When is static routing the right design choice, and when do you need a dynamic routing protocol?
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Static routing fits stub sites with one way out, default routes toward an internet edge and small networks that rarely change. It is simple, predictable and costs no CPU, but it cannot react to a failure it cannot see. Once there are multiple paths, many subnets or frequent changes, a dynamic protocol pays for itself because it detects failures and finds the alternate path on its own.
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The static route's blind spot
A static route stays in the table as long as its exit interface is up. If the failure is beyond the next hop, for example inside the ISP, the route stays and traffic is black-holed. Fix it with IP SLA and object tracking, or use a routing protocol.
Floating static as a backup
A floating static has an administrative distance higher than the primary route, so it is installed only when the primary disappears. A branch that learns its default by OSPF over MPLS (AD 110) can keep a backup over an internet tunnel:
ip route 0.0.0.0 0.0.0.0 Tunnel100 250When dynamic routing is worth it
- More than one path to the same place.
- More than a handful of subnets or sites.
- Frequent adds and moves, where manual routes would drift.
- When you need fast, automatic failover without scripting.
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Your floating static default never takes over when the MPLS link fails. Why might that be?
Three usual reasons. The primary never disappears: if OSPF stays up over a path that no longer forwards, the OSPF default stays and the backup never floats in. Another source still supplies a default with a lower AD, for example a second OSPF neighbor. Or the backup cannot be installed: a static route that points at a tunnel or interface that is down is not put in the table.
Traps
- Thinking a less specific static route can float. AD is compared only between identical prefixes; a less specific route is always installed and wins only by longest match when the specifics vanish.