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Example: Configuring OSPF Overload Mode

OSPF Overload Function Overview

If the time elapsed after the OSPF instance is enabled is less than the specified timeout, overload mode is set.

• If you want the routing device to participate in OSPF routing, but do not want it to be used for transit traffic. This could include a routing device that is connected to the network for analysis purposes, but is not considered part of the production network, such as network management routing devices.
• If you are performing maintenance on a routing device in a production network. You can move traffic off that routing device so network services are not interrupted during your maintenance window.

You configure or disable overload mode in OSPF with or without a timeout. Without a timeout, overload mode is set until it is explicitly deleted from the configuration. With a timeout, overload mode is set if the time elapsed since the OSPF instance started is less than the specified timeout.

A timer is started for the difference between the timeout and the time elapsed since the instance started. When the timer expires, overload mode is cleared. In overload mode, the router link-state advertisement (LSA) is originated with all the transit router links (except stub) set to a metric of 0xFFFF. The stub router links are advertised with the actual cost of the interfaces corresponding to the stub. This causes the transit traffic to avoid the overloaded routing device and to take paths around the routing device. However, the overloaded routing device’s own links are still accessible.

Note: The routing device can also dynamically enter the overload state, regardless of configuring the device to appear overloaded. For example, if the routing device exceeds the configured OSPF prefix limit, the routing device purges the external prefixes and enters into an overload state. You can limit the number of routes exported into OSPF to minimize the load on the routing device and prevent this potential problem.

Example: Configuring OSPF to Make Routing Devices Appear Overloaded

This example shows how to configure a routing device running OSPF to appear to be overloaded.

• Requirements
• Overview
• Configuration
• Verification

Requirements

• Configure the device interfaces. See the Junos® OS Network Interfaces.
• Configure the router identifiers for the devices in your OSPF network. See Example: Configuring an OSPF Router Identifier.
• Control OSPF designated router election. See Example: Controlling OSPF Designated Router Election
• Configure a single-area OSPF network. See Example: Configuring a Single-Area OSPF Network.
• Configure a multiarea OSPF network. See Example: Configuring a Multiarea OSPF Network.

Overview

You can configure a local routing device running OSPF to appear to be overloaded, which allows the local routing device to participate in OSPF routing, but not for transit traffic. When configured, the transit interface metrics are set to the maximum value of 65535.

• overload—Configures the local routing device so it appears to be overloaded. You might configure this if you want the routing device to participate in OSPF routing, but do not want it to be used for transit traffic, or you are performing maintenance on a routing device in a production network.
• timeout seconds—(Optional) Specifies the number of seconds at which the overload is reset. If no timeout interval is specified, the routing device remains in the overload state until the overload statement is deleted or a timeout is set. In this example, you configure 60 seconds as the amount of time the routing device remains in the overload state. By default, the timeout interval is 0 seconds (this value is not configured). The range is from 60 through 1800 seconds.

Configuration

CLI Quick Configuration

To quickly configure a local routing device to appear as overloaded, copy the following command and paste it into the CLI.

Step-by-Step Procedure

1. Enter OSPF configuration mode.

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

   [edit]user@host edit protocols ospf
2. Configure the local routing device to be overloaded.
   [edit protocols ospf]user@host set overload
3. (Optional) Configure the number of seconds at which overload is reset.
   [edit protocols ospf]user@host set overload timeout 60
4. If you are done configuring the device, commit the configuration.
   [edit protocols ospf]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. The output includes the optional timeout statement.

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

Verification

Confirm that the configuration is working properly.

• Verifying Traffic Has Moved Off Devices
• Verifying Transit Interface Metrics
• Verifying the Overload Configuration
• Verifying the Viable Next Hop

Verifying Traffic Has Moved Off Devices

Purpose

Verify that the traffic has moved off the upstream devices.

Action

From operational mode, enter the show interfaces detail command.

Verifying Transit Interface Metrics

Purpose

Verify that the transit interface metrics are set to the maximum value of 65535 on the downstream neighboring device.

Action

From operational mode, enter the show ospf database router detail advertising-router address command for OSPFv2, and enter the show ospf3 database router detail advertising-router address command for OSPFv3.

Verifying the Overload Configuration

Purpose

Verify that overload is configured by reviewing the Configured overload field. If the overload timer is also configured, this field also displays the time that remains before it is set to expire.

Action

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

Verifying the Viable Next Hop

Purpose

Verify the viable next hop configuration on the upstream neighboring device. If the neighboring device is overloaded, it is not used for transit traffic and is not displayed in the output.

Action

From operational mode, enter the show route address command.