Route Failover Between Two Backhaul Providers
BGP path selection, prepends, route maps and last-resort default-route failover.
Redundant backhaul is only useful if failover actually works when the primary circuit fails. There are several ways to route traffic between two providers, ranging from simple default-route floating statics to full BGP multi-homing. The right approach depends on your providers, your IP space, and your tolerance for complexity.
Option 1: Floating static routes
The simplest approach is two default routes with different administrative distances. The primary route has a lower distance, so it is preferred. If the primary next-hop becomes unreachable, the router removes the route and traffic falls back to the secondary.
ip route 0.0.0.0/0 198.51.100.1 10
This works best when both providers give you their own IP space and you do not need inbound failover.
Option 2: IP SLA and tracked routes
A next-hop can be "up" even when the upstream connection to the internet is down. IP SLA (or equivalent probe-based tracking) sends test traffic to a target beyond the provider router. If the probe fails, the route is withdrawn and failover occurs.
- Ping a reliable target (e.g., 1.1.1.1 or 8.8.8.8) through each provider
- Track the SLA result
- Attach the track object to the default route
Option 3: BGP failover
If you have your own IP space and BGP sessions with both providers, failover becomes more automatic and symmetric. You announce your prefixes to both providers and use BGP attributes to influence path selection.
- AS path prepending: Make one path look longer so inbound traffic prefers the other.
- Local preference: Influence outbound path selection within your network.
- MED: Suggest a preferred path to providers.
Design principle: avoid shared fate
Primary and secondary backhaul should not share the same conduit, pole, provider POP, or power feed. If they do, a single construction cut or power event can take down both circuits. This is the most common reason redundant backhaul fails in practice.
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SmashByte Wireless designs diverse-path primary and secondary backhaul and configures failover for WISP networks.
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