Supported Platforms
Configuring LDP Graceful Restart
When you alter the graceful restart configuration at either the [edit routing-options graceful-restart] or [edit protocols ldp graceful-restart] hierarchy levels, any running LDP session is automatically restarted to apply the graceful restart configuration. This behavior mirrors the behavior of BGP when you alter its graceful restart configuration.
By default, graceful restart helper mode is enabled, but graceful restart is disabled. Thus, the default behavior of a router is to assist neighboring routers attempting a graceful restart, but not to attempt a graceful restart itself.
To configure LDP graceful restart, see the following sections:
Enabling Graceful Restart
To enable LDP graceful restart, you also need to enable graceful restart on the router. To enable graceful restart, include the graceful-restart statement:
You can include this statement at the following hierarchy levels:
- [edit routing-options]
- [edit logical-systems logical-system-name routing-options]
The graceful-restart statement enables graceful restart for all protocols supporting this feature on the router. For more information about graceful restart, see the Junos OS Routing Protocols Library for Routing Devices.
By default, LDP graceful restart is enabled when you enable graceful restart at both the LDP protocol level and on all the routing instances. However, you can disable both LDP graceful restart and LDP graceful restart helper mode.
Disabling LDP Graceful Restart or Helper Mode
To disable LDP graceful restart and recovery, include the disable statement:
For a list of hierarchy levels at which you can include this statement, see the statement summary section for this statement.
You can disable helper mode at the LDP protocols level only. You cannot disable helper mode for a specific routing instance. To disable LDP helper mode, include the helper-disable statement:
For a list of hierarchy levels at which you can include this statement, see the statement summary section for this statement.
The following LDP graceful restart configurations are possible:
- LDP graceful restart and helper mode are both enabled.
- LDP graceful restart is disabled but helper mode is enabled. A router configured in this way cannot restart gracefully but can help a restarting neighbor.
- LDP graceful restart and helper mode are both disabled. The router does not use LDP graceful restart or the graceful restart type, length, and value (TLV) sent in the initialization message. The router behaves as a router that cannot support LDP graceful restart.
A configuration error is issued if you attempt to enable graceful restart and disable helper mode.
Configuring Reconnect Time
After the LDP connection between neighbors fails, neighbors wait a certain amount of time for the gracefully restarting router to resume sending LDP messages. After the wait period, the LDP session can be reestablished. You can configure the wait period in seconds. This value is included in the fault tolerant session TLV sent in LDP initialization messages when LDP graceful restart is enabled.
Suppose that Router A and Router B are LDP neighbors. Router A is the restarting Router. The reconnect time is the time that Router A tells Router B to wait after Router B detects that Router A restarted.
To configure the reconnect time, include the reconnect-time statement:
You can set the reconnect time to a value in the range from 30 through 300 seconds. By default, it is 60 seconds.
For a list of hierarchy levels at which you can configure these statements, see the statement summary sections for these statements.
Configuring Recovery Time and Maximum Recovery Time
The recovery time is the amount of time a router waits for LDP to restart gracefully. The recovery time period begins when an initialization message is sent or received. This period is also typically the amount of time that a neighboring router maintains its information about the restarting router, allowing it to continue to forward traffic.
To prevent a neighboring router from being adversely affected if it receives a false value for the recovery time from the restarting router, you can configure the maximum recovery time on the neighboring router. A neighboring router maintains its state for the shorter of the two times. For example, Router A is performing an LDP graceful restart. It has sent a recovery time of 900 seconds to neighboring Router B. However, Router B has its maximum recovery time configured at 400 seconds. Router B will only wait for 400 seconds before it purges its LDP information from Router A.
To configure recovery time, include the recovery-time statement and the maximum-neighbor-recovery-time statement:
For a list of hierarchy levels at which you can configure these statements, see the statement summary sections for these statements.
