Intermediate · OSPF on VyOS: costs and OSPFv3 · section 4 of 5

Checklist and troubleshooting

✓ Lab-tested 17 October 2026 on VyOS 2026.10.16-0024-rolling

Checklist

Router ID set explicitly (the loopback address), and the same for OSPFv2 and OSPFv3.
Loopbacks in area 0 and passive; internal links in area 0 as point-to-point.
External interfaces (transit, IX, customers) never run OSPF.
Costs planned, and changed on both ends of a link, unless you really want asymmetric routing.
OSPFv3 alongside OSPFv2 for IPv6, with global addresses only on loopbacks.
The input firewall accepts OSPF for both IPv4 and IPv6.
BFD on the links if 40 seconds of dead interval is too slow (see BGP on loopbacks).

When it doesn't work

Symptom	Look at	Usual cause
No neighbor at all	`show ip ospf interface`, the input firewall	Different areas, network types or timers; OSPF dropped by the firewall
Stuck in ExStart/Exchange	MTU on both ends	MTU mismatch (`MTU mismatch detection: enabled`)
Neighbor up, route missing	`show ip ospf database`	The interface isn't in OSPF, or the loopback isn't advertised
Traffic takes an odd path	`show ip route` on each router along the way	Costs set on one end only
IPv6 loopbacks unreachable	`show ipv6 ospfv3 neighbor`	OSPFv3 not configured, or dropped by the IPv6 input filter

core1's final OSPF configuration (real output)

vyos@core1:~$ show configuration commands | match "protocols ospf"
set protocols ospf interface eth0 area '0'
set protocols ospf interface eth0 network 'point-to-point'
set protocols ospf interface eth1 area '0'
set protocols ospf interface eth1 network 'point-to-point'
set protocols ospf interface lo area '0'
set protocols ospf interface lo passive
set protocols ospf parameters router-id '198.51.100.241'
set protocols ospfv3 interface eth0 area '0'
set protocols ospfv3 interface eth0 network 'point-to-point'
set protocols ospfv3 interface eth1 area '0'
set protocols ospfv3 interface eth1 network 'point-to-point'
set protocols ospfv3 interface lo area '0'
set protocols ospfv3 interface lo passive
set protocols ospfv3 parameters router-id '198.51.100.241'

Lines 2–8: OSPFv2.
Lines 9–15: OSPFv3, the same shape.