OSPF LSU/LSAck flooding in point to multipoint interface

[Ariyarathna]

I have observed following observations for  OSPF Point to Multipoint interface.

1.It floods received LSU to all connected Neighbors via the Point to Multipoint interface, even to the original sender.

2. LSAck send to the all the connected neighbors through the Point to Multipoint interface ,not only to the original sender     but to all neighbors connected via interface.

3.But if only one Neighbor connected through Point to Multipoint interface, LSU will not send back to the original sender.

Please help me to understand the concept behind the above observations.

Thank you very much for your time and effort

[Giuseppe Larosa · Accepted solution]

Hello @Ariyarathna ,

with point to multipoint network type OSPF  just knows that not every neighbor in the subnet can be directly reachable at L2.

Besides this , no DR/BDR election occurs.

1)   This is probably done for simplicity and efficiency: the LSU is sent to every reachable neighbor regardless if one of them is the LSU originator. For the LSU originator this can be seen as a form of acknowledgment.

2) Again for simplicity the router does not track who is the original sender and so it sends the LSA ack over each L2 circuit/path.

3) OSPF cannot know  how the L2 topology looks like , however it can track tne number of neighbors out of the interface, if it is only one it can revert to a point to point logic and this is waht you see

Hope to help

Giuseppe

[Ariyarathna]

@Giuseppe Larosa  Thank you very much for your clear explanation.