Default Routes and the Gateway of Last Resort

A branch office router has five specific static routes for internal subnets and needs to send everything else to the internet. Do you write a static route for every public IP on the internet? No — you need a default route. One line, and the router knows what to do with traffic to any destination it hasn't learned about explicitly.

The default route is the catch-all — the routing equivalent of 'I don't know exactly where this goes, but I know the general direction.' Every router should have one, either statically configured or learned dynamically from a routing protocol.

#How the Default Route Works#link

The default route `0.0.0.0/0` matches every destination IP. Because of longest-prefix-match, it's used only when no more specific route exists. Traffic to 172.16.5.0/24 uses the specific static; traffic to 8.8.8.8 falls through to the default.

#Configuring the Default Route#link

The floating static is a classic redundancy trick. The primary default route (`AD 1`) wins. The backup route with `AD 250` is installed in the RIB but not used — it only activates if the primary route disappears. Since floating statics can use any AD value greater than the primary, `250` marks it clearly as a backup.

#Default Routes from Dynamic Protocols#link

• ▪OSPF: `default-information originate` — advertises 0.0.0.0/0 to OSPF peers
• ▪EIGRP: `redistribute static` after configuring a static default, or `ip default-network`
• ▪BGP: default route learned from the ISP, or configured with `neighbor X default-originate`
• ▪IS-IS: `default-information originate` similar to OSPF

In a typical enterprise, the core router has a static default to the ISP, and it advertises that default into the IGP so all internal routers know how to reach the internet. One static route at the edge; every other router learns it dynamically.

#Check Your Understanding#link