RFC 2328: OSPF Version 2 defines OSPF network types. In this article, we are going to discuss the two major network types, which are Point-to-Point and Broadcast. As a network engineer working with OSPF, you must understand the differences between each OSPF network type and their compatibility. Some types will function together, but the hello and dead timers must be adjusted. The following list indicates which OSPF network types can communicate with one another:
- Broadcast to Broadcast
- Non-broadcast to Non-broadcast
- Point-to-Point to Point-to-Point
- Broadcast to Non-broadcast Networks (adjust hello and dead timers)
- Point-to-Point to Point-to-Multipoint Networks (adjust hello and dead timers)
Point-to-Point
A Point-to-Point network type is, as its name implies, a connection between two specific points (or OSPF routers). On a point-to-point link, a packet delivered from one of the routers will always have precisely one recipient. It does not maintain a DR/BDR relationship, and it has a 10-second hello and 40-second dead timer. Leased lines running Point-to-Point Protocol (PPP) and High-Level Data Link Control (HDLC) are some examples of point-to-point links.
Broadcast
The broadcast network type is the default network type for an OSPF-enabled Ethernet interface. It requires a link that supports Layer 2 broadcast. It requires the use of a DR/BDR relationship, and it has a 10-second hello and 40-second dead timer.
Configuring the Network Types
OSPF is a very versatile protocol. Take into account its capacity to support both broadcast and non-broadcast multi-access (NBMA) network types. OSPF responds by changing its operation to match the settings you’ve provided.
OSPF makes an attempt to detect physical media and defaults to the appropriate behavior. If you don’t like the default behavior, you can override it using the following command:
Router#conf t Router(config)#int gi0/0 Router(config-if)#ip ospf ne Router(config-if)#ip ospf network ? broadcast Specify OSPF broadcast multi-access network non-broadcast Specify OSPF NBMA network point-to-multipoint Specify OSPF point-to-multipoint network point-to-point Specify OSPF point-to-point network
Let’s have an example. We can see below that the default network type is Broadcast and with DR neighbor.
Router#sh ip ospf int gi0/0 GigabitEthernet0/0 is up, line protocol is up Internet Address 10.1.1.1/30, Area 0, Attached via Network Statement Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
Router#sh ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 2.2.2.2 1 FULL/DR 00:00:34 10.1.1.2 GigabitEthernet0/0
Change the interface network type from Broadcast to Point-to-Point:
Router#conf t Router(config)#int gi0/0 Router(config-if)#ip ospf network point-to-point
Upon checking again, we can see that the network type is now POINT_TO_POINT.
Router#sh ip ospf int gi0/0 GigabitEthernet0/0 is up, line protocol is up Internet Address 10.1.1.1/30, Area 0, Attached via Network Statement Process ID 1, Router ID 1.1.1.1, Network Type POINT_TO_POINT, Cost: 1
If we check the OSPF neighbor again, we will no longer see the DR/BDR relationship.
Router#sh ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 2.2.2.2 0 FULL/ - 00:00:37 10.1.1.2 GigabitEthernet0/0
Why would not having a DR/BDR neighbor be beneficial? Assume you have a point-to-point network. Because there is only one other router to communicate with, you can reduce router memory and CPU consumption by not using neighbors. In this instance, having a basic adjacency works nicely.
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