Failover is the SD-WAN term for automated switching from a primary WAN source to a secondary or back-up WAN. The Internet connection on any network comes in through a WAN (Wide Area Network). A WAN connection can originate from any of several sources: cable, DSL, fiber, satellite, or even cellular. Failover technology is built into a router, but not all routers are failover capable, and different routers have different levels of failover. Every router takes an incoming WAN connection and distributes it to connected users on the LAN (Local Area Network). A failover router can manage two or more incoming WAN sources and dynamically switch between them.
Failover 101
For our examples we will consider a Pepwave MAX-HD4-LTEA-W-T which is a powerhouse multi-WAN router with up to five wired WANs and four embedded LTEA cellular modems for potentially four more WAN sources. While the HD4 is much more router than we will need for these examples, it has all possible capabilities making it flexible enough for any possible deployment.
For the first example we will consider a mobile only deployment such as a maritime application or a remote worksite where wired connections have yet to be established. The example HD4 has active SIM cards for each modem as follows:
Modem 1 – Verizon
Modem 2 – AT&T
Modem 3 – T-Mobile
Modem 4 – Sprint
(In this example we are using all four networks. Failover can be established with only two WAN connections.)
The SIMs are installed in order based on a preference factor like cost or billing. The failover technology allows setting up a priority rule where the device will treat the Verizon connection as the primary WAN. If the Verizon connection becomes unavailable, the router will begin passing traffic over the AT&T connected modem. If both Verizon and AT&T networks become unavailable, the connection will be sent through the T-Mobile modem. If all three of those carriers are unavailable, the router will begin sending sessions through the Sprint connected modem. The switching requires no user input after the initial set up, however the unused connections most often remain connected, they are just not actively used.
When You Need Session Persistence
With this standard failover set-up any active sessions will have to be reestablished when there is a switch from one modem to the other. For example a stalled webpage may have to be reloaded. While establishing a failover connection is fast, there is a brief interval of disconnection during the handoff. Standard failover does not offer session persistence and any sessions that are active at the time a WAN source switches will be reestablished over the new WAN source. For session persistence, you’ll want to look at SpeedFusion Hot Failover.
Our example HD4 is SpeedFusion capable. To establish hot failover, The HD4 must create a SpeedFusion tunnel with a second Peplink or Pepwave device. The two SpeedFusion enabled devices constantly monitor each other’s WAN health. In the standard failover example above the failover connections are only activated once a primary connection failure is detected. With hot failover, at least one secondary connection is kept open by the router and the health of the SpeedFusion tunnel is constantly monitored. Session packets that fail on the primary WAN are kept active by the second connected device and rerouted to the failover connection with minimal packet loss.
In our example above, the remote HD4 maintains a SpeedFusion tunnel with a Peplink Balance 380 at the main office, which is connected to a cable Internet WAN. The remote HD4 uses the Verizon cellular WAN for both SpeedFusion activity and all user traffic. At the same time the HD4 keeps the AT&T, T-Mobile, and Sprint modems connected, although traffic to those connections is limited to minimal data for health checks. If at any time the SpeedFusion network senses a failure on any of the active HD4 connections, it will automatically begin routing sessions over the next available channel. If the Verizon network disconnects, the HD4 will switch the main traffic to the AT&T connection that is already active. Because SpeedFusion is constantly monitoring each connection it will know if the AT&T modem is unavailable and route traffic to the T-Mobile or Sprint connection as the pre-established priority dictates. SpeedFusion hot failover keeps an alternative, secondary WAN connection open and available for instant failover with minimal or no packet loss, maintaining session persistence. To the user on the HD4 LAN, the WAN switching is undetected; the user will not experience a break in the connection.
Hot Failover will use minimally more cellular data than a standard failover set-up, both to establish the SpeedFusion tunnel and to ping the idle connections. The minimally higher data consumption is a worthwhile trade-off in critical applications that require session persistence. SpeedFusion hot failover creates an unbreakable connection.
One Technology, Many Applications
In our example we used only cellular WAN connections, but failover can be created with any type of WAN connections. In open sea maritime applications a VSAT WAN may be added for failover. Many FrontierUS partners use failover for Point of Purchase sales terminals. The terminal will normally process all traffic through a wired cable WAN, but in the rare case that the cable WAN fails, they will failover to a cellular WAN connection to keep processing bank cards, and keep the businesses open. There are countless other application where failover is important, some of them situations where session persistence is critical, as in law enforcement or emergency services. In those applications, SpeedFusion Hot Failover is the best solution.
Topher Lautner is a FrontierUS Technical Support Specialist.
FrontierUS is the largest distributor of Peplink products in the world and carries the full Peplink line.
