When would you use static routes instead of a dynamic routing protocol?
Show the answer
Say this first
Static routes fit small or stub places where there is one way out: a branch with a single WAN link, a default route to an ISP, a route to a firewall or a partner. They cost nothing and do exactly what you typed. They do not notice failures on their own and do not scale. Once there are redundant paths or many prefixes, a dynamic protocol is easier and safer. Most networks use both.
Go deeper
| Static | Dynamic | |
|---|---|---|
| CPU, memory, bandwidth | none | hellos, updates, SPF or DUAL |
| Failover | only if the interface goes down, or with tracking | automatic |
| Scale | every change is manual on every router | adds and removes routes itself |
| Predictability | exactly what you configured | depends on metrics and timers |
| Security | nothing to spoof | needs authentication and passive interfaces |
Typical static uses
- Default route at the internet edge.
- Stub branch: the branch has a default to the hub, and the hub has a static for the branch subnet, often redistributed into the IGP.
- Floating static backup over LTE or a VPN.
- Null0 discard routes for aggregates you announce in BGP.
- Routes to devices that do not run a protocol, such as some firewalls and appliances.
They will ask next
What breaks first when a network built on statics grows?
Consistency. Every new subnet needs edits on many routers, people miss one, and you get black holes or asymmetric paths. Failover also needs IP SLA tracking on every static that matters.
Traps
- Claiming static routes always fail over when the path breaks. They only react to their own interface going down, unless you add tracking.
How did you do?Book 3.4 Routing