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Troubleshooting OSPF network configuration

The network topology is shown in figure 10.1. After the configuration is completed, it is found that the hosts from R1 to R4 and R5 are sometimes blocked, and the network communication fails.

Figure 10.1 BENET network topology diagram

The configuration of each router is as follows:

R1 configuration

Interface FastEthernet0/0

Ip address 10.0.0.1 255.255.255.252

!

Interface FastEthernet1/0

Ip address 20.0.0.1 255.255.255.252

!

Router ospf 1

Network 10.0.0.0 0.0.0.3 area 0

Network 20.0.0.0 0.0.0.3 area 0

R2 configuration

Interface FastEthernet0/0

Ip address 10.0.0.2 255.255.255.252

!

Interface FastEthernet1/0

Ip address 30.0.0.1 255.255.255.0

!

Router ospf 1

Network 10.0.0.0 0.0.0.3 area 0

Network 30.0.0.0 0.0.0.255 area 1

R3 configuration

Interface FastEthernet0/0

Ip address 30.0.0.5 255.255.255.0

!

Interface FastEthernet1/0

Ip address 20.0.0.2 255.255.255.252

!

Router ospf 1

Network 20.0.0.0 0.0.0.3 area 0

Network 30.0.0.0 0.0.0.255 area 2

R4 configuration

Interface FastEthernet0/0

Ip address 30.0.0.2 255.255.255.0

!

Router ospf 1

Network 30.0.0.0 0.0.0.255 area 1

R5 configuration

Interface FastEthernet0/0

Ip address 30.0.0.6 255.255.255.0

!

Router ospf 1

Network 30.0.0.0 0.0.0.255 area 2

When dealing with OSPF faults, generally first check the routing table, through the routing table to find routing anomalies, and then check the configuration of the router, adjacency and other troubleshooting according to the destination address of the abnormal route entry.

Because there is a communication problem between R1 and R4 and R5, view the routing table of R1 as follows:

20.0.0.0/30 is subnetted, 1 subnets

C 20.0.0.0 is directly connected, FastEthernet1/0

10.0.0.0/30 is subnetted, 1 subnets

C 10.0.0.0 is directly connected, FastEthernet0/0

30.0.0.0/24 is subnetted, 1 subnets

O IA 30.0.0.0 [110/2] via 20.0.0.2, 00:32:40, FastEthernet1/0

[110/2] via 10.0.0.2, 00:32:40, FastEthernet0/0

It is found that the route entries of load balancing appear in the routing table, but there is no requirement of load balancing in network planning.

Check the router that is configured with this segment because the 30.0.0.0amp 24 segment failed. After checking the configuration, it is found that the subnet mask of the interface address is misconfigured, and the mask of / 30 is misconfigured to the mask of / 24, which leads to network communication failure.

The network administrator changes the configuration as follows:

R2 configuration:

Interface FastEthernet1/0

Ip address 30.0.0.1 255.255.255.252

R3 configuration:

Interface FastEthernet0/0

Ip address 30.0.0.5 255.255.255.252

R4 configuration:

Interface FastEthernet0/0

Ip address 30.0.0.2 255.255.255.252

R5 configuration:

Interface FastEthernet0/0

Ip address 30.0.0.6 255.255.255.252

After the configuration change is complete, the routing table has returned to normal and network communication has returned to normal.

When the network running OSPF protocol fails, looking at LSDB can also quickly and effectively determine the problem. In the above fault cases, the fault can also be found through LSDB.

When the OSPF network fails, you can use the show ip ospf database command R1 to view the LSDB, as follows:

OSPF Router with ID (10.0.0.1) (Process ID 1)

Router Link States (Area 0)

Link ID ADV Router Age Seq# Checksum Link count

10.0.0.1 10.0.0.1 953 0x80000005 0x00CAFC 2

30.0.0.1 30.0.0.1 605 0x8000000A 0x0011C2 1

30.0.0.5 30.0.0.5 943 0x80000006 0x009723 1

Net Link States (Area 0)

Link ID ADV Router Age Seq# Checksum

10.0.0.1 10.0.0.1 1448 0x80000002 0x0048B3

20.0.0.2 30.0.0.5 943 0x80000002 0x00C60E

Summary Net Link States (Area 0)

Link ID ADV Router Age Seq# Checksum

30.0.0.0 30.0.0.1 97 0x80000004 0x00F00B

30.0.0.0 30.0.0.5 943 0x80000002 0x00DC1D

When different routers send the same LSA in the bold part of the LSDB of R1, you can use the show ip ospf database summary command to further view the type 3 LSA information received by the R1 router as follows:

OSPF Router with ID (10.0.0.1) (Process ID 1)

Summary Net Link States (Area 0)

Routing Bit Set on this LSA

LS age: 163

Options: (No TOS-capability, DC, Upward)

LS Type: Summary Links (Network)

Link State ID: 30.0.0.0 (summary Network Number)

Advertising Router: 30.0.0.1

LS Seq Number: 80000004

Checksum: 0xF00B

Length: 28

Network Mask: / 24

TOS: 0 Metric: 1

Routing Bit Set on this LSA

LS age: 1009

Options: (No TOS-capability, DC, Upward)

LS Type: Summary Links (Network)

Link State ID: 30.0.0.0 (summary Network Number)

Advertising Router: 30.0.0.5

LS Seq Number: 80000002

Checksum: 0xDC1D

Length: 28

Network Mask: / 24

TOS: 0 Metric: 1

The LSA of the Summary NET issued by the two routers is exactly the same, which results in the load balancing of the route. Check that the routing table has the load balance of the network segment 30.0.0.0 and 24. However, the actual network is not a load-balanced network. Check the configuration and find that the misconfiguration of the subnet mask leads to the routing problem.

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