Implementing and Administering Cisco Solutions: OSPF Network Types Explained

OSPF Network Types Explained


A user configured OSPF and advertised the Gigabit Ethernet interface in OSPF. By default, to which type of OSPF network does this interface belong?



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A. B. C. D.


The Broadcast network type is the default for an OSPF enabled ethernet interface (while Point-to-Point is the default OSPF network type for Serial interface with

HDLC and PPP encapsulation).

When a user configures OSPF and advertises the Gigabit Ethernet interface in OSPF, the type of OSPF network to which this interface belongs by default depends on the configuration of the network type on the router interface.

There are four types of OSPF network types: point-to-point, broadcast, non-broadcast, and point-to-multipoint.

  1. Point-to-Point Network: This network type connects two routers directly, using a serial cable, for example. OSPF detects the network as a point-to-point network automatically. When OSPF runs on a point-to-point network, it uses a special type of Hello packet that contains fewer fields, which makes OSPF more efficient.

  2. Broadcast Network: A broadcast network is a multi-access network, such as Ethernet, that supports the transmission of broadcast packets. In a broadcast network, OSPF elects a designated router (DR) and a backup designated router (BDR) to reduce the number of adjacencies. The DR and BDR are responsible for forwarding multicast OSPF packets to all other routers on the network. This reduces network traffic and improves network efficiency.

  3. Non-broadcast Network: This type of network is similar to a broadcast network, but it does not support the transmission of broadcast packets. Non-broadcast networks include Frame Relay, ATM, and X.25 networks. In a non-broadcast network, OSPF must be configured with a list of neighbors because it cannot detect neighbors automatically.

  4. Point-to-Multipoint Network: This network type connects multiple routers using a single physical interface. OSPF treats each router as a separate entity and does not elect a DR or BDR. OSPF also sends a special type of Hello packet that contains a list of neighbors.

So, if the Gigabit Ethernet interface is advertised in OSPF without any network type configuration, it is classified as a broadcast network by default.