Problem: OSPF Neighbor Stuck in 2-WAY-Cause: Priority 0 Is Configured on All Routers

Problem: OSPF Neighbor Stuck in 2-WAY ‚ Cause: Priority 0 Is Configured on All Routers

It is normal in broadcast media to have a 2-WAY state because not every router becomes adjacent on broadcast media. Every router enters into FULL state with the DR and the BDR.

In this example, there are only two routers on Ethernet; both are configured with priority 0. Priority 0 means that this router will not take part in DR/BDR election process. This configuration is useful when there are "low-end" routers on the segment and the desire is not to make those low-end routers DRs. For this purpose, you should configure priority 0. By default, the priority is set to 1. A router with the highest priority on a segment wins a DR election. If all priorities are kept to the default, the router with the highest router ID becomes the DR. For more information on DR and BDR election, refer to Chapter 8.

If all the routers on an Ethernet segment are configured with priority 0, no routers on the segment will be in FULL state with any other router. This creates problems. At least one router on the segment must have a priority that is not set to 0.

Figure 9-30 shows the network setup suffering from this problem.

Figure 9-30. Network Setup Used to Produce OSPF Neighbor Stuck in 2-WAY Problem

Figure 9-31 shows the flowchart to follow to solve this problem.

Figure 9-31. Problem-Resolution Flowchart

Debugs and Verification

Example 9-82 shows the output of show ip ospf neighbor. No neighbors on this interface are in FULL state with each other.

Example 9-82 show ip ospf neighbor Command Output Determines That Neighbors Are in 2-WAY State with Each Other

 R2#  show ip ospf neighbor  Neighbor ID  Pri  State           Dead Time  Address       Interface 131.108.2.1  0  2-WAY/DROTHER  00:00:32   131.108.1.1   Ethernet0 

Example 9-83 shows that both R1 and R2 Ethernet interfaces are configured with priority 0.

Example 9-83 Priority Settings on Ethernet0 Interfaces of R1 and R2

 R1#  interface Ethernet0   ip address 131.108.1.1 255.255.255.0    ip ospf priority 0   _____________________________________________________________________________________ R2#  interface Ethernet0   ip address 131.108.1.1 255.255.255.0   ip ospf priority 0  

Solution

To fix this problem, remove the priority 0 command on at least one router so that router becomes a DR and forms a FULL adjacency . Example 9-84 shows the configuration change on R1 that fixes this problem.

Example 9-84 Removing priority 0 from R1 So That It Can Form FULL Adjacency with R1

 R1#  interface Ethernet0   ip address 131.108.1.1 255.255.255.0   no ip ospf priority 0  

Example 9-85 shows that after removing the priority 0 command on R1, the problem is fixed and OSPF forms an adjacency with its neighbor.

Example 9-85 Verifying That Removing priority 0 on R1 Has Fixed the Problem

 R2#  show ip ospf neighbor  Neighbor ID  Pri  State     Dead Time  Address       Interface  131.108.2.1  1    FULL/DR   00:00:32   131.108.1.1   Ethernet0