OSPF Packets and Neighbor Discovery

Lessons Discussion
62

Hello Markos

Concerning question 1, there is no clear cut answer given for the destination IP address for DBDs from DROTHER routers in the RFC2328 describing OSPFv2. It states what kinds of packets use what kind of address (unicast/multicast) except for the DBD. Take a look at Pages 58-59 on the RFC. However, researching further, there seems to be a consensus that the DBD packets are sent to the unicast addresses of the DR and BDR. Now according to the RFC:

On physical point-to-point networks, the IP destination is always set to the address AllSPFRouters. On all other network types (including virtual links), the majority of OSPF packets are sent as unicasts, i.e., sent directly to the other end of the adjacency. In this case, the IP destination is just the Neighbor IP address associated with the other end of the adjacency … The only packets not sent as unicasts are on broadcast networks; on these networks Hello packets are sent to the multicast destination AllSPFRouters, the Designated Router and its Backup send both Link State Update Packets and Link State Acknowledgment Packets to the multicast address AllSPFRouters, while all other routers send both their Link State Update and Link State Acknowledgment Packets to the multicast address AllDRouters.

So it seems the DR and BDR send LSUs and LSAs to the AllSPFRouters multicast address.

For question 2, DBD packets will be sent to a neighbour depending on the neighbour’s state:

  1. In ExSTart, the router sends an empty DBD.
  2. In the Exchange state the DBD contain summaries of the link state information. Whether or not it is sent depends on whether the router is master or slave.
  3. In loading and full states, the slave must resend its last DBD in rspose to duplicate DBD packets received from the master, but this is done only once, as soon as these states have been reached.

Now when receiving DBD packets under various states, routers will either accept them or drop them, depending on the state of the neighbor adjacency. The conditions under which DBDs are sent or recieved are described in detail in the following sections of the RFC:

  • 10.6. Receiving Database Description Packets
  • 10.8. Sending Database Description Packets

The RFC link once again:

https://www.ietf.org/rfc/rfc2328.txt

I hope this has been helpful!

Laz