iproute_ospf

Enabling OSPFv2 on an Interface Basis

This document describes how to enable Open Shortest Path First version 2 (OSPFv2) on a per-interface basis to simplify the configuration of unnumbered interfaces. The ip ospf area command allows you to enable OSPFv2 explicitly on an interface. The ip ospf area command is an alternative to enabling OSPFv2 through the address of the interface that matches the address range specified by the network area command.

Finding Feature Information

For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the "Feature Information for Enabling OSPFv2 on an Interface Basis" section.

Use Cisco Feature Navigator to find information about platform support and Cisco IOS XE software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Contents

•Prerequisites for Enabling OSPFv2 on an Interface Basis

•Restrictions on Enabling OSPFv2 on an Interface Basis

•Information About Enabling OSPFv2 on an Interface Basis

•How to Enable OSPFv2 on an Interface Basis

•Configuration Example for Enabling OSPFv2 on an Interface

•Additional References

•Feature Information for Enabling OSPFv2 on an Interface Basis

Prerequisites for Enabling OSPFv2 on an Interface Basis

OSPFv2 must be running on your network.

Restrictions on Enabling OSPFv2 on an Interface Basis

The ip ospf area command is supported only for OSPFv2.

Information About Enabling OSPFv2 on an Interface Basis

This section contains the following information:

•Benefits of Enabling OSPFv2 on an Interface Basis

•Implications of Configuring OSPFv2 On a Router Basis or an Interface Basis

Benefits of Enabling OSPFv2 on an Interface Basis

OSPF is enabled on an interface when the network address for the interface matches the range of addresses that is specified by the network area command, which is entered in router configuration mode. Alternatively, you can enable OSPFv2 explicitly on an interface by using the ip ospf area command, which is entered in interface configuration mode. This capability simplifies the configuration of unnumbered interfaces with different areas.

Because the ip ospf area command is configured explicitly for an interface, it supersedes the effects of the network area command, which is entered at the network level to affect the interfaces whose addresses fall within the address range specified for the network area command.

If you later disable the ip ospf area command, the interface still will run OSPFv2 as long as its network address matches the range of addresses that is specified by the network area command.

Implications of Configuring OSPFv2 On a Router Basis or an Interface Basis

Before you use the ip ospf area command to enable OSPFv2 on an interface, we recommend that you understand the following scenarios and command behavior. There are implications to using the network area command (configuring OSPFv2 in router configuration mode) versus using the ip ospf area command (configuring OSPFv2 in interface configuration mode).

Interface Is Already OSPFv2-Enabled by network area Command with Same Area and Process

If you enter the ip ospf area command on an interface that is enabled in OSPFv2 by the network area command, the process ID or area ID of the interface does not change, and the interface status will not be changed. However, the interface will be flagged as being configured from interface configuration mode, and the configuration data will be saved in the interface description block (IDB).

Interface Is Already Configured by network area Command with Different Area or Process

If you enter the ip ospf area command on an interface that is enabled in OSPFv2 by the network area command, but you change the configuration by changing the process ID and area ID of the interface, after the new configuration information is stored in the IDB, the interface will be removed and reattached. Therefore, the interface will be removed from the original area and process and be added to the new ones. The state of the interface will also be reset.

Interface Is Not Configured by network area Command

If the interface is not enabled in OSPFv2 by the network area command, the area and OSPF router instance will be created if needed. When the router is reloaded, the OSPF process will not begin running until system initialization is complete. To remove an OSPF router instance, enter the no router ospf command. Removing the ip ospf area command in interface mode will not result in removing an OSPF router instance.

Removing an ip ospf area Command

When the ip ospf area command is removed, the interface will be detached from the area. The area will be removed if it has no other attached interfaces. If the interface address is covered by the network area command, the interface will be enabled once again in the area for the network that it is in.

New Processes

If an OSPF process does not already exist, and a router ID cannot be chosen when either the router ospf command or the interface command is configured, a Proximity Database (PDB) and a process will be created, but the process will be inactive. The process will become active when a router ID is chosen, either when it is explicitly configured using the router-id command or when an IP address becomes available. Note that the router ospf command will now be accepted even if a router ID cannot be chosen, putting the command-line interface (CLI) into the OSPF configuration context. Therefore, the router-id command is to be entered before an IP address is available. If the process is not active and the show ip ospf command is entered, the message "%OSPF: Router process X is not running, please provide a router-id" will be displayed.

Link-State Advertisements and Shortest Path First

If a state change occurs as a result of the ip ospf area command, new router link-state advertisements (LSAs) will be generated (also for the old area, if the interface is changing areas) and shortest path first (SPF) will be scheduled to run in both the old and new areas.

How to Enable OSPFv2 on an Interface Basis

Enabling OSPFv2 on an Interface

SUMMARY STEPS

1. enable

2. configure terminal

3. interface type number

4. ip ospf process-id area area-id [secondaries none]

5. end

6. show ip ospf interface [type number]

DETAILED STEPS

Configuration Example for Enabling OSPFv2 on an Interface

Example: Enabling OSPFv2 on an Interface

In the following example, OSPFv2 is configured explicitly on GigabitEthernet interface 0/0/0:

Router(config)# interface GigabitEthernet 0/0/0

Router(config-if)# bandwidth 10000

Router(config-if)# ip address 172.16.1.1 255.255.255.0

Router(config-if)# ip ospf hello-interval 1

Router(config-if)# ip ospf 1 area 0

When the show ip ospf interface command is entered, the following output shows that GigabitEthernet interface 0/0/0 was configured in interface configuration mode to run OSPFv2. The secondary IP addresses on the interface will also be advertised:

Router# show ip ospf interface GigabitEthernet 0/0/0

GigabitEthernet0/0/0 is up, line protocol is up 

  Internet Address 172.16.1.1/24, Area 0 

  Process ID 1, Router ID 172.16.11.11, Network Type BROADCAST, Cost: 10

  Enabled by interface config, including secondary ip addresses

  Transmit Delay is 1 sec, State DR, Priority 1 

  Designated Router (ID) 172.16.11.11, Interface address 172.16.1.1

  Backup Designated router (ID) 172.16.22.11, Interface address 172.16.1.2

  Timer intervals configured, Hello 1, Dead 4, Wait 4, Retransmit 5

    oob-resync timeout 40

    Hello due in 00:00:00

  Supports Link-local Signaling (LLS)

  Index 2/2, flood queue length 0

  Next 0x0(0)/0x0(0)

  Last flood scan length is 1, maximum is 1

  Last flood scan time is 0 msec, maximum is 0 msec

  Neighbor Count is 1, Adjacent neighbor count is 1 

    Adjacent with neighbor 172.26.22.11  (Backup Designated Router)

  Suppress hello for 0 neighbor(s)

Additional References

The following sections provide references related to enabling OSPFv2 on an interface.

Related Documents

Standards

MIBs

RFCs

Technical Assistance

Feature Information for Enabling OSPFv2 on an Interface Basis

Table 1 lists the release history for this feature.

Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which Cisco IOS XE software images support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Note Table 1 lists only the Cisco IOS XE software release that introduced support for a given feature in a given Cisco IOS XE software release train. Unless noted otherwise, subsequent releases of that Cisco IOS XE software release train also support that feature.

Table 1 Feature Information for Enabling OSPFv2 on an Interface Basis

Feature Name	Releases	Feature Information
Enabling OSPFv2 on an Interface Basis Note This feature was originally named "Area Command in Interface Mode for OSPFv2."	Cisco IOS XE Release 2.1 Cisco IOS XE Release 2.6	This document describes how to enable OSPFv2 on a per-interface basis to simplify the configuration of unnumbered interfaces. The ip ospf area command allows you to enable OSPFv2 explicitly on an interface. The ip ospf area command is an alternative to enabling OSPFv2 through the address of the interface that matches the address range specified by the network area command. The following commands are introduced or modified in the feature documented in this module: •ip ospf area.

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Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be actual addresses and phone numbers. Any examples, command display output, network topology diagrams, and other figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses or phone numbers in illustrative content is unintentional and coincidental.

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