OSPF Neighbor Troubleshooting
Overview
This lab simulates real-world OSPF deployment and troubleshooting across a five-router network. Key tasks included configuring serial and Ethernet interfaces, enabling OSPF using interface and network commands, correcting mismatched OSPF network types, and aligning hello/dead timer values. We resolved missing route propagation issues and ensured default route advertisement from the internet-connected edge router. The lab concludes with LSDB verification to confirm successful link-state advertisement.
| Skills Demonstrated |
|---|
| Configuring serial interfaces and setting clock rates |
| Enabling OSPF via interface and network commands |
| Troubleshooting OSPF adjacency failures |
| Resolving mismatched OSPF network types and timer settings |
| Verifying OSPF neighbor relationships and LSDB entries |
| Configuring and validating default route propagation with Type 5 LSAs |
| Testing end-to-end connectivity including external pings |
| Tools Used |
|---|
| Cisco Packet Tracer |
| Route CLI |
| OSPF Show Commands |
| Ping Utility |
1. Network Topology
I have configured a five-router network with different connection types and certain errors to simulate troubleshooting a real network using OSPF.
For this lab, let's assume:
- The connection between R1 and R2 has been newly added, it does not have OSPF configured, and we want to set the clock rate of 128000.
- Only R3 has a route to 10.0.2.0/24
- R2 and R4 won't become OSPF neighbors with R5
- PC1 and PC2 can't ping external server 8.8.8.8
- We must examine the LSDB to see which LSAs are present
2. R1 and R2 OSPF Configuration and Connection
First, let's configure IP addresses on R1 and R2 and enable OSPF on the interface this time opposed to using the network command like in the previous lab.
On R1 and R2 I run the following commands:
R1> en
R1# conf t
R1(config)# int s0/0/0
R1(config)# ip address 192.168.12.1 255.255.255.252
R1(config)# ip ospf 1 area 0
R2> en
R2# conf t
R2(config)# int s0/0/0
R2(config)# ip address 192.168.12.2 255.255.255.252
R2(config)# ip ospf 1 area 0
Now let's configure the clock rate of 128000 between the serial connection of R1 and R2. To configure clock rate, it must be done on the DCE side.
With that being said, if I run the command show controllers s0/0/0 we get this output and can see that it is the DCE, so I will set clock rate here:
Interface Serial0/0/0
Hardware is PowerQUICC MPC860
DCE V.35, clock rate 2000000
idb at 0x81081AC4, driver data structure at 0x81084AC0
SCC Registers:
General [GSMR]=0x2:0x00000000, Protocol-specific [PSMR]=0x8
Events [SCCE]=0x0000, Mask [SCCM]=0x0000, Status [SCCS]=0x00
Transmit on Demand [TODR]=0x0, Data Sync [DSR]=0x7E7E
Interrupt Registers:
Config [CICR]=0x00367F80, Pending [CIPR]=0x0000C000
Mask [CIMR]=0x00200000, In-srv [CISR]=0x00000000
Command register [CR]=0x580
Port A [PADIR]=0x1030, [PAPAR]=0xFFFF
[PAODR]=0x0010, [PADAT]=0xCBFF
Port B [PBDIR]=0x09C0F, [PBPAR]=0x0800E
[PBODR]=0x00000, [PBDAT]=0x3FFFD
Port C [PCDIR]=0x00C, [PCPAR]=0x200
[PCSO]=0xC20, [PCDAT]=0xDF2, [PCINT]=0x00F
Receive Ring
rmd(68012830): status 9000 length 60C address 3B6DAC4
rmd(68012838): status B000 length 60C address 3B6D444
...
R1> en
R1# conf t
R1(config)# int s0/0/0
R1(config-if)# clock rate 128000
Now when running show controllers s0/0/0 once more we can see the clock rate has changed:
Interface Serial0/0/0
Hardware is PowerQUICC MPC860
DCE V.35, clock rate 128000
idb at 0x81081AC4, driver data structure at 0x81084AC0
SCC Registers:
General [GSMR]=0x2:0x00000000, Protocol-specific [PSMR]=0x8
Events [SCCE]=0x0000, Mask [SCCM]=0x0000, Status [SCCS]=0x00
Transmit on Demand [TODR]=0x0, Data Sync [DSR]=0x7E7E
Interrupt Registers:
Config [CICR]=0x00367F80, Pending [CIPR]=0x0000C000
Mask [CIMR]=0x00200000, In-srv [CISR]=0x00000000
Command register [CR]=0x580
Port A [PADIR]=0x1030, [PAPAR]=0xFFFF
[PAODR]=0x0010, [PADAT]=0xCBFF
Port B [PBDIR]=0x09C0F, [PBPAR]=0x0800E
[PBODR]=0x00000, [PBDAT]=0x3FFFD
Port C [PCDIR]=0x00C, [PCPAR]=0x200
[PCSO]=0xC20, [PCDAT]=0xDF2, [PCINT]=0x00F
Receive Ring
rmd(68012830): status 9000 length 60C address 3B6DAC4
rmd(68012838): status B000 length 60C address 3B6D444
...
And when running show ip protocols on both R1 and R2 we can see they are using OSPF now as well:
Routing Protocol is "ospf 1"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Router ID 192.168.12.1
Number of areas in this router is 1. 1 normal 0 stub 0 nssa
Maximum path: 4
Routing for Networks:
Routing Information Sources:
Gateway Distance Last Update
192.168.12.1 110 00:01:12
192.168.34.1 110 00:14:37
192.168.245.1 110 00:01:12
192.168.245.2 110 00:14:37
Distance: (default is 110)
3. Resolve R3
When running show ip route on R3, we see that it only has a route to 10.0.2.0/24:
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route
Gateway of last resort is not set
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 10.0.2.0/24 is directly connected, GigabitEthernet0/0
L 10.0.2.254/32 is directly connected, GigabitEthernet0/0
192.168.34.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.34.0/30 is directly connected, GigabitEthernet0/1
L 192.168.34.1/32 is directly connected, GigabitEthernet0/1
For two routers to form an OSPF neighbor adjacency, several conditions must be met:
- OSPF is enabled on the interfaces
- Interface IP addresses are in the same subnet
- OSPF Hello and Dead timers must match
- Area ID must match
- Authentication settings must match
- OSPF Network Type must be compatible
- MTU size must match
- Router IDs must be unique
- Interfaces have to be up/up
- The interface should not be passive
When I run the command show ip ospf interface g0/1 on R3 and R4, there is an OSPF Network Type mismatch:
R3:
GigabitEthernet0/1 is up, line protocol is up
Internet address is 192.168.34.1/30, Area 0
Process ID 1, Router ID 192.168.34.1, Network Type POINT-TO-POINT, Cost: 1
Transmit Delay is 1 sec, State POINT-TO-POINT,
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:03
Index 1/1, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 1
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 1 , Adjacent neighbor count is 1
Adjacent with neighbor 192.168.245.2
Suppress hello for 0 neighbor(s)
R4:
GigabitEthernet0/1 is up, line protocol is up
Internet address is 192.168.34.2/30, Area 0
Process ID 1, Router ID 192.168.245.2, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 192.168.245.2, Interface address 192.168.34.2
Backup Designated Router (ID) 192.168.34.1, Interface address 192.168.34.1
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:04
Index 2/2, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 1
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 192.168.34.1 (Backup Designated Router)
Suppress hello for 0 neighbor(s)
As we can see R3 G0/1 is using Point-to-Point network type while R4 G0/1 is using a Broadcast network type.
I will remove the Point-to-Point network type on R3 running:
R3> en
R3# conf t
R3(config)# int g0/1
R3(config-if)# no ip ospf network point-to-point
When running show ip route on R3 we now see OSPF working:
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route
Gateway of last resort is not set
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 10.0.2.0/24 is directly connected, GigabitEthernet0/0
L 10.0.2.254/32 is directly connected, GigabitEthernet0/0
192.168.12.0/30 is subnetted, 1 subnets
O 192.168.12.0/30 [110/66] via 192.168.34.2, 00:00:09, GigabitEthernet0/1
192.168.34.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.34.0/30 is directly connected, GigabitEthernet0/1
L 192.168.34.1/32 is directly connected, GigabitEthernet0/1
192.168.245.0/29 is subnetted, 1 subnets
O 192.168.245.0/29 [110/2] via 192.168.34.2, 00:00:09, GigabitEthernet0/1
4. R2 and R4 OSPF Neighbor Troubleshooting with R5
When we run the show ip ospf neighbor command on R2, we can see that it's only neighbors with R1 and R4, not R5:
Neighbor ID Pri State Dead Time Address Interface
192.168.245.2 1 FULL/DR 00:00:38 192.168.245.2 GigabitEthernet0/0
192.168.12.1 0 FULL/ - 00:00:38 192.168.12.1 Serial0/0/0
When running the command show ip ospf interface g0/0 on both R4 and R2, we can see that they are both in the correct subnet, are in the same area, have the same network type, and the same Hello and Dead timers.
Since there is no issue there, I'm going to run the command show ip ospf interface g0/0 on R5 to see any OSPF neighbor mismatch. This is the output I get:
GigabitEthernet0/0 is up, line protocol is up
Internet address is 192.168.245.3/29, Area 0
Process ID 1, Router ID 203.0.113.1, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 203.0.113.1, Interface address 192.168.245.3
No backup designated router on this network
Timer intervals configured, Hello 5, Dead 20, Wait 20, Retransmit 5
Hello due in 00:00:00
Index 1/1, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 1
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 0, Adjacent neighbor count is 0
Suppress hello for 0 neighbor(s)
From the output, we can see the issue is with a misconfiguration of the Hello and Dead timers, they are set to Hello 5 and Dead 20.
Because of this, I'm going to run the command no ip ospf hello-interval and no ip ospf dead-interval on R5's G0/0 interface to reset them to the defaults.
Now when running the command show ip ospf neighbor on R2, we can see R5 is now showing up as an OSPF neighbor and we fixed the OSPF neighbor mismatch:
Neighbor ID Pri State Dead Time Address Interface
192.168.245.2 1 FULL/DROTHER 00:00:37 192.168.245.2 GigabitEthernet0/0
203.0.113.1 1 FULL/DR 00:00:37 192.168.245.3 GigabitEthernet0/0
192.168.12.1 0 FULL/ - 00:00:37 192.168.12.1 Serial0/0/0
5. PC1 and PC2 Can't Reach External Server
Lastly, the final misconfiguration is that PC1 and PC2 can't reach external server 8.8.8.8:
When troubleshooting issues reaching an external server, it usually involves the default gateway and whether a default route was set. Because of this, I check the show ip route settings on R5:
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route
Gateway of last resort is not set
10.0.0.0/24 is subnetted, 2 subnets
O 10.0.1.0/24 [110/66] via 192.168.245.1, 00:03:32, GigabitEthernet0/0
O 10.0.2.0/24 [110/3] via 192.168.245.2, 00:31:16, GigabitEthernet0/0
192.168.12.0/30 is subnetted, 1 subnets
O 192.168.12.0/30 [110/65] via 192.168.245.1, 00:31:16, GigabitEthernet0/0
192.168.34.0/30 is subnetted, 1 subnets
O 192.168.34.0/30 [110/2] via 192.168.245.2, 00:31:16, GigabitEthernet0/0
192.168.245.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.245.0/29 is directly connected, GigabitEthernet0/0
L 192.168.245.3/32 is directly connected, GigabitEthernet0/0
203.0.113.0/24 is variably subnetted, 2 subnets, 2 masks
C 203.0.113.0/30 is directly connected, GigabitEthernet0/0/0
L 203.0.113.1/32 is directly connected, GigabitEthernet0/0/0
We can see that a default route has not been set so I will set a static route and initiate default-information originate so all the routers in the network will know the default route. I run the following commands:
R5> enable
R5# conf t
R5(config)# ip route 0.0.0.0 0.0.0.0 203.0.113.2
R5(config)# router ospf 1
R5(config-router)# default-information originate
Now when running show ip route once again we can see a default route has been set:
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route
Gateway of last resort is 203.0.113.2 to network 0.0.0.0
10.0.0.0/24 is subnetted, 2 subnets
O 10.0.1.0/24 [110/66] via 192.168.245.1, 00:12:28, GigabitEthernet0/0
O 10.0.2.0/24 [110/3] via 192.168.245.2, 00:40:12, GigabitEthernet0/0
192.168.12.0/30 is subnetted, 1 subnets
O 192.168.12.0/30 [110/65] via 192.168.245.1, 00:40:12, GigabitEthernet0/0
192.168.34.0/30 is subnetted, 1 subnets
O 192.168.34.0/30 [110/2] via 192.168.245.2, 00:40:12, GigabitEthernet0/0
192.168.245.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.245.0/29 is directly connected, GigabitEthernet0/0
L 192.168.245.3/32 is directly connected, GigabitEthernet0/0
203.0.113.0/24 is variably subnetted, 2 subnets, 2 masks
C 203.0.113.0/30 is directly connected, GigabitEthernet0/0/0
L 203.0.113.1/32 is directly connected, GigabitEthernet0/0/0
S* 0.0.0.0/0 [1/0] via 203.0.113.2
And when we ping 8.8.8.8 we get successful replies:
We have successfully troubleshooted the OSPF network setup.