Troubleshooting checklist
Start with the neighbour state. It tells you which step is failing.
Neighbour problems
| Symptom | Likely cause | Check |
|---|---|---|
| No neighbour at all | Interface not in OSPF, passive, ACL dropping protocol 89 / multicast, Hello field mismatch | show ip ospf interface eth1, debug ospf packet hello |
| Stuck in Init | One-way: they hear us but we don't hear them (or vice versa) | ACLs, multicast filtering, unidirectional link |
| Stuck in 2-Way on a LAN | Normal for DROther ↔ DROther | Only a problem if nobody is DR (all priority 0) |
| Stuck in ExStart / Exchange | MTU mismatch | show interface on both ends; fix MTU (ip ospf mtu-ignore is a workaround, not a fix) |
| Flapping every ~40s | Hellos lost, dead timer expiring | Interface errors, CPU, timers |
| Adjacency won't form after a change | Duplicate router ID | show ip ospf on both routers |
Route problems
| Symptom | Likely cause |
|---|---|
| A prefix is missing everywhere | Interface not in OSPF, or in the wrong area |
| Inter-area routes missing | Area not connected to area 0, or a filter / range ... not-advertise on the ABR |
| External routes missing in an area | It's a stub/totally stubby area — use the default route |
| Traffic takes a "long" path | Reference bandwidth not raised, so all fast links cost 1 |
| Routes installed but traffic black-holes | Network type mismatch (broadcast vs point-to-point) |
Useful FRR commands
show ip ospf neighbor
show ip ospf interface eth1
show ip ospf database
show ip ospf database router 10.255.0.3
show ip ospf route
show ip route ospf
debug ospf packet helloYou now know enough OSPF to read and build an IGP underlay. Continue with BGP Fundamentals, where OSPF carries the loopbacks for iBGP.
Check your understanding
R1 shows R2 in Init. R2 shows no neighbour at all. Where do you look first?
- R2's inbound filtering: R2 isn't receiving R1's Hellos
- MTU on both interfaces
- The area IDs
- The reference bandwidth
Where next
- BGP on loopbacks vs interface addressesBuilds on this lesson · Intermediate · 45 min
Why BGP sessions between loopbacks survive link failures and use parallel links, and what they need to work: an IGP or static routes, update-source, the eBGP connected check, and BFD. Every failure is reproduced on two real VyOS routers.
- OSPF on VyOS: costs and OSPFv3Builds on this lesson · Intermediate · 30 min
OSPF the VyOS way: interface-based configuration, reading the interface and database output, steering traffic with costs (and the asymmetric routing that follows), and OSPFv3 for IPv6 over link-local addresses. Captured on the three-router edge/core lab.
- BGP FundamentalsBuilds on this lesson · Intermediate · 75 min
How the Border Gateway Protocol glues the internet together: autonomous systems, sessions, path attributes, best-path selection, iBGP scaling and a hands-on eBGP lab with real FRR configuration.
- Edge and core: building out a VyOS networkBuilds on this lesson · Intermediate · 60 min
Split one BGP router into edge routers that face the outside world and a core that carries traffic inside: loopbacks, OSPF for infrastructure, iBGP between loopbacks, next-hop-self, and what happens when a link fails. Built and broken on three real VyOS routers.