OSPF Network Types

OSPF network types

There are three types of networks that are known to OSPF:

  • point to point
  • broadcast
  • non broadcast multiaccess NBMA

For each one of these network types, OSPF has an operational mode. The default operational modes are not always suitable for a specific network. So that’s why sometimes we need to change it.

To change the operational mode of OSPF on a particular network type:

(config-if)#ip ospf network {operational mode}

Adjacency behaviour on point-to-point network types

OSPF-point-to-point-network-type

OSPF point to point network type

  • This network type encompasses the following technologies
    • physical serial interfaces
    • serial point-to-point sub-interfaces
  • No DR or BDR elections
  • Neighbors dynamically become adjacent
  • Hello timer = 10 sec
  • Dead Interval = 40 sec (4 times the Hello timer)

 Adjacency behaviour on broadcast network types

OSPF-broadcast-network-type

OSPF broadcast network type

  • On each broadcast network a DR and a BDR are elected.
  • routers establish full adjacency with the DR only
  • routers communicate with the DR through the multicast address 224.0.0.6
  • DR communicate with routers through the multicast address 224.0.0.5
  • all routers have the same synchronized LSDB

 

Adjacency behavior on non-broadcast network types

ospf-non-broadcast-multiaccess-network-type

OSPF Non Broadcast MultiAccess network type

In this type of networks, OSPF provides 5 operational modes. Two of them are RFCs and the rest is Cisco-proprietary:

OSPF RFC-based operational modes:

  • Non Broadcast
  • Point to Multipoint

Cisco-based operational modes:

  • Point to Multipoint non Broadcast
  • Broadcast
  • Point to multipoint

In NBMA networks, a DR and a BDR are elected.

If there is a tie, then the router with highest Router ID wins.

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