Unlike Private Peering, which uses your internal private IP space, Microsoft Peering requires you to use public IP addresses that you own and have registered with a Regional Internet Registry (RIR). You advertise these public prefixes to Microsoft, and Microsoft advertises its public service prefixes to you.

Why Use Microsoft Peering?

• Performance: By using the ExpressRoute path for Microsoft 365, you avoid the variable latency and congestion of the public internet.
• Predictability: Traffic to cloud services stays on the Microsoft backbone, providing a consistent user experience for employees.
• Security: You can restrict access to your Azure public endpoints so that they are only reachable via your ExpressRoute circuit, effectively locking out the public internet from your cloud resources.

Important Considerations

You cannot simply enable Microsoft Peering without validation. Microsoft requires you to prove ownership of the public IP addresses you intend to use. Furthermore, you must ensure that your firewall and security policies allow traffic to these public endpoints. Because this peering involves public IP space, it is highly recommended to implement strict route filters to ensure you are only receiving the routes you actually need.

Warning: Route Filtering is Mandatory Microsoft advertises thousands of public IP prefixes through Microsoft Peering. If you do not implement a route filter, your local router may attempt to install all of these routes into its routing table, which can easily overwhelm the memory and processing capabilities of standard networking hardware. Always apply a route filter to limit the routes to only the specific services you require (e.g., only Azure Storage or only Microsoft 365).

3. Public Peering (Deprecated)

It is important to address Public Peering, even though it is largely a legacy concept. Historically, Public Peering was used to access public Azure services before Microsoft Peering was fully refined. Today, Microsoft strongly recommends using Microsoft Peering for all public-service connectivity.

If you encounter documentation referring to Public Peering, treat it as an obsolete configuration. Modern Azure deployments should rely exclusively on Private Peering for VNet connectivity and Microsoft Peering for SaaS and public endpoint connectivity.

Comparison Table: Peering Types at a Glance

Implementation Best Practices

Designing a successful ExpressRoute implementation requires more than just technical configuration; it requires adherence to industry-standard networking practices.

1. High Availability (HA)

Never rely on a single ExpressRoute circuit. If the circuit goes down, your hybrid connectivity disappears. Always implement a redundant design:

• Dual Circuits: Use two different ExpressRoute circuits from two different service providers connected to different peering locations.
• BGP Optimization: Configure your BGP attributes (AS-Path Prepending or Local Preference) to prefer one circuit while keeping the other as a standby.

2. Monitoring and Logging

You cannot manage what you do not measure. Use Azure Monitor and Network Watcher to track the health of your peering connections. Monitor your BGP state transitions and packet drops. If a peering session flaps, you need to know immediately.

3. Security Hardening

Even though ExpressRoute is a "private" connection, you should still treat the traffic as potentially untrusted.

• Encryption: Consider using MACsec for Layer 2 encryption on the circuit, or implement IPsec tunnels over the ExpressRoute peering if your compliance requirements demand end-to-end encryption.
• Firewalling: Ensure that all traffic traversing the ExpressRoute circuit passes through a Network Virtual Appliance (NVA) or an Azure Firewall to inspect packets for malicious activity.

Technical Example: Configuring Private Peering via CLI

Below is an example of how you might configure Private Peering using the Azure CLI. This assumes you have already created the ExpressRoute circuit.

# Define the variables for your peering
RG="MyResourceGroup"
CIRCUIT_NAME="MyExpressRouteCircuit"

# Create the Private Peering configuration
az network express-route peering create \
    --resource-group $RG \
    --circuit-name $CIRCUIT_NAME \
    --name AzurePrivatePeering \
    --peering-type AzurePrivatePeering \
    --peer-asn 65000 \
    --primary-peer-subnet 10.0.0.0/30 \
    --secondary-peer-subnet 10.0.0.4/30 \
    --vlan-id 100

Explanation:

• --peer-asn: This is the Autonomous System Number of your on-premises edge router.
• --primary-peer-subnet and --secondary-peer-subnet: These are the /30 ranges that connect your router to Microsoft.
• --vlan-id: The tag used for the logical sub-interface on your router.

Common Pitfalls and How to Avoid Them

Pitfall 1: Asymmetric Routing

Asymmetric routing occurs when traffic leaves your network via one path but returns via another. This often happens if you have both a Site-to-Site VPN and an ExpressRoute circuit connected to the same VNet. Azure will prefer the ExpressRoute path due to BGP metrics, but your local router might prefer the VPN.

• Solution: Ensure your BGP metrics are tuned so that ExpressRoute is always the preferred path, and use the "Local Gateway" settings to ensure the return traffic follows the same path.

Pitfall 2: BGP Route Limits

Every ExpressRoute circuit has a limit on the number of prefixes it can accept. If you exceed this limit, the BGP session will drop.

• Solution: Use route summarization on your on-premises routers to minimize the number of prefixes you advertise to Azure. Instead of advertising 50 individual /24 subnets, advertise one aggregate /19 subnet.

Pitfall 3: Ignoring MTU Sizes

Standard Ethernet MTU is 1500 bytes. However, some ExpressRoute providers support Jumbo Frames (up to 9000 bytes). If your router is configured for Jumbo Frames but the provider or Azure is not, you will experience packet fragmentation and severe performance degradation.

• Solution: Always verify the MTU settings with your connectivity provider before enabling Jumbo Frames. If in doubt, stick to the standard 1500-byte MTU.

Callout: The Importance of BGP Path Selection Understanding how BGP selects a path is crucial for hybrid networking. BGP makes decisions based on attributes like AS-Path, Local Preference, and MED (Multi-Exit Discriminator). If you have a complex network with multiple exits to the internet and multiple ExpressRoute circuits, you must explicitly define these attributes to prevent traffic from "tromboning" through inefficient paths. A common mistake is leaving these at default values, which can lead to unexpected traffic flows across your most expensive circuits.

Deep Dive: Managing Route Filters for Microsoft Peering

As mentioned earlier, Microsoft Peering requires careful management. Because Microsoft advertises a massive number of routes, you must use Route Filters to keep your routing table manageable.

A Route Filter is a resource in Azure that allows you to specify which services you want to access. Once created, you attach the filter to your ExpressRoute peering.

Steps to Implement a Route Filter:

1. Create the Filter: Define the resource in your Azure subscription.
2. Add Rules: Add rules to the filter that specify the "Community Value" for the services you need (e.g., the community value for all Microsoft 365 services).
3. Attach: Link the filter to the Microsoft Peering configuration of your ExpressRoute circuit.

By doing this, your router will only receive the specific routes associated with those community values, drastically reducing the strain on your hardware.

Troubleshooting Connectivity Issues

When things go wrong, the first step is always to check the status of the BGP session. Use the following diagnostic approach:

1. Check Physical Layer: Is the light on your fiber interface blinking? Is the cross-connect patched correctly?
2. Check Layer 2: Can you ping the IP address of the Microsoft peer on your local router? If you cannot ping the peer IP, there is an issue with your VLAN or local sub-interface configuration.
3. Check Layer 3 (BGP): Use the command show ip bgp summary (on Cisco-style devices) to see if the BGP state is "Established." If it is "Idle" or "Active," your BGP parameters (ASN, IP, or MD5 password) are likely incorrect.
4. Check Routes: Once the session is established, use show ip bgp neighbors <IP> routes to verify if you are actually receiving prefixes from Microsoft.

Future-Proofing Your Network

As your organization grows, your connectivity needs will change. The beauty of ExpressRoute is its modularity. You can start with a basic Private Peering setup and add Microsoft Peering later as you adopt more SaaS products. You can also move from a standard ExpressRoute circuit to ExpressRoute Direct (where you connect at 100 Gbps directly to the Microsoft edge) without re-architecting your entire peering strategy.

Always keep your documentation updated. Network diagrams that reflect the current state of your BGP advertisements and peering configurations are invaluable when you are troubleshooting a mid-night outage. Ensure that every member of your team understands the difference between the peering types and knows how to safely update the routing configurations.

Key Takeaways

• Understand the Purpose: Private Peering is for your internal VNet resources; Microsoft Peering is for public services like Office 365 and Azure PaaS. Never confuse the two.
• Prioritize Security: Treat your ExpressRoute circuit as an extension of your internal network. Implement firewalling, monitoring, and consider encryption if your security policy dictates it.
• Manage Routes Actively: Use route summarization and mandatory route filters for Microsoft Peering to keep your on-premises routing tables efficient and functional.
• Plan for Redundancy: Never deploy a single ExpressRoute circuit in a production environment. Use dual providers and redundant physical paths to ensure uptime.
• Monitor BGP Health: Proactively monitor your BGP sessions. The most common issues are session flaps caused by configuration mismatches or physical interface instability.
• Avoid Overlap: Always verify that your peering subnets and your advertised address spaces do not conflict with existing networks. This is the single most common cause of "silent" connectivity failure.
• Use Modern Standards: Focus on Private and Microsoft Peering. Disregard legacy documentation regarding Public Peering, as it is no longer the recommended or supported path for modern Azure cloud connectivity.

By following these guidelines and maintaining a deep understanding of how peering functions at the BGP level, you will be well-equipped to design and maintain a reliable, high-performance hybrid network that serves your organization's needs for years to come.