Factors that affect OSPF neighbor relationships, OSPF neighbor issues: network and subnet mask

1. OSPF troubleshooting: Multi-point access networks require consistent masks. Inconsistency causes OSPF neighbors to fail to establish

Ethernet also has multi-point access. Here we take SW1 and R3 as examples for demonstration:

 [SW1]display current-configuration interface Vlanif 30 
 
 IP address 10.1.30.10 255.255.255.0 
 
 ospf enable 1 area 0.0.0.34 
 
 [R3]display current-configuration interface g 0/0/0 
 
 ip address 10.1.35.3 255.255.255.0 
 
 ospf enable 1 area 0.0.0.35

R3 and SW1 establish OSPF neighbor relationship normally

At this time, we change the mask of VLAN30 interface of SW1 to 30

 [SW1]interface Vlanif 30 
 
 [SW1-Vlanif30]ip address 10.1.30.10 28

At this time, the direct connection between SW1 and R3 interfaces can still be pinged normally, but the OSPF neighbor establishment fails.

By looking at the ospf error you can see that the network masks do not match:

At this point we can restore the mask of SW1 to 24 to establish the OSPF neighbor relationship normally.

 [SW1]interface Vlanif 30 
 
 [SW1-Vlanif30]ip address 10.1.30.10 24

2. Inconsistency of point-to-point network masks does not affect the establishment of OSPF neighbor relationships

Next, we configure different masks for the PPP link between R3 and R4.

The mask is 24, and R3 and R4 establish an OSPF neighbor relationship normally.

At this time, we configure the mask of R3's S2/0/0 interface to 28

 [R3]interface Serial 2/0/0 
 
 [R3-Serial2/0/0]ip address 10.1.34.3 28

At this time, R3 and R4 can still establish OSPF neighbor relationship normally