OSPF is a very versatile dynamic routing protocol, not only does it provide redundancy, fast convergence, scalability, automatic failover but it works with all types of network types as well. Today I am going to talk about broadcast and point-to-point OSPF networks and how they function.
Broadcast- Broadcast OSPF network is one of the most common types of topology. An OSPF network is considered an OSPF broadcast network if every router is connected to the same segment either by a switch or an ethernet hub. When routers are connected to the same segment, if one router were to send a broadcast or a multi-cast, every other router on that segment would receive that message. Broadcast networks usually have more than 2 or more routers connected to the segment. Even if there are only two routers on a broadcast network that segment still has the potential for more routers to be added to that same segment unlike point-to-point networks. Broadcast networks are known to support mostly Ethernet and FDDI (Fiber Distributed Data Interface). OSPF assigns network type based on the interface’s data link technology so when it detects ethernet or fiber interfaces it will become a broadcast network. There is an exception if an ethernet link is explicitly configured via CLI to function as a point-to-point network. Broadcast networks are the networks that have DR and BDR elections. DR and BDR routers form full adjacencies with all routers. DROthers only reach full adjacencies with the DR and BDR, but remain in a two-way state when linked to other DROther routers.
Point-to-Point- Point-to-point networks on the other hand operate very differently from broadcast networks. First they always form an adjacency with each other without any option for a third router to join the network. Point-to-point networks do not perform an DR/BDR election. Two routers that are connected to each other via serial interface will automatically become a point-to-point network. Serial interfaces use PPP (Point-to-Point) and HDLC (High Level Data Link Control) to encapsulate data. Whenever OSPF protocol detects that the data link technology is either PPP or HDLC that network type will automatically be viewed as point-to-point topology.
Point-to-point networks use the ALLSPFRouters (224.0.0.5) multi-cast address to communicate while broadcast networks use both AllDRouters (224.0.0.6) and ALLSPFRouter (224.0.0.5) multicast addresses. In broadcast networks, routers are capable of having multiple neighbor adjacencies depending on how many participating interfaces they have while point-to-point networks only form one adjacency. Although broadcast and point-to-point have many differences, one thing they do have in common is that their hello and dead timers are the same (10 second Hello, 40 second Dead).
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