Topology: R4680-- branch R2809B, 2M link as active, and ISDN link as standby.
Failure symptom: when using the standby link, both routers could ping through to each other, but OSPF neighbor cannot be established.
Revise the OSPF TIMER of dial 0 interface of AR46 to: hello time 10s, dead time 40s. It succeeds in neighbor establishment.
At AR46, execute dis ospf int dial 0 command, and the network type is shown as P2MP, hello time as 30s, dead time as 120s.
At AR2809B, execute dis ospf int dial 0 command, and the network type is shown as P2P, hello time as 10s, and dead time as 40s.
Diversity in configuration of OSPF TIMER of both sides is responsible for why OSPF neighbor cannot be established.
For the network types of P2P and P2MP, OSPF neighbor could not be established unless OSPF TIMER at both sides are the same.