OSPF Media Type

The OSPF protocol runs over several media types and its functionality is different from each one.   Usually, the media defines the default network type.

Broadcast Multi-access

Include Ethernet, Fast Ethernet, Gigabit Ethernet, FDDI and Token Ring.  This network type requires that the link supports Layer 2 broadcast and the election of DR and BDR takes place by default.  No specific configuration is required because is on by default in these media.

OSPF-Broadcast

Point-to-Point

Include PPP and HDLC encapsulated links such T1, ATM point-to-point subinterfaces, and similar P2P interfaces. In this network type, DR and BDR election does not take place. The OSPF packets are multicast based because in some cases unnumbered links are used and the destination address is not known. The default OSPF network type of point-to-point is on by default over these media.

OSPF-P2P

Nonbroadcast Multiaccess (NBMA)

Include Frame Relay, ATM, X.25 and others.   This network type requires additional configuration to operate.   DR and BDR take place in this network type.  The default OSPF network type of nonbroadcast is on by default over these media.   The OSPF neighbors must be manually defined thus all traffic is unicast.

This network type can be run in 3 modes under OSPF:

  • Broadcast mode – In this mode, the broadcast network is simulated. DR and BDR are elected.   It can be configured by configuring the network-type as broadcast or statically configuring the neighbors.
  • Point-to-Point mode – In this mode, each virtual circuit (VC) must be defined as a separate point-to-point subinterface; for instance, separate subnets for each point-to-point subinterfaces are required. The network type is not required to be set because, by default, all subinterfaces have a point-to-point network type.
  • Point-to-Multipoint mode – In this mode, no DR/BDR election occurs. Multicast hellos are sent to discover neighbors, so the layer 2 must support multicast/broadcast capability.   Only one subnet is required.   This is the recommended network type to use in non-full mesh NBMA networks.
  • Point-to-Multipoint non-broadcast –In this mode the NBMA cloud don’t support multicast, thus, the neighbors should be manually defined. In this mode, no DR/BDR election occurs.

OSPF-NBMA

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