OSPF: Reference Bandwidth Mismatch
Nothing is down, nothing is missing, and the return path is wrong. Find the two routers that genuinely disagree about distance, live, right below.
Symptom
Every adjacency is FULL and every route is present, but traffic takes a slow path in one direction. Compare how each router costs the same links.
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 · R2
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.
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
All OSPF neighbors are FULL and every prefix is in every routing table, but traffic between two sites leaves over the gigabit link and returns over a much slower backup path. One router was recently updated to a higher reference bandwidth.
What you'll practice
- Explain how OSPF derives interface cost from reference bandwidth
- Identify inconsistent reference bandwidth across a domain
- Recognise asymmetric routing as a symptom rather than a cause
- Apply a consistent reference bandwidth and verify path symmetry
1. Explain how OSPF derives interface cost from reference bandwidth
Cost is reference bandwidth divided by interface bandwidth, rounded down to a minimum of 1. With the 100 Mbps default, every link at or above 100 Mbps costs exactly 1, which erases the distinction between a gigabit link and a fast ethernet one.
2. Identify inconsistent reference bandwidth across a domain
show ip ospf reports the reference bandwidth per router. Checking it everywhere is the only reliable way to catch a partial rollout.
3. Recognise asymmetric routing as a symptom rather than a cause
Asymmetry itself is not always a fault, but when it appears after a change it usually points at a metric disagreement. Knowing that saves a great deal of time.
4. Apply a consistent reference bandwidth and verify path symmetry
You will confirm the fix by tracing the path in both directions, which is the only proof that both routers now agree.
The topology
Two sites are joined by a fast primary link and a slower secondary path, with several routers participating in a single OSPF area.
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 ospf | include Referenceshow ip ospf interface brieftraceroute 192.168.20.1Topology diagram
Fact-checked references
The commands and behaviour in this lab were checked against these primary references.
Frequently asked
Why not just set the cost manually on each interface?
You can, and ip ospf cost overrides the calculation entirely. But manual costs must be maintained by hand on every link, whereas a consistent reference bandwidth scales automatically as interface speeds change.
What reference bandwidth should I choose?
High enough that your fastest link still costs more than 1, so future upgrades remain distinguishable. The value matters far less than applying the same one to every router in the domain.
Ready to run this lab yourself?
No setup, no image sourcing. Book a session or ask for a live demo.