Help us improve your experience.

Let us know what you think.

Do you have time for a two-minute survey?

Example: Configuring OSPF Stub and Totally Stubby Areas

 

This example shows how to configure an OSPF stub area and a totally stubby area to control the advertisement of external routes into an area.

Requirements

Before you begin:

Overview

The backbone area, which is 0 in Figure 1, has a special function and is always assigned the area ID 0.0.0.0. Area IDs are unique numeric identifiers, in dotted decimal notation. Area IDs need only be unique within an autonomous system (AS). All other networks or areas (such as 3, 7, and 9) in the AS must be directly connected to the backbone area by area border routers (ABRs) that have interfaces in more than one area.

Stub areas are areas through which or into which OSPF does not flood AS external link-state advertisements (Type 5 LSAs). You might create stub areas when much of the topology database consists of AS external advertisements and you want to minimize the size of the topology databases on the internal routers in the stub area.

The following restrictions apply to stub areas:

  • You cannot create a virtual link through a stub area.

  • A stub area cannot contain an AS boundary router.

  • You cannot configure the backbone as a stub area.

  • You cannot configure an area as both a stub area and an not-so-stubby area (NSSA).

In this example, you configure each routing device in area 7 (area ID 0.0.0.7) as a stub router and some additional settings on the ABR:

  • stub—Specifies that this area become a stub area and not be flooded with Type 5 LSAs. You must include the stub statement on all routing devices that are in area 7 because this area has no external connections.

  • default-metric—Configures the ABR to generate a default route with a specified metric into the stub area. This default route enables packet forwarding from the stub area to external destinations. You configure this option only on the ABR. The ABR does not automatically generate a default route when attached to a stub. You must explicitly configure this option to generate a default route.

  • no-summaries—(Optional) Prevents the ABR from advertising summary routes into the stub area by converting the stub area into a totally stubby area. If configured in combination with the default-metric statement, a totally stubby area only allows routes internal to the area and advertises the default route into the area. External routes and destinations to other areas are no longer summarized or allowed into a totally stubby area. Only the ABR requires this additional configuration because it is the only routing device within the totally stubby area that creates Type 3 LSAs used to receive and send traffic from outside of the area.

Note

In Junos OS Release 8.5 and later, the following applies:

  • A router-identifier interface that is not configured to run OSPF is no longer advertised as a stub network in OSPF LSAs.

  • OSPF advertises a local route with a prefix length of 32 as a stub link if the loopback interface is configured with a prefix length other than 32. OSPF also advertises the direct route with the configured mask length, as in earlier releases.

Figure 1: OSPF Network Topology with Stub Areas and NSSAs
OSPF Network
Topology with Stub Areas and NSSAs

Configuration

CLI Quick Configuration

  • To quickly configure an OSPF stub area, copy the following command and paste it into the CLI. You must configure all routing devices that are part of the stub area.

  • To quickly configure the ABR to inject a default route into the area, copy the following command and paste it into the CLI. You apply this configuration only on the ABR.

  • (Optional) To quickly configure the ABR to restrict all summary advertisements and allow only internal routes and default route advertisements into the area, copy the following command and paste it into the CLI. You apply this configuration only on the ABR.

Step-by-Step Procedure

To configure OSPF stub areas:

  1. On all routing devices in the area, configure an OSPF stub area.Note

    To specify an OSPFv3 stub area, include the ospf3 statement at the [edit protocols] hierarchy level.

  2. On the ABR, inject a default route into the area.
  3. (Optional) On the ABR, restrict summary LSAs from entering the area. This step converts the stub area into a totally stubby area.
  4. If you are done configuring the devices, commit the configuration.

Results

Confirm your configuration by entering the show protocols ospf command. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

Configuration on all routing devices:

Configuration on the ABR (the output also includes the optional setting):

To confirm your OSPFv3 configuration, enter the show protocols ospf3 command.

Verification

Confirm that the configuration is working properly.

Verifying the Interfaces in the Area

Purpose

Verify that the interface for OSPF has been configured for the appropriate area. Confirm that the output includes Stub as the type of OSPF area.

Action

From operational mode, enter the show ospf interface detail command for OSPFv2, and enter the show ospf3 interface detail command for OSPFv3.

Verifying the Type of OSPF Area

Purpose

Verify that the OSPF area is a stub area. Confirm that the output displays Normal Stub as the Stub type.

Action

From operational mode, enter the show ospf overview command for OSPFv2, and enter the show ospf3 overview command for OSPFv3.