ExpressRoute Gateway Scalable

Express Route Scalable Gateway: A Detailed Overview

Key Concepts:

• ExpressRoute: A dedicated private network connection from on-premises to Azure.
• Virtual Network (VNet): A logically isolated network in Azure.
• Private Peering: Connecting an on-premises network’s IP space to an Azure VNet via ExpressRoute.
• Gateway Subnet: A dedicated subnet within a VNet used for deploying ExpressRoute gateways.
• SKU (Stock Keeping Unit): A specific configuration of a service, defining its capacity and features (e.g., ER Gateway 1A, ER Gateway 3A).
• Zone Redundancy: Distributing resources across multiple Availability Zones within an Azure region for higher availability.
• FastPath: A mechanism to bypass the ExpressRoute gateway for direct connectivity to resources, reducing latency and increasing throughput.
• Scale Units: The fundamental unit of throughput for the Scalable Gateway, each providing 1 Gbps.
• BGP (Border Gateway Protocol): A standardized exterior gateway protocol designed to exchange routing and reachability information among autonomous systems on the Internet.

1. Traditional ExpressRoute Architecture & Limitations

The video begins by outlining the traditional ExpressRoute setup. Customers establish a connection from their on-premises network to Azure via an ISP, which then connects to Microsoft’s global network at “meet me peering points” (carrier-neutral facilities). This connection utilizes a pair of redundant routers on both the customer and Microsoft sides. Purchasing an ExpressRoute circuit provides dedicated connectivity between the customer’s network and the Azure backbone.

To extend the customer’s IP space into the VNet, private peering is used, requiring a dedicated gateway subnet within the VNet. Historically, ExpressRoute gateways were offered in fixed SKUs (1A, 2A, 3A) designed for zone redundancy, meaning multiple instances of the gateway operate across different Availability Zones within a region. These SKUs differ in throughput: 1A offers 1 Gbps, 2A provides 2 Gbps, and 3A delivers 10 Gbps. The video emphasizes the limitation of these fixed SKUs – you are constrained by the chosen throughput, even if your circuit capacity is higher.

2. ExpressRoute Gateway Functionality: Routing & Traffic Flow

The ExpressRoute gateway performs two key functions:

• Route Plumbing: The gateway uses BGP to learn routes from the on-premises network and propagates them into the VNet and any connected VNets (hub-and-spoke topology). This allows resources within Azure to route traffic to the on-premises network.
• Traffic Enablement: The gateway facilitates the flow of traffic into Azure from the on-premises network. Inbound traffic enters via the gateway, which then forwards it to the target resource. The gateway acts as a bandwidth throttle, limiting inbound traffic to its SKU’s maximum throughput.

FastPath Exception: The video introduces FastPath, available with the 3A SKU (Ultra). If the target resource is FastPath-capable, traffic bypasses the gateway entirely, flowing directly to the resource. This reduces latency and allows throughput to exceed the gateway’s 10 Gbps limit, provided the ExpressRoute circuit supports it. However, the gateway still handles route plumbing and serves as a fallback mechanism. Outbound traffic never goes through the gateway; it always bypasses it.

3. Introducing the Scalable Gateway: Dynamic Throughput

The core of the video focuses on the new Scalable Gateway. This represents a significant shift from the fixed SKU model. The Scalable Gateway eliminates fixed SKUs, instead utilizing scale units. Each scale unit provides 1 Gbps of throughput.

Configuration involves specifying a minimum and maximum number of scale units. The gateway dynamically adjusts the number of scale units based on traffic demand, within the defined range. The Scalable Gateway is always zone redundant, with multiple instances distributed across Availability Zones.

Scale Unit Performance: The first 10 scale units provide 1 Gbps throughput and up to 200,000 packets per second. The subsequent 30 scale units also offer 1 Gbps throughput, but with a reduced maximum packet rate. The number of VM connections also decreases for the last 30 scale units.

4. Autoscaling Behavior & FastPath Compatibility

The video details the autoscaling behavior of the Scalable Gateway:

• Scaling Up (Scaling Out): The gateway scales up when sustained throughput exceeds its current capacity for more than 5 minutes. It then doubles its capacity and pauses further scaling actions for 20 minutes to prevent flapping.
• Scaling Down (Scaling In): The gateway scales down when sustained underutilization persists for 15 minutes, and all relevant metrics (throughput, flow count, CPU) are below a certain threshold. It removes one instance and pauses further scaling for 30 minutes.

FastPath and Minimum Scale Units: To utilize FastPath with the Scalable Gateway, a minimum of 10 scale units is required, equivalent to the throughput of the previous 3A SKU.

5. Cost & Migration Considerations

The Scalable Gateway offers a cost advantage over the fixed SKUs. The pricing is approximately $0.21 per scale unit per hour, making it consistently cheaper than the equivalent fixed SKU.

Migration:

• Existing Zone Redundant SKUs: Switching from a fixed zone-redundant SKU to the Scalable Gateway is a seamless process with no downtime. Traffic configuration is preserved.
• Non-AZ SKUs (Standard, High, Ultra): Migration requires the ExpressRoute Gateway Migration Tool and may involve brief connectivity interruptions as a new gateway is created, configured, and connections are switched over.

6. Availability & Future Outlook

The Scalable Gateway is not yet available in all Azure regions, but its availability is expanding. The video concludes by stating that the Scalable Gateway is the future of ExpressRoute gateways, eventually replacing the fixed SKU model. Its dynamic scalability, cost-effectiveness, and inherent zone redundancy make it the preferred option for most scenarios.

Notable Quote:

“There is not a technical or financial reason to not use it [Scalable Gateway]. I mean check the documentation has a few limitations today but outside of that you're probably going to want to go ahead and switch.” – The video presenter.

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