Configuring OSPF Interfaces

You can configure OSPF attributes for either a single OSPF network using the address commands, or for all OSPF networks on a particular media interface using the ip ospf commands.

OSPF MTU size is negotiated rather than configured. OSPF database description exchange uses the interface MTU to signal the largest OSPF MTU that can be sent over an OSPF interface without fragmentation.

Perform the following tasks to configure OSPF attributes for the OSPF network(s):

• Set the cost.
• Set the dead interval.
• Set the hello interval.
• Set router priority.
• Set the retransmit interval.
• Set the transmit delay.

Note: Before using the address or ip ospf commands, see Precedence of Commands on p 8-18 for information on the relationship between these commands.

address Commands

You can use the address area command to create a new OSPF interface. Use the other address commands to configure parameters for OSPF interfaces that already exist.

Note: You must first issue the address area command before issuing any other address command.

Note: The address commands configure OSPF attributes for a single OSPF network. The ip ospf commands configure OSPF attributes for all OSPF networks in the given interface context—for example, in a multinet environment where multiple IP networks sit on top of an Ethernet interface.

    address area

• Use to create a new OSPF interface and configure the area ID.
• The interface can have an IP address, or it can be unnumbered.
• Example

host1(config-router)#address 10.12.10.2 area 3

• You must first issue the address area command before issuing any other address commands.
• Use the no version to delete the area ID from the specified interface.

    address cost

• Use to specify the cost metric for the interface. The cost is used in calculating the SPF routing table and can range from 0-65535 seconds.
• The interface can have an IP address, or it can be unnumbered.
• Example

host1(config-router)#address unnumbered area 3

host1(config-router)#address unnumbered atm 4/0.1 cost 50

• Use the no version to reset the path cost to the default value, 10.

    address dead-interval

• Use to specify the time period that the system's neighbors should wait without seeing hello packets from the system before they declare the system to be down.
• The dead interval can range from 1-65535 seconds, and is advertised by the system's hello packets.
• For the OSPF routers to become adjacent, the dead interval has to be identical on each router.
• The interface can have an IP address, or it can be unnumbered.
• Example

host1(config-router)#address 192.168.10.32 area 6

host1(config-router)#address 192.168.10.32 dead-interval 60 

• Use the no version to reset the dead interval to the default value, 40.

    address hello-interval

• Use to specify the interval between hello packets that the system sends on the interface.
• The hello interval can range from 1-65535 seconds.
• The interface can have an IP address, or it can be unnumbered.
• Example

host1(config-router)#address 192.168.1.1 area 5

host1(config-router)#address 192.168.1.1 hello-interval 25

• Use the no version to reset the hello interval to the default value, 10.

    address passive-interface

• Use to disable the transmission of routing updates on the interface, meaning that OSPF routing information is neither sent by nor received through the interface.
• The interface can have an IP address, or it can be unnumbered.
• Example

host1(config-router)#address 192.168.100.20 area 5

host1(config-router)#address 192.168.100.20 
passive-interface 

• Use the no version to reenable the transmission of routing updates.

    address priority

• Use to specify the router priority, an 8-bit number from 1 to 255. Used in determining the designated router for the particular network.
• Applies only to nonbroadcast multiaccess (NBMA) networks. Every broadcast and NBMA network has a designated router.
• The interface can have an IP address, or it can be unnumbered.
• Example

host1(config-router)#address unnumbered area 6

host1(config-router)#address unnumbered loopback 0 priority 

• Use the no version to restore the default value, 1.

    address retransmit-interval

• Use to specify the time between LSA retransmissions for the interface when an acknowledgment for the LSA is not received.
• Specify an interval in the range 1-65535 seconds; the default value is 5.
• The interface can have an IP address, or it can be unnumbered.
• Example

host1(config-router)#address 192.168.10.200 area 6

host1(config-router)#address 192.168.10.200 
retransmit-interval 

• Use the no version to restore the default value, 5.

    address transmit-delay

• Use to specify the estimated time it takes to transmit a link state update packet on the interface.
• Specify an interval in the range 1-65535 seconds; the default value is 1.
• The interface can have an IP address, or it can be unnumbered.
• Example

host1(config-router)#address 10.100.25.38 area 7

host1(config-router)#address 10.100.25.38 transmit-delay 

• Use the no version to restore the default value, 1.

ip ospf Commands

The ip ospf commands have two effects on interface configuration. These effects apply to all ip ospf commands:

• Configuration per logical IP interface (for example, Fast Ethernet 0/1.3 or ATM 5/0.1):

The ip ospf command configures the specified OSPF parameter(s) for all networks configured on the given IP interface—for example, all multinetted addresses on an interface.

The no version of the command resets the specified parameter(s) to unspecified.

If the no version of the command takes effect, then for the specified IP interface there is no default value for the specified parameter(s). The parameter is set back to unspecified values. However, the value of the specified parameter for the OSPF interface is set back to the default value or the value previously specified by the address command.

• Configuration per OSPF interface:

The ip ospf command configures the specified OSPF parameter(s) for each OSPF interface which sits on top of the IP interface.

The no version of the command restores the specified parameter(s) to the default value(s).

Note: If you use ip ospf commands to configure OSPF interfaces, you must first have created the interfaces with the network area command.

Note: The ip ospf commands configure OSPF attributes for all OSPF networks in the given interface context—for example, in a multinet environment where multiple IP networks sit on top of an Ethernet interface. The address commands configure OSPF attributes for a single OSPF interface.

    ip ospf cost

• Use to configure the cost of sending a packet on the network.
• Cost is a metric value in the range 0-65535; the default value is 1.
• The router LSA advertises the link state metric as the link cost.
• Example

host1(config)#interface fastethernet 0/0

host1(config-if)#ip ospf cost 50

• Use the no version to reset the path cost to the default value, 1.

    ip ospf dead-interval

• Use to configure the interval since the last hello packet was seen.
• Specify an interval in the range 1-65535 seconds; the default value is 40.
• For the OSPF routers to become adjacent, the dead interval has to be identical on each router.
• The router's hello packets advertise this interval.
• Example

host1(config-if)#ip ospf dead-interval 60

• Use the no version to restore the default value, 40.

    ip ospf hello-interval

• Use to configure the interval between hello packets.
• Specify an interval in the range 1-65535 seconds; the default value is 10.
• For the OSPF routers to become adjacent, the hello interval has to be identical on each router.
• Example

host1(config-if)#ip ospf hello-interval 8

• Use the no version to restore the default value, 10.

    ip ospf priority

• Use to configure the router's priority.
• Select a priority level in the range 0-255; the default value is 1.
• This setting determines the designated router for the particular network.
• A router whose priority is set to 0 cannot be a designated router.
• Configure priority only for interfaces to multiaccess networks.
• Example

host1(config-if)#ip ospf priority 2

• Use the no version to restore the default value, 1.

    ip ospf retransmit-interval

• Use to configure the time interval between retransmission of an LSA.
• Specify an interval in the range 1-65535 seconds; the default value is 5.
• Example

host1(config-if)#ip ospf retransmit-interval 10

• Use the no version to return to the default value, 5.

    ip ospf transmit-delay

• Use to configure the time it takes to transmit a link state update on the interface.
• This is the time between transmissions of LSAs.
• Specify an interval in the range 1-65535 seconds; the default value is 1.
• In setting the time, consider the interface's transmission and propagation delays.
• Example

host1(config-if)#ip ospf transmit-delay 4

• Use the no version to return to the default value, 1.

Comparison Example

Suppose you configure a range of OSPF interfaces with the network area command as follows:

host1(config)#interface fastEthernet 0/0

host1(config-if)#ip address 1.1.1.1 255.255.255.0

host1(config-if)#ip address 2.2.2.2 255.255.255.0 secondary

host1(config-if)#exit

host1(config)#router ospf 1

host1(config-router)#network 1.1.1.0 0.0.0.255 area 0

host1(config-router)#network 2.2.2.0 0.0.0.255 area 0

If you want to specify the cost, you can do so for both interfaces simultaneously:

host1(config)#interface fastEthernet 0/0

host1(config-if)#ip ospf cost 30

You can use address commands to create a third OSPF interface over the Ethernet interface. When you specify a cost, it is set for only that interface:

host1(config)#interface fastEthernet 0/0

host1(config-if)#ip address 3.3.3.3 255.255.255.0 secondary

host1(config-if)#exit

host1(config)#router ospf 1

host1(config-router)#address 3.3.3.3 area 0

host1(config-router)#address 3.3.3.3 cost 25

Precedence of Commands

For a single OSPF interface, when you modify the same OSPF attribute by issuing both the ip ospf command and the address command, the value configured with the address command takes precedence. In other words, the most specific command for a single OSPF interface takes precedence.

Consider the following example. Suppose you have a numbered IP interface with an IP address of 10.10.1.1/24 sitting on top of Fast Ethernet interface 0/0. Configure a single OSPF interface on top of the IP interface.

host1(config)#router ospf 100

host1(router-config)#address 10.10.1.1 area 0

The default cost for this OSPF interface is 10. Change the cost for this OSPF interface using the address cost command:

host1(router-config)#address 10.10.1.1 cost 45

The cost for OSPF interface 10.10.1.1 is now 45.

Now use the ip ospf cost command to change the cost for this OSPF interface:

host1(config)#int FastEthernet 0/0

host1(config-if)#ip ospf cost 23

The cost of OSPF interface 10.10.1.1 does not change. The previously issued address cost command is more specific for the interface and takes precedence over the ip ospf cost command. You must use the address cost command if you want to change the cost again:

host1(router-config)#address 10.10.1.1 cost 23