OSPF network types

OSPF uses different network types for interfaces.

OSPF relies on mechanisms such as multicast in order for various functions that are crucial to its operation to take place successfully (DR/BDR elections, neighbor adjacencies, sending of updates...). However, not all network infrastructures support these capabilities. OSPF must thus be configured to adjust the way it fulfils these functions in order to successfully operate on such topologies.

However, it's crucial to distinguish between the OSPF protocol's own mechanism of sending multicasts and the underlying network's support (or lack thereof) for multicast. OSPF will attempt to multicast where possible, but in certain topologies and network types (like point-to-multipoint), it will emulate multicast behavior by sending individual unicast packets to each neighbor. This emulation ensures that OSPF can function on networks that might not natively support multicast.

The following list explains the characteristics of the OSPF network types and the situations in which each should be configured:

Broadcast - Used on normal Ethernet networks

  • The Broadcast network type is the default for an OSPF enabled ethernet interface.
  • The Broadcast network type requires that a link support Layer 2 Broadcast capabilities.
  • The Broadcast network type has a 10 second hello and 40 second dead timer.
  • An OSPF Broadcast network type requires the use of a DR/BDR which are automatically elected.

Non-Broadcast - used for frame relay physical interfaces

  • The Non-Broadcast network type is the default for OSPF enabled Frame-Relay physical interfaces.
  • Non-Broadcast networks requires the configuration of static neighbors because broadcast traffic cannot be sent
  • The Non-Broadcast network type has a 30 second hello and 120 second dead timer.
  • An OSPF Non-Broadcast network type requires the use of a DR, but the hub must always be DR for it to work so the spokes should have a priority of 0

Point-to-Point - used between two directly connected routers regardless of the technology used

  • A Point-to-Point OSPF network type does not maintain a DR/BDR relationship.
  • The Point-to-Point network type has a 10 second hello and 40 second dead timer.
  • Point-to-Point network types are intended to be used between 2 directly connected routers.

Point-to-Multipoint - used for frame relay with the broadcast keyword for frame relay maps

  • OSPF treats Point-to-Multipoint networks as a collective of point-to-point links, thus no DR/BDR elections take place.
  • Point-to-Multipoint networks do not maintain a DR/BDR relationship.
  • Point-to-Multipoint networks will have an automatic neighbor discovery, so no need to manually configure them.
  • The Point-to-Multipoint network type has a 30 second hello and 120 second dead timer.

Point-to-Multipoint Non-Broadcast - non broadcast layer 2 topologies

  • Same as Point-to-Multipoint but requires static neighbors. Used on Non-broadcast layer 2 topologies.
  • Gives you the ability to define link cost on a per neighbor basis.

Some additional notes that may be useful include: