Configuring Synchronization Between LDP and IGP
Pre-configuration Tasks
Before configuring synchronization between LDP and IGP, configure a local LDP session. For details, see Configuring Basic Functions of MPLS LDP.
Synchronization between LDP and IGP applies to MPLS networks where primary and backup LSPs exist. LSPs are established between LSRs based on IGP. When the LDP session on the primary LSP fails (not due to a link failure) or the faulty primary LSP is restored, you can enable synchronization between LDP and IGP to prevent traffic interruption caused by the active/standby switchover.
Enabling Synchronization Between LDP and IGP
Context
Enable this function in the interface view.
This mode allows synchronization between LDP and IGP to be enabled on interfaces. This mode applies to the scenario where a few interfaces need to support this function.
Enable this function in an IGP process.
This mode allows synchronization between LDP and IGP to be enabled on all interfaces in the IGP process. This mode applies to the scenario where many interfaces on a node need to support this function.
Synchronization between LDP and IGP can be enabled in IS-IS processes, not in the interface view.
If the synchronization status between LDP and IS-IS is different on an interface and in an IS-IS process, the synchronization status on the interface takes effect.
(Optional) Blocking Synchronization Between LDP and IS-IS on an Interface
Context
The ldp-sync enable command run in an IS-IS process enables synchronization between LDP and IS-IS on all local IS-IS interfaces. On an IS-IS interface transmits importance services, LDP and IS-IS synchronization may affect service transmission. If the link is working properly and an LDP session over the link fails, IS-IS sends link state PDUs (LSPs) to advertise the maximum cost of the link. As a result, IS-IS does not select the route for the link, which affects important service transmission.
To prevent the preceding problem, block LDP and IS-IS synchronization on an IS-IS interface that transmits important services.
Procedure
- Run system-view
The system view is displayed.
- Run interface interface-type interface-number
The IS-IS interface view is displayed.
- Run isis ldp-sync block
Synchronization between LDP and IS-IS is blocked on the interface.
By default, synchronization between LDP and IS-IS is not blocked on an interface.
(Optional) Setting the Hold-down Timer Value
Context
On a device that has LDP-IGP synchronization enabled, if the active physical link recovers, an IGP enters the Hold-down state, and a Hold-down timer starts. Before the Hold-down timer expires, the IGP delays establishing an IGP neighbor relationship until an LDP session is established over the active link so that the LDP session over and IGP route for the active link can become available simultaneously.
If IS-IS is used, you can set the value of the Hold-down timer on a specified interface or set the value of the Hold-down timer for all IS-IS interfaces in the IS-IS view.
If different Hold-down values on an interface and in an IS-IS process are set, the setting on the interface takes effect.
(Optional) Setting the Hold-max-cost Timer Value
Context
If an LDP session over the active link fails but an IGP route for the active link is reachable, a node that has LDP-IGP synchronization enabled uses a Hold-max-cost timer to enable an IGP to advertise LSAs or LSPs carrying the maximum route cost, which delays IGP route convergence until an LDP session is established. Therefore, an IGP route for a standby link and an LDP session over the standby link can become available simultaneously.
Setting the Hold-max-cost timer value in the interface view
You can set the Hold-max-cost timer value on a specified interface. This mode applies to the scenario where a few interfaces need to use the Hold-max-cost timer.
Setting the Hold-max-cost timer value in the IGP process
After you set the Hold-max-cost timer value in the IGP process, the Hold-max-cost timers on all interfaces in the IGP process are set to this value. This mode applies to the scenario where many interfaces on a node need to use the Hold-max-cost timer.
A Hold-max-cost timer can be set on either an OSPF or IS-IS interface and can only be set in an IS-IS process, not an OSPF process.
If different Hold-max-cost values on an interface and in an IS-IS process are set, the setting on the interface takes effect.
- If an IGP carries only LDP services, configure the parameter infinite to ensure that a selected IGP route is kept consistent with the LDP LSP.
- If an IGP carries multiple types of services including LDP services, set the value of the parameter value to ensure that a teardown of LDP sessions does not affect IGP route selection or other services.
(Optional) Setting the Delay Timer Value
Context
When an LDP session is reestablished on a faulty link, LDP starts the Delay timer to wait for the establishment of an LSP. After the Delay timer times out, LDP notifies the IGP that synchronization between LDP and IGP is complete.
Procedure
- Run system-view
The system view is displayed.
- Run interface interface-type interface-number
The interface view is displayed.
- Run mpls ldp timer igp-sync-delay value
The period of waiting for the LSP setup after the establishment of the LDP session is set.
By default, the value of the delay timer is 10s.
Verifying the Synchronization Between LDP and IGP
Procedure
- Run the display ospf ldp-sync interface { all | interface-type interface-number } command to check information about synchronization between LDP and OSPF on an interface.
- Run the display isis [ process-id ] ldp-sync interface command to check information about synchronization between LDP and IS-IS on the interface.
- Run the display rm interface [ interface-type interface-number | vpn-instance vpn-instance-name ] command to check information about the route management.