ospf network type point-to-Multipoint non-broadcast behavior

[Cisco Shark]

Hi All,

I am a bit puzzled with the behavior of point-to-Multipoint non-broadcast. I have ospf configured over WAN ( WAN is MPLS L2 managed by the provider).

What I expected with point-to-Multipoint non-broadcast, you have to have the command neighbor neighbor-ip-address for each neighbor so  the router starts sending Hellos and form adjacency with other routers. but what I found out is if you configure neighbor command on only one of the routers the second router will start responding to these Hellos and form neighbor relationship, is this the correct behavior? .. please see the config example below:

R1:

interface GigabitEthernet0/0
 ip address 10.100.1.1 255.255.255.0
 ip ospf network point-to-multipoint non-broadcast
 
 router ospf 1
 router-id 1.1.1.1
 network 10.100.1.0 0.0.0.255 area 0
 
**************************************
 R2:

 interface GigabitEthernet0/0
 ip address 10.100.1.2 255.255.255.0
 ip ospf network point-to-multipoint non-broadcast

 router ospf 1
 router-id 2.2.2.2
 network 10.100.1.0 0.0.0.255 area 0  
 neighbor 10.100.1.1

Thank you in advance

[Simon Darlington · Accepted solution]

Yes. It's the correct behaviour for ospf adjacencies to establish with just a single neighbor statement at one end. It's not best practice though.

As you've pointed out, ospf network type point-to-point non-broadcast needs neighbor statements. Non-broadcast needs neighbors. Our non-broadcast network won't allow multicast (or broadcasts) which is how ospf usually established adjacencies by multicasting hellos to the all routers address 224.0.0.5. No multicast hellos will be sent with this neighbor type configured. Instead neighbor statement(s) are configured to establish unicast comms between ospf speakers to the ip address specified in the neighbor statement. This neighborship can be established unidirectionally. 

https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/13690-18.html

"Though configuration of the neighbor statement on one end is sufficient to form adjacency, it is a good practice to have it configured on both the ends as shown".

Please rate if this helps. Thanks

[Cisco Shark]

Thank you Simon.. This is solves the mystery.... drove me mad for couple days