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Example: Controlling OSPF Route Preferences

This example shows how to control OSPF route selection in the forwarding table. This example also shows how you might control route selection if you are migrating from OSPF to another IGP.

Requirements

This example assumes that OSPF is properly configured and running in your network, and you want to control route selection because you are planning to migrate from OSPF to a different IGP.

Overview

Route preferences are used to select which route is installed in the forwarding table when several protocols calculate routes to the same destination. The route with the lowest preference value is selected.

By default, internal OSPF routes have a preference value of 10, and external OSPF routes have a preference value of 150. You might want to modify this setting if you are planning to migrate from OSPF to a different IGP. Modifying the route preferences enables you to perform the migration in a controlled manner.

This example makes the following assumptions:

  • OSPF is already running in your network.
  • You want to migrate from OSPF to IS-IS.
  • You configured IS-IS per your network requirements and confirmed it is working properly.

In this example, you increase the OSPF route preference values to make them less preferred than IS-IS routes by specifying 168 for internal OSPF routes and 169 for external OSPF routes. IS-IS internal routes have a preference of either 15 (for Level1) or 18 (for Level 2), and external routes have a preference of 160 (for Level 1) or 165 (for Level 2). In general, it is preferred to leave the new protocol at its default settings to minimize complexities and simplify any future addition of routing devices to the network. To modify the OSPF route preference values, configure the following settings:

  • preference—Specifies the route preference for internal OSPF routes. By default, internal OSPF routes have a value of 10. The range is from 0 through 4,294967,295 (232 – 1).
  • external-preference—Specifies the route preference for external OSPF routes. By default, external OSPF routes have a value of 150. The range is from 0 through 4,294967,295 (232 – 1).

Configuration

CLI Quick Configuration

To quickly configure the OSPF route preference values, copy the following command and paste it into the CLI.

[edit]set protocols ospf preference 168 external-preference 169

Step-by-Step Procedure

To configure route selection:

  1. Enter OSPF configuration mode and set the external and internal routing preferences.

    Note: To specify OSPFv3, include the ospf3 statement at the [edit protocols] hierarchy level.

    [edit]user@host# set protocols ospf preference 168 external-preference 169
  2. If you are done configuring the device, commit the configuration.
    [edit]user@host# commit

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.

user@host# show protocols ospf preference 168;external-preference 169;

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

Verification

Confirm that the configuration is working properly.

Verifying the Route

Purpose

Verify that the IGP is using the appropriate route. After the new IGP becomes the preferred protocol (in this example, IS-IS), you should monitor the network for any issues. After you confirm that the new IGP is working properly, you can remove the OSPF configuration from the routing device by entering the delete ospf command at the [edit protocols] hierarchy level.

Action

From operational mode, enter the show route command.

 

Related Documentation

 

Published: 2012-12-08

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