Routing Table: The Host Route That Wins
The OSPF route is correct. The router ignores it for one address. Work out why by reading the table the way the forwarding lookup does, live, right below.
Symptom
The OSPF route to the data center is correct and one server is still unreachable. Read the routing table the way the forwarding lookup does.
Your mission
Inspect, recover, then verify
Format
Cisco IOS CLI
Investigate before you configure.
Use the CLI evidence to isolate the fault, make the smallest safe correction, then verify the network state changed.
Observe the symptom and link state.
Diagnose by comparing the protocol evidence.
Verify the expected device state and confirm the original symptom is resolved.
Console access: R1
This workspace stacks for portrait phones. Rotate to landscape for the full split-screen console.
Need a hint?
Reveal the root cause when you're ready.
Try the investigation first, then use this as your escape hatch, not a dead end.
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Lab debrief
Turn the session into a repeatable troubleshooting pattern.
Use this reference after your attempt: first explain the symptom, then verify the evidence, then confirm the repair.
The problem
After a data center migration, everything in 10.20.0.0/16 is reachable from R1 except one server, 10.20.5.10. The OSPF route for the subnet points at the new path through R3.
What you'll practice
- Read prefix, administrative distance, metric and next hop in show ip route
- Explain why longest prefix match is applied before administrative distance
- Find the entry used for one address with show ip route <address>
- Remove a stale static host route and verify the new path
The topology
R1 connects to R2 (the old path) on 10.0.12.0/24 and to R3 (the new path) on 10.0.13.0/24. R3 advertises 10.20.0.0/16 and 10.30.0.0/16 into OSPF.
Commands to run yourself
The real diagnostic commands for this lab, copy-paste ready. Run these in the same order to reproduce the investigation on your own session.
show ip routeshow ip route 10.20.5.10show running-config | include ip routeTopology diagram
Fact-checked references
The commands and behaviour in this lab were checked against these primary references.
Frequently asked
Static routes have a lower AD than OSPF. Isn't that why it wins?
Not here. Administrative distance only compares routes to the same prefix. 10.20.5.10/32 and 10.20.0.0/16 are different prefixes, so the router simply takes the longest match that contains the destination.
Why does only one server fail?
The host route covers exactly one address. Every other address in 10.20.0.0/16 only matches the OSPF /16 and follows it correctly.
What is a host route?
A route with a /32 mask (255.255.255.255) that matches a single IPv4 address. It is useful for steering one destination, and easy to forget after the reason for it has gone.
Ready to run this lab yourself?
No setup, no image sourcing. Book a session or ask for a live demo.