A fiber cut occurs between Branch-12 and the metro provider's aggregation point. Branch-12 has two WAN paths: this primary metro fiber circuit, and a secondary broadband/LTE circuit, both terminated into an SD-WAN edge running continuous path monitoring.
FailureThe branch router's WAN interface loses carrier signal within milliseconds and transitions to a "down" state. This is the fastest possible detection layer — pure hardware, no protocol involved yet.
L1Not every WAN failure is this clean. A gray failure — the physical link stays up (carrier present) but traffic upstream is silently dropped, e.g. a provider-side routing issue — produces no interface-down event at all. Relying purely on physical link state would leave the router believing this path is still healthy while every packet sent over it vanishes.
Gray FailureA lightweight hello protocol — BFD (Bidirectional Forwarding Detection) or an SD-WAN-native health probe — runs continuously over the link, often every 50-300ms, independent of the routing protocol's own hello timers. This is what actually catches a gray failure: the probes simply stop getting replies, regardless of whether the physical interface still reports "up."
BFDOnce a configured number of consecutive probe intervals are missed (a detection multiplier, e.g. 3 missed at 150ms = ~450ms), the path is formally declared down — typically within a few hundred milliseconds to a couple of seconds. This is well before a human notices anything, and crucially, works the same whether the underlying cause was a hard link failure or a gray one.
CheckpointIf dynamic routing is used between branch and HQ (e.g. eBGP per transport), the route learned over the failed path is withdrawn. On an SD-WAN overlay, the affected underlay member is simply marked unusable for path selection — either way, the failed path stops being a candidate for new traffic decisions.
Route WithdrawA route lookup for HQ/internet-bound traffic now resolves to the surviving path — the broadband/LTE circuit — either via a pre-provisioned lower-preference static route now promoted, or SD-WAN's path selection algorithm automatically choosing the next best available member. No human intervention is required for this step.
ReconvergeAny packet already in transit over the failed link at the moment of failure is simply lost — rerouting doesn't retrieve it. Existing TCP sessions bound to that specific path (e.g. an active RDP session) typically break and must reconnect; some SD-WAN platforms with per-flow tracking can reroute successive packets of the same flow onto the new path transparently, but this depends heavily on platform capability, not something to assume universally.
In-FlightNew sessions, and any reconnecting old ones, now traverse the broadband/LTE circuit. If an IPsec/SD-WAN tunnel wasn't already active over this backup transport, it establishes now — adding a small additional delay on top of the raw path-selection time.
ResumedThe backup path typically has materially different characteristics — lower bandwidth, higher latency, sometimes metered/capped data on LTE. Users regain connectivity but may notice real performance degradation; a fast failover is not the same claim as a transparent one.
DegradedWithout a secondary WAN transport, the branch loses connectivity to HQ and the internet entirely until the fiber is physically repaired. Critically, purely local traffic — same-VLAN and inter-VLAN communication within the branch — keeps working fine, since none of that depends on the WAN. The outage's blast radius is scoped exactly to anything requiring HQ or internet reachability.
No BackupMonitoring (NMS/SD-WAN controller) detects the interface-down and BFD-down events and alerts the NOC. A fault ticket is raised with the metro fiber provider referencing the circuit ID; physical repair — often requiring a field technician to splice the cut — restores the primary circuit.
AlertOnce the primary circuit comes back up and passes health checks for a stabilization period, traffic can shift back if policy prefers the primary — but immediate failback isn't always configured, deliberately, to avoid "flapping" traffic back and forth if the repaired link is still intermittently unstable.
Failback| Term | Meaning |
|---|---|
| Gray Failure | A link that stays physically up but silently fails to pass traffic correctly |
| BFD | Bidirectional Forwarding Detection — a fast, lightweight hello protocol for sub-second failure detection |
| Detection Multiplier | The number of consecutive missed health-probe intervals required before declaring a path down |
| Route Withdrawal | Removing a route learned over a now-failed path from the routing table |
| SD-WAN Member / Underlay | One of the physical transports (e.g. MPLS, broadband) an SD-WAN overlay can route traffic across |
| Failover / Failback | Switching to a backup path on failure, and later switching back once the primary recovers |
| Dampening / Stabilization Window | A delay requiring sustained health before trusting a recovered link again, to prevent flapping |
| Circuit ID | The provider's unique reference for a specific WAN circuit, used when raising fault tickets |
| NOC | Network Operations Center — the team monitoring and responding to infrastructure alerts |
| Blast Radius | The actual scope of what's impacted by a given failure, as distinct from what merely sounds impacted |