Troubleshooting

Understand

Inspect interface/address health, OSPF activation/area, neighbor state, LSDB records, calculated/installed routes, then forwarding and return paths. Full does not prove a prefix is enabled; an LSA does not prove usable reachability.

These faults are independent. Restore the complete baseline between them. Scoped debugging can help in a lab; sustained broad debugging can overload a production control plane.

Lab setup

Use a fresh lab for this lesson; changes from other lessons are not cumulative. All outputs are illustrative excerpts, not recorded router tests. Transit cost is explicitly 10 and Loopback0 cost is 1. Interface names may vary by image.

Loopback0 is 10.255.0.<router number>/32; area assignments: R1: area 0, R2: area 0.

configure terminal
hostname R1
router ospf 1
 router-id 10.255.0.1
 passive-interface Loopback0
 exit
interface Loopback0
 ip address 10.255.0.1 255.255.255.255
 ip ospf 1 area 0
 ip ospf cost 1
 exit
interface GigabitEthernet1
 ip address 192.0.2.1 255.255.255.252
 ip ospf network point-to-point
 ip ospf cost 10
 ip ospf 1 area 0
 no shutdown
 exit
end

configure terminal
hostname R2
router ospf 1
 router-id 10.255.0.2
 passive-interface Loopback0
 exit
interface Loopback0
 ip address 10.255.0.2 255.255.255.255
 ip ospf 1 area 0
 ip ospf cost 1
 exit
interface GigabitEthernet1
 ip address 192.0.2.2 255.255.255.252
 ip ospf network point-to-point
 ip ospf cost 10
 ip ospf 1 area 0
 no shutdown
 exit
end

Apply

Fault A · wrong area

R2 · inject

configure terminal
interface GigabitEthernet1
 ip ospf 1 area 1
end

R1# show ip ospf interface GigabitEthernet1 | include Area
  Internet Address 192.0.2.1/30, Area 0
R2# show ip ospf interface GigabitEthernet1 | include Area
  Internet Address 192.0.2.2/30, Area 1

No stable adjacency survives expiry. Compare link settings; process numbers are not the problem.

R2 · repair

configure terminal
interface GigabitEthernet1
 ip ospf 1 area 0
end

Fault B · MTU

On an image supporting ip mtu, lower R2 Gi1 IP MTU and re-form only this link. ExStart or Exchange symptoms vary by image.

R2 · inject on fresh baseline

configure terminal
interface GigabitEthernet1
 ip mtu 1400
 shutdown
 no shutdown
end

R1# show ip ospf neighbor
Neighbor ID     Pri State      Dead Time Address     Interface
10.255.0.2        0 EXSTART/-  00:00:36  192.0.2.2  GigabitEthernet1

Hellos pass while DBD negotiation fails. Compare interface/IP MTUs; mtu-ignore can hide the symptom while larger packets still fail.

R2 · repair

configure terminal
interface GigabitEthernet1
 no ip mtu
end

Verify and interpret

Fault C · unadvertised loopback

R2 · inject on fresh baseline

configure terminal
interface Loopback0
 no ip ospf 1 area 0
end

R1# show ip ospf neighbor
Neighbor ID     Pri State   Dead Time Address     Interface
10.255.0.2        0 FULL/-  00:00:37  192.0.2.2  GigabitEthernet1
R1# show ip route 10.255.0.2
% Network not in table
R2# show ip ospf interface Loopback0
%OSPF: OSPF not enabled on Loopback0

Router ID remains 10.255.0.2; identity does not advertise an address.

R2 · repair

configure terminal
interface Loopback0
 ip ospf 1 area 0
end

R1# show ip route ospf | include 10.255.0.2
O 10.255.0.2/32 [110/11] via 192.0.2.2, GigabitEthernet1
R1# ping 10.255.0.2 source Loopback0
!!!!!

Route and return reachability recover.

Fault D · local installation filter

R1 · inject on fresh baseline

configure terminal
ip prefix-list LOCAL-IN seq 5 deny 10.255.0.2/32
ip prefix-list LOCAL-IN seq 10 permit 0.0.0.0/0 le 32
router ospf 1
 distribute-list prefix LOCAL-IN in
end

R1# show ip ospf database router 10.255.0.2
  Link State ID: 10.255.0.2
  Advertising Router: 10.255.0.2
R1# show ip route 10.255.0.2
% Network not in table

The LSDB remains synchronized while a local route filter blocks installation. It does not stop area-wide LSA flooding.

R1 · repair

configure terminal
router ospf 1
 no distribute-list prefix LOCAL-IN in
end

Repeat neighbor, route and sourced ping checks after repair. Across ABRs, trace where Type 3 reachability stops. For externals, add import policy, area type and forwarding-address checks.

Technical references

OSPFv2 standard · Router configuration guide