Core
Default Routes
A default route is the gateway of last resort used to forward traffic when no more specific route matches, commonly used for internet-bound traffic or stub sites.
What you need to know
- A default route is written as network 0.0.0.0 with mask 0.0.0.0, matching any destination.
- The router always prefers the most specific (longest prefix) match over the default route.
- Default routes can be static or learned dynamically and redistributed via a routing protocol.
- In OSPF, a router originates a default route into the domain using default-information originate.
- The default-information originate always keyword advertises a default even if the originating router lacks one itself.
- show ip route shows the default route marked with an asterisk and the label gateway of last resort.
- Stub and totally stubby OSPF areas rely on an injected default route to reach networks outside the area.
Configuration commands
| Command | Mode | Purpose |
|---|---|---|
| ip route 0.0.0.0 0.0.0.0 203.0.113.1 | config | Configure a static default route pointing to the ISP next hop. |
| ip default-network 172.16.0.0 | config | Flag a network as a candidate default route for classful protocols. |
| router ospf 1 | config | Enter OSPF configuration to advertise a default route. |
| default-information originate | config-router | Advertise a default route into OSPF if one exists in the routing table. |
| default-information originate always | config-router | Force advertisement of a default route into OSPF regardless of local table. |
| ip route 0.0.0.0 0.0.0.0 gigabitethernet0/0 | config | Configure a default route using an exit interface. |
Verify it
- show ip route
- show ip route 0.0.0.0
- show ip protocols
- show ip ospf database external
- ping 8.8.8.8
Common mistakes
- Forgetting default-information originate so OSPF never advertises the default route.
- Assuming default-information originate works without always when no default route exists locally.
- Configuring a default route on the wrong interface, sending traffic to a dead end.
- Expecting the default route to override a more specific matching route.
- Not testing reachability beyond the immediate next hop after adding a default route.
Practise Default Routes in a real CLI
Generated labs, graded against the simulated network state.