What is OSPF Network Type?

The OSPF network type is a classification of OSPF enabled interfaces. The main network types are as follows

  • BROADCAST
  • POINT_TO_POINT
  • NON_BROADCAST
  • POINT_TO_MULTIPOINT
This is a notation of the network type that can be viewed with show ip ospf interface.

The OSPF enabled interface determines the default network type.

Interface type OSPF Network Tyep
Ethernet BROADCAST
Point-to-point POINT_TO_POINT
Frame Relay/ATM NON_BROADCAST
Unless explicitly configured, the network type will never be POINT_TO_MULTIPOINT.
Frame Relay/ATM is rarely used anymore. As for network types, it is enough to know only BROADCAST and POINT_TO_POINT.

What is determined by network type

The following three points are determined by the network type.

  • How to discover neighbor(auto/manual)
  • Hell/Dead interval default value
  • DR/BDR election

How to discover neighbor(auto/manual)

The method of discovering the neighbor is the destination IP address of the Hello packet.

Auto: Destination IP address of Hello packet = 224.0.0.5
Manual: Destination IP address of Hello packet = The IP address specified by the neighbor command

Automatic neighbor discovery means that you don’t need to know the IP address of the neighbor. The destination IP address of the Hello packet will be 224.0.0.5, which represents all OSPF routers, instead of the individual IP address of the router that will be the neighbor.

On the other hand, manual neighbor discovery means that the IP address of the router to be the neighbor is specified in the configuration in advance. The configuration for this is the following command in OSPF configuration mode.

Specify the neighbor

(config)#router ospf <process-id>
(config-router)#neighbor <ip-address>

<process-id> : OSPF process ID
<ip-address> : IP address of neighbor

Hell/Dead interval default value

Depending on the network type, there are two default values for the Hello/Dead interval.

  • 10/40 sec
  • 30/120 sec

For the BROADCAST and POINT_TO_POINT network types, the default value of the Hello/Dead interval is 10/40 seconds; for the NON_BROADCAST network type, the default value of the Hello/Dead interval is 30/120 seconds.

DR/BDR election

DR/BDR is a router for efficient synchronization of LSDB on a multi-access network. Examples of multi-access networks are Ethernet and Frame Relay/ATM. Therefore, DR/BDR is elected for Ethernet and Frame Relay/ATM network types BROADCAST or NON_BROADCAST.

On the other hand, a serial point-to-point interface should have only two OSPF routers in the same network. the POINT_TO_POINT network type does not need to elect DR/BDR.

Summary of what is determined by network type

The following table summarizes what is determined by network type.

Network Tyep Neighbor discovery Hello/Dead DR/BDR
BROADCAST Auto 10/40 Elected
POINT_TO_POINT Auto 10/40 Not required
NON_BROADCAST Manual 30/120 Elected
POINT_TO_MULTIPOINTAuto30/120Not required

Configuring and verifying the network type

Configuring the network type

The network type is automatically determined by the OSPF enabled interface, but it can also be changed by configuration. To change the network type, enter the following command in interface configuration mode.

Configuring the network type

(config)#interface <interface-name>
(config-if)#ip ospf network {broadcast|point-to-point|non-broadcast|point-to-multipoint}

Changing the network type may change the default value of the Hello/Dead interval. Please be aware that the Hello/Dead interval must match in order to be a neighbor.

Also, the neighbors must be in agreement whether to elect DR/BDR or not. If the recognition of whether to elect DR/BDR is not matched, the SPF calculation cannot be done correctly. When changing the network type, make sure that the DR/BDR election between neighbors also matches.

If the default network type is used, in principle, the network type will match between neighbors.

Verifying the network type

To verify the network type, use the show ip ospf interface command.

R1#show ip ospf interface FastEthernet 0/0
FastEthernet0/0 is up, line protocol is up
  Internet Address 192.168.12.1/24, Area 0
  Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
  Transmit Delay is 1 sec, State DR, Priority 1
  Designated Router (ID) 1.1.1.1, Interface address 192.168.12.1
  No backup designated router on this network
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    oob-resync timeout 40
    Hello due in 00:00:01
  Supports Link-local Signaling (LLS)
  Index 1/1, flood queue length 0
  Next 0x0(0)/0x0(0)
  Last flood scan length is 0, maximum is 0
  Last flood scan time is 0 msec, maximum is 0 msec
  Neighbor Count is 1, Adjacent neighbor count is 0
  Suppress hello for 0 neighbor(s)

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