Understanding Route Server in Azure Networking

A route server in Azure Networking is a type of network service that allows you to manage and optimize your network traffic.

It's essentially a virtual network appliance that helps you control how traffic flows between your virtual networks and the internet.

A route server is particularly useful for organizations that have multiple virtual networks and need to manage traffic between them.

By using a route server, you can simplify your network architecture and reduce complexity.

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Azure Route Server works by receiving an on-premises route from the SDWAN appliance and a default route from the firewall, which are then automatically configured on the VMs in the virtual network.

It establishes BGP peering to receive these routes. The SDWAN appliance receives the virtual network address, which it can then propagate further to the on-premises network.

The Route Server takes advantage of a BGP feature called "third party next-hop", where the IP address of the next router is unchanged when it is passed on to the other peers on the same LAN.

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It doesn't insert its Autonomous System Number into the AS Path, making it appear as though the IXP members are peering directly with each other.

To work with a Route Server, BGP implementations need to disable the AS adjacency check, which is required by the BGP specification as per RFC4271.

On Cisco IOS, you can do this by using the command "no bgp enforce-first-as" in the router bgp configuration.

Azure Route Server simplifies configuration, management, and deployment of your Network Virtual Appliance (NVA) in your virtual network. This is because you no longer need to manually update the routing table on your NVA whenever your virtual network addresses are updated.

You can peer multiple instances of your NVA with Azure Route Server, allowing you to configure the BGP attributes in your NVA and let Azure Route Server know which NVA instance is active or passive.

Here are the key benefits of using Azure Route Server:

Azure is a cloud computing platform that offers a wide range of services to help you manage and scale your applications. It's designed to be highly available, with features like zone-level redundancy for services like Azure Route Server.

Azure Route Server is a fully managed service that allows you to easily manage routing between your network virtual appliance (NVA) and your virtual network. This makes it a great option for businesses with complex network requirements.

Azure Route Server has zone-level redundancy if you deploy it in an Azure region that supports Availability Zones, ensuring that your route server remains online even in the event of a zone failure. This level of redundancy gives you peace of mind, knowing that your application will remain available to users.

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Azure supports only one routing protocol, which is Border Gateway (BGP) Protocol.

Using BGP with Azure requires your network virtual appliance (NVA) to support multi-hop external BGP.

The ASN you choose for your NVA must be different from the Route Server ASN.

Azure Route Server must be deployed in a dedicated subnet in your virtual network.

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Using a Route Server can simplify your life in many ways. You no longer need to manually update the routing table on your Network Virtual Appliance (NVA) whenever your virtual network addresses are updated.

One of the main benefits of Azure Route Server is that it eliminates the need for manual updates. This means you can focus on other important tasks without worrying about tedious configuration changes.

You can peer multiple instances of your NVA with Azure Route Server, which is especially useful for performance or resiliency. This allows you to configure the BGP attributes in your NVA and let Azure Route Server know which instance is active or passive.

Here are some key benefits of using Azure Route Server:

Overall, Azure Route Server can save you time and effort by automating many of the tasks associated with NVA configuration and management.

You can only create one Route Server in a virtual network, and it must be deployed in a dedicated subnet called RouteServerSubnet. This is a requirement for using the Route Server service.

The total number of routes advertised from virtual network address space and Route Server towards ExpressRoute circuit, when Branch-to-branch enabled, must not exceed 1,000.

If your NVA advertises more routes than the limit, the BGP session gets dropped. This is a hard limit that you should be aware of when setting up your Route Server.

Here are the key limitations to keep in mind:

Azure does support virtual network peering, which allows you to connect two virtual networks together. This enables communication between resources in both networks.

To enable peering, you need to peer a virtual network hosting the Azure Route Server to another virtual network. Azure Route Server will then learn the address spaces of the peered virtual network.

Enabling Use the remote virtual network's gateway or Route Server on the second virtual network allows Azure Route Server to send the address spaces to all the peered network virtual appliances (NVAs). This way, the NVAs can learn the routes from the remote virtual network.

Azure Route Server programs the routes from the NVAs into the route table of the virtual machines in the peered virtual network. This ensures that traffic is routed correctly between the two networks.

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You can create only one Route Server in a virtual network. This is a hard limit, so you can't deploy multiple Route Servers in the same virtual network.

Azure Route Server has specific limits on the number of BGP peers it can support. You can have up to 8 BGP peers at a time.

If your NVA advertises more routes than the limit, the BGP session gets dropped. This can cause connectivity issues and is something to watch out for.

You must deploy the route server in a dedicated subnet called RouteServerSubnet. This is a requirement for Azure Route Server to function properly.

Here are the key limitations to keep in mind:

The total number of routes advertised from VNet address space and Route Server towards ExpressRoute circuit, when Branch-to-branch enabled, must not exceed 1,000. This is a critical limit to keep in mind when designing your network.

The setup of a route server is crucial for redundancy purposes. The SwissIX route server setup consists of two machines.

Only one machine is required to provide the route server service, but connecting to both machines is necessary to ensure the service can still be used if one machine is out of order. This can happen during maintenance, for example.

The software used to provide the route server service is BIRD. BIRD is a reliable and efficient choice.

If a BGP announcement is marked as a Blackhole, the route servers add the NO_EXPORT community and the BLACKHOLE Community if they are not already present. This prevents the Blackhole announcement from spreading widely in the Internet routing system.

You can control various functions of the route server using Action BGP Communities. These communities allow you to control the redistribution of advertised prefixes.

With Action BGP Communities, you can also prepend your own ASN up to three times. This helps in various routing scenarios.

To monitor the availability of your peer, it's advised to set up BGP sessions to both route servers, even if you don't want to peer with them. This is an operational best practice.

If you don't want to establish BGP sessions due to your peering policy, you can contact SwissIX staff to set up a monitoring only session. You won't have to advertise any prefixes and won't receive any prefixes from them on this session.

To ensure smooth monitoring, it's advised to set up BGP sessions to both route servers, even if you don't plan to peer with them. This helps staff quickly monitor the availability of each peer.

Setting up these sessions also allows staff to troubleshoot any connectivity issues that may arise.

If your decision not to establish BGP sessions with route servers was made due to your peering policy, you can contact SwissIX staff to establish a monitoring only session.

A monitoring only session doesn't require you to advertise any prefixes or receive prefixes from the route servers.

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Control is a crucial aspect of monitoring and managing your network. With BGP Communities, you can control various functions of the route server.

You can use Action BGP Communities to control the redistribution of advertised prefixes. This means you can dictate which prefixes are shared with other networks and which are kept private.

Action BGP Communities also allow you to prepend your own ASN up to three times. This is useful for maintaining a consistent identity across your network.

This feature is particularly useful when performing maintenance, as it enables the calculation of a new alternate path for your advertised prefixes before the maintenance begins. This ensures minimal disruption to your network and its users.

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