Blog ENG - MS Azure - Post 1 2025
Managing a hybrid network can become increasingly complex as you deploy more ExpressRoute circuits and establish more connections to your workloads in different Azure regions. To help manage this complexity and route traffic from Azure to on-premises efficiently, you can configure BGP communities on your Azure virtual networks.
What is a BGP Community?
A Border Gateway Protocol (BGP) community is a group of IP prefixes that share a common property called a BGP community tag or value. In Azure, you can now:
- Set a custom BGP community value on each of your virtual networks.
- Access a predefined regional BGP community value for all your virtual networks deployed in a region.
Once these values are configured on your virtual networks, ExpressRoute preserves them on the corresponding private IP prefixes shared with your on-premises. When these prefixes are learned on-premises, they’re learned along with the configured BGP community values.
Using Community Values for Multi-Region Networks
A common scenario for using ExpressRoute is accessing workloads deployed in an Azure virtual network. ExpressRoute facilitates the exchange of Azure and on-premises private IP address ranges using a BGP session over a private connection. This feature enables a seamless extension of your existing networks into the cloud.
When you have multiple ExpressRoute connections to virtual networks in different Azure regions, traffic can take more than one path, leading to suboptimal routing. For example, traffic intended for Region A might take a less efficient path through ER Circuit 2 instead of the optimal path through ER Circuit 1.
To ensure traffic going to Region A takes the optimal path over ER Circuit 1, you can configure a route filter on-premises. This filter ensures that Region A routes are only learned at the customer edge from ER Circuit 1 and not from ER Circuit 2. However, this approach requires you to maintain a comprehensive list of IP prefixes in each region and regularly update this list whenever a new virtual network is added or a private IP address space is expanded in the cloud. As your cloud presence grows, this task can become increasingly burdensome.
When virtual network IP prefixes are learned on-premises with custom and regional BGP community values, you can configure your route filters based on these values instead of specific IP prefixes. This means that when you expand your address spaces or create more virtual networks in an existing region, you don’t need to modify your route filter. The route filter already has rules for the corresponding community values. With the use of BGP communities, your multi-region hybrid networking is simplified.
Benefits of Configuring BGP Community Values
Another reason to configure a BGP community value on a virtual network connected to ExpressRoute is to understand where traffic is originating from within an Azure region. As you deploy more virtual networks and adopt more complex network topologies within an Azure region, troubleshooting connectivity and performance issues can become more difficult. With custom BGP community values configured on each virtual network within a region, you can quickly identify where the traffic is originating from in Azure. Being able to identify the source virtual network helps you narrow down your investigation.
By leveraging BGP communities, you can streamline the management of your hybrid network, ensure optimal routing, and simplify troubleshooting, making your network more efficient and easier to manage as it grows.