How do I make a backup route that only activates when the primary fails?
We have a fast primary link and a slow backup. I want traffic on the primary normally, failing over automatically. Can statics do this without a routing protocol?
1 Answer
Yes — a floating static route. Configure the backup with a worse administrative distance than the primary: ip route 0.0.0.0 0.0.0.0 203.0.113.1 (primary, AD 1) and ip route 0.0.0.0 0.0.0.0 198.51.100.1 250 (backup, AD 250). The routing table installs only the best AD, so the backup "floats" invisible until the primary is withdrawn — which happens when its exit interface goes down. That's the catch: if the primary fails somewhere the router can't see (the far end of a carrier circuit, interface still up/up), the static never withdraws and the float never engages. That failure mode is exactly what IP SLA tracking or a real routing protocol solves; for the classic lab/exam scenario, interface-down failover with AD 250 is the expected answer.