During the planning phase of a Windows 10 Always On VPN implementation the administrator must decide between two tunneling options for VPN client traffic – split tunneling or force tunneling. When split tunneling is configured, only traffic for the on-premises network is routed over the VPN tunnel. Everything else is sent directly to the Internet. With force tunneling, all client traffic, including Internet traffic, is routed over the VPN tunnel. There’s been much discussion recently on this topic, and this article serves to outline the advantages and disadvantages for both tunneling methods.
Force Tunneling
Force tunneling is typically enabled to meet the following requirements.
Visibility and Control
By routing all the client’s Internet traffic over the VPN tunnel, administrators can inspect, filter, and log Internet traffic using existing on-premises security solutions such as web proxies, content filters, or Next Generation Firewalls (NGFW).
Privacy
Enabling force tunneling ensures privacy and protection of all Internet communication. By routing all Internet traffic over the VPN, administrators can be certain that all communication from the Always On VPN client is encrypted, even when clients access unencrypted web sites or use untrusted or insecure wireless networks.
Force Tunneling Drawbacks
While configuring force tunneling for Always On VPN has some advantages, it comes with some serious limitations as well.
Poor User Experience
User experience is often degraded when all Internet traffic is routed over the VPN. These suboptimal network paths increase latency, and VPN encapsulation and encryption overhead increase fragmentation, leading to reduced throughput. Most Internet traffic is already encrypted in some form, and encrypting traffic that is already encrypted makes the problem even worse. In addition, force tunneling short-circuits geographic-based Content Delivery Networks (CDNs) further reducing Internet performance. Further, location-based services are often broken which can lead to improper default language selection or inaccurate web search results.
Increased Resource Consumption
Additional resources may need to be provisioned to support force tunneling. With corporate and Internet traffic coming over the VPN, more CPU, memory, and network resources may be required. Deploying additional VPN servers and higher throughput load balancers to support the increase in network traffic may also be necessary. Force tunneling also places higher demands on Internet Service Provider (ISP) links to the corporate datacenter.
Split Tunneling
The alternative to force tunneling is “split tunneling”. With split tunneling configured, only traffic destined for the internal corporate network is routed over the VPN. All other traffic is sent directly to the Internet. Administrators define IP networks that should be routed over the VPN, and those networks are added to the routing table on the VPN client.
Security Enforcement
The challenge of providing visibility and control of Internet traffic with split tunneling enabled can be met using a variety of third-party security solutions. Microsoft Defender ATP recently introduced support for web content filtering. Also, there are numerous cloud-based security offerings from many vendors that allow administrators to monitor and control client-based Internet traffic. Zscaler and Cisco Umbrella are two popular solutions, and no doubt there are many more to choose from.
Recommendations
The general guidance I provide customers is to use split tunneling whenever possible, as it provides the best user experience and reduces demands on existing on-premises infrastructure. Enabling split or force tunneling is ultimately a design decision that must be made during the planning phase of an Always On VPN implementation project. Both configurations are supported, and they each have their merits.
In today’s world, with many applications accessible via public interfaces, force tunneling is an antiquated method for providing visibility and control for managed devices in the field. If required, investigate the use of Microsoft or other third-party solutions that enforce security policy in place without the requirement to backhaul client Internet traffic to the datacenter over VPN for inspection, logging, and filtering.
Additional Information
Whitepaper: Enhancing VPN Performance at Microsoft
Whitepaper: How Microsoft Is Keeping Its Remote Workforce Connected
The Windows Server 2016 Routing and Remote Access Service (RRAS) is commonly deployed as a VPN server for Windows 10 Always On VPN deployments. Using RRAS, Always On VPN administrators can take advantage of Microsoft’s proprietary 
Performance issues with DirectAccess are not uncommon. In fact, there are numerous 
DirectAccess has been around for many years, and with Microsoft now moving in the direction of Always On VPN, I’m often asked “What’s the difference between DirectAccess and Always On VPN?” Fundamentally they both provide seamless and transparent, always on remote access. However, Always On VPN has a number of advantages over DirectAccess in terms of security, authentication and management, performance, and supportability.
