Multicast and Unicast in Ospf
Question:
I am currently studying OSPF and I got confused by reading the book (Foundation Guide) and then aplying what learned to my 3-layer switch WS-C3560X-48T-L.
The book refers that Update and ACK packets are sent to multicast address alongside with hello's. But when trying this in PT I see that Update and ACKs are sent to unicast as well. So in packet tracer only hellos are sent always to multicast. Everything written so far has refers to a point to point network.
When I tried in an ethernet LAN, same behavior occured until the 2-way and Full states got handled. Then except hellos, everything else was BR/BDR adress and to 224.0.0.5 when LSAs were originating from BDR/BDR.
Answer:
I guess one thing good to keep in mind is that OSPF on Cisco routers is Ciscosimplementation of OSPF...not sayng thats a factor in this issue, but just something to keep in mind.
I was looking at RFC2328 - here is a general statement on unicast/multicast:
The IP destination address for the packet is selected as follows. 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 (see Section 10). 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.
"Depending on the state of the sending interface and the source of the advertisements being acknowledged, a Link State Acknowledgment packet is sent either to the multicast address AllSPFRouters, to the multicast address AllDRouters, or as a unicast."
I think she is refering to the DR multicast address (224.0.0.6), since on that network there has been elected the DR/BDR, so all nieghbors after receiving the lsus from the DR they have to send the acks so they will send those lsacks (acks) via multicast address 224.0.0.6, since they can comuniate only with the DR, so they have to use that multicast address to send even the acks in that case. Having said that, I would answer now, the lsacks are sent as multicast except in NBMA and point-to-multipoint networks.