ip route 0.0.0.0 0.0.0.0 [next-hop]

When troubleshooting connectivity issues, the first step is often to verify the presence of a default route. Use `show ip route` and look for a line starting with 'S*' (static default route) or 'O*' (OSPF default), etc. If no default route exists, the router will drop packets destined for unknown networks, resulting in 'Destination Unreachable' ICMP messages. A healthy output shows a default route with a valid next-hop or interface.

Problem indicators include: no default route at all, a default route pointing to an unreachable next-hop, or a default route with a high administrative distance that is not preferred. Focus on the 'via' field (next-hop IP) or interface, and the administrative distance. Common symptoms this command helps diagnose include: inability to reach the internet, intermittent connectivity to external sites, or asymmetric routing.

The diagnostic flow: 1) Check `show ip route` for default route. 2) If missing, check if a static route is configured with `show running-config | include ip route 0.0.0.0`. 3) If configured but not in table, verify next-hop reachability with `ping` or `show ip arp`. 4) If next-hop is reachable, check for routing table conflicts (e.g., a more specific route that might be preferred). 5) Use `show ip route 0.0.0.0` to see detailed information. 6) Correlate with `show ip interface brief` to ensure the exit interface is up/up. 7) Use `debug ip routing` (with caution) to see route installation events. The default route can also be learned via dynamic protocols; check `show ip protocols` to see if a default route is being redistributed. In complex scenarios, use `traceroute` to see the path taken and identify where traffic stops.

The `show ip cef` command can show the forwarding entry for the default route. Remember that the default route is only used if no other more specific route matches; verify that there are no overlapping routes that might cause unexpected behavior.