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Configuring Graceful Restart for OSPF

Graceful Restart for OSPF Overview

Graceful restart allows a routing device undergoing a restart to inform its adjacent neighbors and peers of its condition. During a graceful restart, the restarting device and its neighbors continue forwarding packets without disrupting network performance. Because neighboring devices assist in the restart (these neighbors are called ), the restarting device can quickly resume full operation without recalculating algorithms.

Note:

On a broadcast link with a single neighbor, when the neighbor initiates an OSPFv3 graceful restart operation, the restart might be terminated at the point when the local routing device assumes the role of a helper. A change in the LSA is considered a topology change, which terminates the neighbor’s restart operation.

Graceful restart is disabled by default. You can either globally enable graceful restart for all routing protocols, or you can enable graceful restart specifically for OSPF.

This topic describes the following information:

Helper Mode for Graceful Restart

When a device enabled for OSPF graceful restart restarts, it retains routes learned before the restart in its forwarding table. The device does not allow new OSPF link-state advertisements (LSAs) to update the routing table. This device continues to forward traffic to other OSPF neighbors (or helper routers), and sends only a limited number of LSAs during the restart period. To reestablish OSPF adjacencies with neighbors, the restarting device must send a grace LSA to all neighbors. In response, the helper routers enter helper mode (the ability to assist a neighboring device attempting a graceful restart) and send an acknowledgment back to the restarting device. If there are no topology changes, the helper routers continue to advertise LSAs as if the restarting device had remained in continuous OSPF operation.

Note:

Helper mode is enabled by default when you start the routing platform, even if graceful restart is not enabled. You can disable helper mode specifically for OSPF.

When the restarting device receives replies from all the helper routers, the restarting device selects routes, updates the forwarding table, and discards the old routes. At this point, full OSPF adjacencies are reestablished and the restarting device receives and processes OSPF LSAs as usual. When the helper routers no longer receive grace LSAs from the restarting device or when the topology of the network changes, the helper routers also resume normal operation.

Beginning with Junos OS Release 11.4, you can configure restart signaling-based helper mode for OSPFv2 graceful restart configurations. The Junos OS implementation is based on RFC 4811, OSPF Out-of-Band Link State Database (LSDB) Resynchronization, RFC 4812, OSPF Restart Signaling, and RFC 4813, OSPF Link-Local Signaling. In restart signaling-based helper mode implementations, the restarting device informs its restart status to its neighbors only after the restart is complete. When the restart is complete, the restarting device sends hello messages to its helper routers with the restart signal (RS) bit set in the hello packet header. When a helper router receives a hello packet with the RS bit set in the header, the helper router returns a hello message to the restarting device. The reply hello message from the helper router contains the ResyncState flag and the ResyncTimeout timer that enable the restarting device to keep track of the helper routers that are syncing up with it. When all helpers complete the synchronization, the restarting device exits the restart mode.

Note:

Restart signaling-based graceful restart helper mode is not supported for OSPFv3 configurations.

Planned and Unplanned Graceful Restart

OSPF supports two types of graceful restart: planned and unplanned. During a planned restart, the restarting routing device informs the neighbors before restarting. The neighbors act as if the routing device is still within the network topology, and continue forwarding traffic to the restarting routing device. A grace period is set to specify when the neighbors should consider the restarting routing device as part of the topology. During an unplanned restart, the routing device restarts without warning.

Example: Configuring Graceful Restart for OSPF

This example shows how to configure graceful restart specifically for OSPF.

Requirements

Before you begin:

Overview

Graceful restart enables a routing device undergoing a restart to inform its adjacent neighbors and peers of its condition. During a graceful restart, the restarting routing device and its neighbors continue forwarding packets without disrupting network performance. By default, graceful restart is disabled. You can globally enable graceful restart for all routing protocols by including the graceful-restart statement at the [edit routing-options] hierarchy level, or you can enable graceful restart specifically for OSPF by including the graceful-restart statement at the [edit protocols (ospf|ospf3)] hierarchy level.

The first example shows how to enable graceful restart and configure the optional settings for the grace period interval. In this example, interfaces fe-1/1/1 and fe-1/1/2 are in OSPF area 0.0.0.0, and you configure those interfaces for graceful restart. The grace period interval for OSPF graceful restart is determined as equal to or less than the sum of the notify-duration time interval and the restart-duration time interval. The grace period is the number of seconds that the routing device’s neighbors continue to advertise the routing device as fully adjacent, regardless of the connection state between the routing device and its neighbors.

The notify-duration statement configures how long (in seconds) the routing device notifies helper routers that it has completed graceful restart by sending purged grace link-state advertisements (LSAs) over all interfaces. By default, the routing device sends grace LSAs for 30 seconds. The range is from 1 through 3600 seconds.

The restart-duration statement configures the amount of time the routing device waits (in seconds) to complete reacquisition of OSPF neighbors from each area. By default, the routing device allows 180 seconds. The range is from 1 through 3600 seconds.

The second example shows how to disable graceful restart for OSPF by including the disable statement.

Topology

Configuration

Enabling Graceful Restart for OSPF

CLI Quick Configuration

To quickly enable graceful restart for OSPF, copy the following commands and paste them into the CLI.

Step-by-Step Procedure

To enable graceful restart for OSPF:

  1. Configure the interfaces.

    Note:

    For OSPFv3, use IPv6 addresses.

  2. Configure OSPF on the interfaces.

    Note:

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

  3. Configure graceful restart globally

  4. Configure OSPF graceful restart.

  5. (Optional) Configure the restart duration time.

  6. (Optional) Configure the notify duration time.

  7. If you are done configuring the device, commit the configuration.

Results

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

To confirm an OSPFv3 configuration, enter the show interfaces and the show protocols ospf3 commands.

Disabling Graceful Restart for OSPF

CLI Quick Configuration

To quickly disable graceful restart for OSPF, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, copy and paste the commands into the CLI at the [edit] hierarchy level, and then enter commit from configuration mode.

Step-by-Step Procedure

To disable graceful restart for OSPF:

  1. Disable graceful restart for the OSPF protocol only.

    This command does not affect the global graceful restart configuration setting.

    Note:

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

  2. If you are done configuring the device, 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.

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

Verification

Confirm that the configuration is working properly.

Verifying the OSPF Graceful Restart Configuration

Purpose

Verify information about your OSPF graceful restart configuration.

Action

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

Meaning

The Restart field displays the status of graceful restart as either enabled or disabled. The Restart duration field displays how much time the restarted routing device requires to complete reacquisition of OSPF neighbors. The Restart grace period field displays how much time the neighbors should consider the restarted routing device as part of the topology.

Verifying Graceful Restart Status

Purpose

Verify the status of graceful restart.

Action

From operational mode, enter the show route instance detail command.

Meaning

The Restart State field displays Pending if the restart has not been completed or Complete if the restart has finished. The Path selection timeout field indicates the amount of time remaining until graceful restart is declared complete. There is a more detailed Restart State field that displays a list of protocols that have or have not yet completed graceful restart for the specified routing table.

Example: Configuring the Helper Capability Mode for OSPFv2 Graceful Restart

This example shows how to disable and reenable the helper mode capability for OSPFv2 graceful restart.

Requirements

Before you begin:

Overview

The OSPF graceful restart helper capability assists a neighboring routing device attempting a graceful restart. By default, the helper capability is globally enabled when you start the routing platform. This means that the helper capability is enabled when you start OSPF, even if graceful restart is not globally enabled or specifically enabled for OSPF. You can further modify your graceful restart configuration to disable the helper capability.

Beginning with Junos OS Release 11.4, you can configure restart signaling-based helper mode for OSPFv2 graceful restart configurations. Both the standard and restart signaling-based helper modes are enabled by default.

In the first example, interfaces fe-1/1/1 and fe-1/1/2 are in OSPFv2 area 0.0.0.0, and you configure those interfaces for graceful restart. You then disable the standard OSPFv2 graceful restart helper capability by including the helper-disable standard statement. This configuration is useful if you have an environment that contains other vendor equipment that is configured for restart signaling-based graceful restart.

Note:

The helper-disable statement and the no-strict-lsa-checking statement cannot be configured at the same time. If you attempt to configure both statements at the same time, the routing device displays a warning message when you enter the show protocols ospf command.

The second example shows how to reenable the standard OSPFv2 restart helper capability that you disabled in the first example.

Topology

Configuration

Disabling Helper Mode for OSPFv2

CLI Quick Configuration

To quickly enable graceful restart for OSPFv2 with helper mode disabled, copy the following commands and paste them into the CLI.

Step-by-Step Procedure

To enable graceful restart for OSPFv2 with helper mode disabled:

  1. Configure the interfaces.

  2. Configure OSPFv2 on the interfaces

  3. Disable the OSPFv2 graceful restart helper capability.

    If you disable the OSPFv2 graceful restart helper capability, you cannot disable strict LSA checking.

  4. If you are done configuring the device, commit the configuration.

Results

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

Reenabling Helper Mode for OSPFv2

CLI Quick Configuration

To quickly reenable standard helper-mode for OSPFv2, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, copy and paste the commands into the CLI at the [edit] hierarchy level, and then enter commit from configuration mode.

Note:

To reenable restart signaling-based helper mode, include the restart-signaling statement. To reenable both standard and restart signaling-based helper mode, include the both statement.

Step-by-Step Procedure

To reenable standard helper mode for OSPFv2:

  1. Delete the standard helper-mode statement from the OSPFv2 configuration.

  2. If you are done configuring the device, commit the configuration.

Results

After you reenable standard helper mode, the show protocols ospf command no longer displays the graceful restart configuration.

Verification

Confirm that the configuration is working properly.

Verifying the OSPFv2 Graceful Restart Configuration

Purpose

Verify information about your OSPFv2 graceful restart configuration. The Restart field displays the status of graceful restart as either enabled or disabled, the Graceful restart helper mode field displays the status of the standard helper mode capability as enabled or disabled, and the Restart-signaling helper mode field displays the status of the restart signaling-based helper mode as enabled or disabled. By default, both standard and restart signaling-based helper modes are enabled.

Action

From operational mode, enter the show ospf overview command.

Verifying Graceful Restart Status

Purpose

Verify the status of graceful restart. The Restart State field displays Pending if the restart has not completed, or Complete if the restart has finished. The Path selection timeout field indicates the amount of time remaining until graceful restart is declared complete. There is a more detailed Restart State field that displays a list of protocols that have completed graceful restart or have not yet completed graceful restart for the specified routing table.

Action

From operational mode, enter the show route instance detail command.

Example: Configuring the Helper Capability Mode for OSPFv3 Graceful Restart

This example shows how to disable and reenable the helper mode capability for OSPFv3 graceful restart.

Requirements

Before you begin:

Overview

The OSPF graceful restart helper capability assists a neighboring routing device attempting a graceful restart. By default, the helper capability is globally enabled when you start the routing platform. This means that the helper capability is enabled when you start OSPF, even if graceful restart is not globally enabled or specifically enabled for OSPF. You can further modify your graceful restart configuration to disable the helper capability.

In the first example, interfaces fe-1/1/1 and fe-1/1/2 are in OSPFv3 area 0.0.0.0, and you configure those interfaces for graceful restart. You then disable the OSPFv3 graceful restart helper capability by including the helper-disable statement.

Note:

The helper-disable statement and the no-strict-lsa-checking statement cannot be configured at the same time. If you attempt to configure both statements at the same time, the routing device displays a warning message when you enter the show protocols ospf command.

The second example shows how to reenable the OSPFv3 restart helper capability that you disabled in the first example.

Topology

Configuration

Disabling Helper Mode for OSPFv3

CLI Quick Configuration

To quickly enable graceful restart for OSPFv3 with helper mode disabled, copy the following commands and paste them into the CLI.

Step-by-Step Procedure

To enable graceful restart for OSPFv3 with helper mode disabled:

  1. Configure the interfaces.

  2. Configure OSPFv3 on the interfaces

  3. Disable the OSPFv3 graceful restart helper capability.

    If you disable the OSPFv3 graceful restart helper capability, you cannot disable strict LSA checking.

  4. If you are done configuring the device, commit the configuration.

Results

Confirm your configuration by entering the show interfaces and the show protocols ospf3 commands. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

Reenabling Helper Mode for OSPFv3

CLI Quick Configuration

To quickly reenable helper-mode for OSPFv3, copy the following command and paste it into the CLI.

Step-by-Step Procedure

To reenable helper mode for OSPFv3:

  1. Delete the standard helper-mode statement from the OSPFv3 configuration.

  2. If you are done configuring the device, commit the configuration.

Results

After you reenable standard helper mode, the show protocols ospfs command no longer displays the graceful restart configuration.

Verification

Confirm that the configuration is working properly.

Verifying the OSPFv3 Graceful Restart Configuration

Purpose

Verify information about your OSPFv3 graceful restart configuration. The Restart field displays the status of graceful restart as either enabled or disabled, and the Helper mode field displays the status of the helper mode capability as either enabled or disabled.

Action

From operational mode, enter the show ospf3 overview command.

Verifying Graceful Restart Status

Purpose

Verify the status of graceful restart. The Restart State field displays Pending if the restart has not completed, or Complete if the restart has finished. The Path selection timeout field indicates the amount of time remaining until graceful restart is declared complete. There is a more detailed Restart State field that displays a list of protocols that have completed graceful restart or have not yet completed graceful restart for the specified routing table.

Action

From operational mode, enter the show route instance detail command.

Example: Disabling Strict LSA Checking for OSPF Graceful Restart

This example shows how to disable strict link-state advertisement (LSA) checking for OSPF graceful restart.

Requirements

Before you begin:

Overview

You can disable strict LSA checking to prevent the termination of graceful restart by a helping router. You might configure this option for interoperability with other vendor devices. The OSPF graceful restart helper capability must be enabled if you disable strict LSA checking. By default, LSA checking is enabled.

In this example, interfaces fe-1/1/1 and fe-1/1/2 are in OSPF area 0.0.0.0, and you configure those interfaces for graceful restart. You then disable strict LSA checking by including the no-strict-lsa-checking statement.

Note:

The helper-disable statement and the no-strict-lsa-checking statement cannot be configured at the same time. If you attempt to configure both statements at the same time, the routing device displays a warning message when you enter the show protocols ospf command.

Topology

Configuration

Procedure

CLI Quick Configuration

To quickly enable graceful restart for OSPF with strict LSA checking disabled, copy the following commands and paste them into the CLI.

Step-by-Step Procedure

To enable graceful restart for OSPF with strict LSA checking disabled:

  1. Configure the interfaces.

    Note:

    For OSPFv3, use IPv6 addresses.

  2. Configure OSPF on the interfaces

    Note:

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

  3. Disable strict LSA checking.

    If you disable the strict LSA checking, OSPF graceful restart helper capability must be enabled (which is the default behavior).

  4. If you are done configuring the device, commit the configuration.

Results

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

To confirm your OSPFv3 configuration, enter the show interfaces and the show protocols ospf3 commands.

Verification

Confirm that the configuration is working properly.

Verifying the OSPF Graceful Restart Configuration

Purpose

Verify information about your OSPF graceful restart configuration. The Restart field displays the status of graceful restart as either enabled or disabled.

Action

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

Verifying Graceful Restart Status

Purpose

Verify the status of graceful restart. The Restart State field displays Pending if the restart has not completed, or Complete if the restart has finished. The Path selection timeout field indicates the amount of time remaining until graceful restart is declared complete. There is a more detailed Restart State field that displays a list of protocols that have completed graceful restart or have not yet completed graceful restart for the specified routing table.

Action

From operational mode, enter the show route instance detail command.