Routing and SD-WAN
Route selection and SD-WAN
Static routes carry a destination/prefix, an optional gateway, an outgoing interface, a distance and a priority. Lower distance wins first; if distances tie, lower priority wins; if both tie, the routes are equal-cost and traffic is load-balanced (ECMP). Configuring a gateway on the wrong interface, or leaving distance at default when a backup link should be preferred only on failure, are common errors. Policy routes override the routing table for matching traffic, so a stale policy route can defeat an otherwise correct static route.
Dynamic routing (OSPF, BGP) feeds the same table and is compared by administrative distance. OSPF neighbour problems usually trace back to area, network type or authentication mismatches.
SD-WAN bundles member interfaces into an SD-WAN zone, monitors them with health checks (ping, HTTP, DNS, TCP echo) that evaluate latency, jitter and packet loss against SLA targets, and steers traffic with SD-WAN rules evaluated top-down. Strategies include manual (priority order of members), best quality, lowest cost, maximize bandwidth and load balance. Firewall policies must reference the SD-WAN zone (or the appropriate member) for steering to apply.
Traps: a member interface needs a working gateway/route before health checks can succeed; a health check not bound to the member will not affect steering; and the default rule at the bottom of the SD-WAN rule list still applies when nothing else matches.
Rule of thumb: when traffic takes the wrong WAN link, check in order - SD-WAN rule order, SLA target results, then the underlying static route distance.