OSPF Network Types:

OSPF Network Types and Timers

Network Type	Explanation	Hello Timer	Dead Timer
Broadcast	Commonly used for Ethernet or FDDI LANs. Routers can multicast Hello packets.	10 seconds	40 seconds
Non-Broadcast	Suitable for networks with no broadcast capability. Routers unicast Hello packets to neighbors.	30 seconds	120 seconds
Point-to-Point	Typically used for serial links. Only two routers are involved, and Hello packets are unicast.	10 seconds	40 seconds
Point-to-Multipoint	Suitable for multiple routers on a single network. Hello packets are multicast.	30 seconds	120 seconds
Point-to-Multipoint NBMA	Similar to Point-to-Multipoint but designed for non-broadcast multi-access networks (NBMA). Hello packets are unicast.	30 seconds	120 seconds

Broadcast networks

Characteristics of the Broadcast Network Type

The broadcast network type is characterized by the following features:
1. Default Network Type on Ethernet Interfaces: This network type is the default choice for OSPF on Ethernet interfaces.
2. Multicast Hellos: Hellos can be multicast to the 224.0.0.5 OSPF multicast address, allowing routers to discover each other.
3. Hello and Dead Timers: Hello and Dead timers are configured with values of 10 seconds and 40 seconds, respectively, to control neighbor interactions.
4. Next Hop: The next hop is determined by the router that has a link to the route in the Link-State Database (LSDB).
5. DR and BDR Election: To organize the exchange of Link-State Advertisements (LSAs) and facilitate connectivity, a Designated Router (DR) and a Backup Designated Router (BDR) are elected. They play a crucial role in generating Type-2 LSAs, which link all routers on the segment together.
6. LSA Exchange: Routers exchange Link-State information with the DR and BDR using the 224.0.0.6 "All-OSPF-DR/BDR" multicast group, ensuring efficient communication on the broadcast network.

Representation of Non-Broadcast Networks in OSPF

OSPF can operate over non-broadcast networks in two modes: NBMA and Point-to-MultiPoint. The mode chosen impacts how Hello protocol, flooding, and the network representation in the link-state database work.
- NBMA Mode:
  - In NBMA mode, OSPF emulates operation over a broadcast network.
  - A Designated Router (DR) is elected for the NBMA network, and it originates an LSA for the network.
  - The graph representation for broadcast and NBMA networks is identical.
  - Efficient for link-state database size and routing protocol traffic.
  - Requires all routers on the NBMA network to communicate directly, which can be limiting in some non-broadcast networks.
  - Breakdown into logical subnets may be needed for networks with indirect communication, but this introduces administrative complexity and misconfiguration risks.
Summary of NBMA Adjacency Type in OSPF

In NBMA networks, unicast communication is utilized since broadcast communication is typically not feasible.
Multicast hellos for dynamic neighbor discovery are disabled. Instead, you should manually configure OSPF neighbor adjacencies using static neighbor commands.
The next-hop IP address is configured to match the router that owns the Link State Advertisement (LSA) on the LAN segment.
OSPF employs a hello interval of 30 seconds along with a dead interval of 120 seconds.
The network assumes full-mesh unicast connectivity between routers.
The process of Designated Router (DR) and Backup Designated Router (BDR) election takes place. Routers lacking full-mesh unicast connectivity to one another should be excluded from participating in DR/BDR elections by setting the OSPF priority to 0. This can be accomplished using the "ip ospf priority 0" command in interface configuration mode.

Point-to-MultiPoint Mode:
- In Point-to-MultiPoint mode, OSPF treats all router-to-router connections as if they were point-to-point links.
- No DR is elected, and no LSA is generated for the network.
- The network representation in the link-state database excludes the Point-to-MultiPoint network.
- Routers that can communicate directly over the network are connected by bidirectional edges.
- Autodiscovery of OSPF neighbors may be achieved using data-link protocols like Inverse ARP on some non-broadcast networks where broadcast support is unavailable.
Summary of Point-to-Point Adjacency Type in OSPF

The point-to-point adjacency type in OSPF is characterized as follows:

Adjacencies are established using multicast hello messages.
By default, it employs a Hello/Dead timer of 10/40.
There is no Designated Router (DR) or Backup Designated Router (BDR) election in this type.
The next-hop is directed to the remote router on the point-to-point link.
The IP address assigned to the link is conceptualized as a distinct IP address node connected to the advertising router.