MPLS LDP-IGP Synchronization

First Published: November 20, 2004
Last Updated: May 4, 2009

The MPLS LDP-IGP Synchronization feature ensures that the Label Distribution Protocol (LDP) is fully established before the Interior Gateway Protocol (IGP) path is used for switching.

Finding Feature Information

For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the "Feature Information for MPLS LDP-IGP Synchronization" section.

Use Cisco Feature Navigator to find information about platform support and Cisco IOS XE software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Contents

•Prerequisites for MPLS LDP-IGP Synchronization

•Restrictions for MPLS LDP-IGP Synchronization

•Information About MPLS LDP-IGP Synchronization

•How to Configure MPLS LDP-IGP Synchronization

•Configuration Examples for MPLS LDP-IGP Synchronization

•Additional References

•Feature Information for MPLS LDP-IGP Synchronization

Prerequisites for MPLS LDP-IGP Synchronization

•This feature is supported only on interfaces running Open Shortest Path First (OSPF) or Intermediate System-to-System (IS-IS) processes.

•This feature works when LDP is enabled on interfaces with either the mpls ip or mpls ldp autoconfig command.

Restrictions for MPLS LDP-IGP Synchronization

•This feature is not supported on tunnel interfaces or LC-ATM interfaces.

•This feature is not supported with interface-local label space or downstream-on-demand (DoD) requests.

•This feature does not support targeted LDP sessions. Therefore, Any Transport over MPLS (AToM) sessions are not supported.

Information About MPLS LDP-IGP Synchronization

To configure the LDP-IGP synchronization feature, you should understand the following concepts:

•How MPLS LDP-IGP Synchronization Works

•MPLS LDP-IGP Synchronization with Peers

•MPLS LDP-IGP Synchronization Delay Timer

•MPLS LDP-IGP Synchronization Incompatibility with IGP Nonstop Forwarding

•MPLS LDP-IGP Synchronization Compatibility with LDP Graceful Restart

How MPLS LDP-IGP Synchronization Works

Packet loss can occur because the actions of the IGP and LDP are not synchronized. Packet loss can occur in the following situations:

•When an IGP adjacency is established, the router begins forwarding packets using the new adjacency before the LDP label exchange completes between the peers on that link.

•If an LDP session closes, the router continues to forward traffic using the link associated with the LDP peer rather than an alternate pathway with a fully synchronized LDP session.

The MPLS LDP-IGP Synchronization feature does the following:

•Provides a means to synchronize LDP and IGPs to minimize Multiprotocol Label Switching (MPLS) packet loss.

•Enables you to globally enable LDP-IGP synchronization on each interface associated with an IGP OSPF or IS-IS process.

•Provides a means to disable LDP-IGP synchronization on interfaces that you do not want enabled.

•Prevents MPLS packet loss due to synchronization conflicts.

•Works when LDP is enabled on interfaces using either the mpls ip or mpls ldp autoconfig command.

To enable LDP-IGP synchronization on each interface that belongs to an OSPF or IS-IS process, enter the mpls ldp sync command. If you do not want some of the interfaces to have LDP-IGP synchronization enabled, issue the no mpls ldp igp sync command on those interfaces.

If the LDP peer is reachable, the IGP waits indefinitely (by default) for synchronization to be achieved. To limit the length of time the IGP session must wait, enter the mpls ldp igp sync holddown command. If the LDP peer is not reachable, the IGP establishes the adjacency to enable the LDP session to be established.

When an IGP adjacency is established on a link but LDP-IGP synchronization is not yet achieved or is lost, the IGP advertises the max-metric on that link.

MPLS LDP-IGP Synchronization with Peers

When the MPLS LDP-IGP Synchronization feature is enabled on an interface, LDP determines if any peer connected by the interface is reachable by looking up the peer's transport address in the routing table. If a routing entry (including longest match or default routing entry) for the peer exists, LDP assumes that LDP-IGP synchronization is required for the interface and notifies the IGP to wait for LDP convergence.

LDP-IGP synchronization with peers requires that the routing table be accurate for the peer's transport address. If the routing table shows there is a route for the peer's transport address, that route must be able to reach the peer's transport address. However, if the route is a summary route, a default route, or a statically configured route, it may not the correct route for the peer. You must verify that the route in the routing table can reach the peer's transport address.

When the routing table has an inaccurate route for the peer's transport address, LDP cannot set up a session with the peer, which causes the IGP to wait for LDP convergence unnecessarily for the sync hold-down time.

MPLS LDP-IGP Synchronization Delay Timer

The MPLS LDP-IGP Synchronization feature provide the option to configure a delay time for MPLS LDP and IGP synchronization on an interface-by-interface basis. If you want to configure a delay time on an interface, use the mpls ldp igp sync delay delay-time command in interface configuration mode. To remove the delay timer from a specified interface, enter the no mpls ldp igp sync delay command. This command sets the delay time to 0 seconds, but leaves MPLS LDP IGP synchronization enabled.

When LDP is fully established and synchronized, LDP checks the delay timer:

•If you configured a delay time, LDP starts the timer. When the timer expires, LDP checks that synchronization is still valid and notifies the OSPF process.

•If you did not configure a delay time, if synchronization is disabled or down, or if an interface was removed from an IGP process, LDP stops the timer and immediately notifies the OSPF process.

If you configure a new delay time while a timer is running, LDP saves the new delay time but does not reconfigure the running timer.

MPLS LDP-IGP Synchronization Incompatibility with IGP Nonstop Forwarding

The MPLS LDP-IGP Synchronization feature is not supported during the startup period if IGP nonstop forwarding (NSF) is configured. The MPLS LDP-IGP Synchronization feature conflicts with IGP NSF when the IGP is performing NSF during startup. After the NSF startup is complete, the MPLS LDP-IGP Synchronization feature is supported.

MPLS LDP-IGP Synchronization Compatibility with LDP Graceful Restart

LDP Graceful Restart protects traffic when an LDP session is lost. If an interface that supports a Graceful Restart-enabled LDP session fails, MPLS LDP-IGP synchronization is still achieved on the interface while it is protected by Graceful Restart. MPLS LDP-IGP synchronization is eventually lost under the following circumstances:

•If LDP fails to restart before the LDP Graceful Restart reconnect timer expires.

•If an LDP session restarts through other interfaces, but the LDP session on the protected interface fails to recover when the LDP Graceful Restart recovery timer expires.

How to Configure MPLS LDP-IGP Synchronization

This section contains the following procedures:

•Configuring MPLS LDP-IGP Synchronization with OSPF Interfaces (required)

•Disabling MPLS LDP-IGP Synchronization from Some OSPF Interfaces (optional)

•Verifying MPLS LDP-IGP Synchronization with OSPF (optional)

•Configuring MPLS LDP-IGP Synchronization with IS-IS Interfaces (required)

•Disabling MPLS LDP-IGP Synchronization from Some IS-IS Interfaces (optional)

Configuring MPLS LDP-IGP Synchronization with OSPF Interfaces

To configure MPLS LDP-IGP synchronization with OFPF interfaces, perform the following steps.

SUMMARY STEPS

1. enable

2. configure terminal

3. mpls ip

4. mpls label protocol ldp

5. interface type slot/subslot/port[.subinterface-number]

6. ip address prefix mask

7. mpls ip

8. exit

9. router ospf process-id

10. network ip-address wildcard-mask area area-id

11. mpls ldp sync

12. exit

13. exit

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

•Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

mpls ip

Example:

Router(config)# mpls ip

Globally enables hop-by-hop forwarding.

Step 4 

mpls label protocol ldp

Example:

Router(config)# mpls label protocol ldp

Specifies LDP as the default label distribution protocol.

Step 5 

interface type slot/subslot/port[.subinterface-number]

Example:

Router(config)# interface POS0/3/0

Specifies the interface to configure and enters interface configuration mode.

Step 6 

ip address prefix mask

Example:

Router(config-if)# ip address 10.25.0.11 255.255.255.255

Assigns an IP address to the interface.

Step 7 

mpls ip

Example:

Router(config-if)# mpls ip

Enables hop-by-hop forwarding on the interface.

Step 8 

exit

Example:

Router(config-if)# exit

Exits interface configuration mode.

Step 9 

router ospf process-id

Example:

Router(config)# router ospf 1

Enables OSPF routing and enters router configuration mode.

Step 10 

network ip-address wildcard-mask area area-id

Example:

Router(config-router)# network 10.0.0.0 0.255.255.255 area 3

Defines an interface on which OSPF runs and defines the area ID for that interface.

Step 11 

mpls ldp sync

Example:

Router(config-router)# mpls ldp sync

Enables MPLS LDP-IGP synchronization for interfaces for an OSPF or an IS-IS process.

Step 12 

exit

Example:

Router(config-router)# exit

Exits router configuration mode and returns to global configuration mode.

Step 13 

exit

Example:

Router(config)# exit

Exits global configuration mode and returns to privileged EXEC mode.

Disabling MPLS LDP-IGP Synchronization from Some OSPF Interfaces

When you issue the mpls ldp sync command, all of the interfaces that belong to an OSPF process are enabled for LDP-IGP synchronization. To remove LDP-IGP synchronization from some interfaces, use the no form of the mpls ldp igp sync command on those interfaces.

Perform the following task to disable LDP-IGP synchronization from some OSPF interfaces after they are configured with LDP-IGP synchronization through the mpls ldp sync command.

SUMMARY STEPS

1. enable

2. configure terminal

3. interface type slot/subslot/port[.subinterface-number]

4. no mpls ldp igp sync

5. exit

6. exit

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

•Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

interface type slot/subslot/port[.subinterface-number]

Example:

Router(config)# interface POS0/3/0

Specifies the interface to configure and enters interface configuration mode.

Step 4 

no mpls ldp igp sync

Example:

Router(config-if)# no mpls ldp igp sync

Disables MPLS LDP-IGP synchronization for that interface.

Step 5 

exit

Example:

Router(config-if)# exit

Exits interface configuration mode and returns to global configuration mode.

Step 6 

exit

Example:

Router(config)# exit

Exits global configuration mode and returns to privileged EXEC mode.

Verifying MPLS LDP-IGP Synchronization with OSPF

After you configure the interfaces for LDP, OSPF, and LDP-IGP synchronization, verify that the configuration is working correctly using the show mpls ldp igp sync and show ip ospf mpls ldp interface commands.

SUMMARY STEPS

1. enable

2. show mpls ldp igp sync

3. show ip ospf mpls ldp interface

4. exit

DETAILED STEPS

Step 1 enable

Use this command to enable privileged EXEC mode. Enter your password if prompted. For example: 

Router> enable

Router#

Step 2 show mpls ldp igp sync

Use this command to show that MPLS LDP-IGP synchronization is configured correctly, because LDP is configured and the SYNC status shows that synchronization is enabled. 

Router# show mpls ldp igp sync


FastEthernet0/0/0:

 LDP configured;  SYNC enabled.

 SYNC status: sync achieved; peer reachable.

 IGP holddown time: infinite.

 Peer LDP Ident: 10.0.0.1:0

 IGP enabled: OSPF 1

If MPLS LDP-IGP synchronization is not enabled on an interface, the output appears as follows: 

FastEthernet0/3/1:

 LDP configured; LDP-IGP Synchronization not enabled.

Step 3 show ip ospf mpls ldp interface

Use the output of the show ip ospf mpls ldp interface command to show that the interfaces are properly configured: 

Router# show ip ospf mpls ldp interface


FastEthernet0/3/1

  Process ID 1, Area 0

  LDP is configured through LDP autoconfig

  LDP-IGP Synchronization: Yes

  Holddown timer is not configured

  Timer is not running

FastEthernet0/0/2

  Process ID 1, Area 0

  LDP is configured through LDP autoconfig

  LDP-IGP Synchronization: Yes

  Holddown timer is not configured

  Timer is not running

Step 4 exit

Use this command to exit from privileged EXEC mode. For example: 

Router# exit

Router>

Configuring MPLS LDP-IGP Synchronization with IS-IS Interfaces

The following sections contain the steps and examples for configuring MPLS LDP-IGP synchronization for interfaces running IS-IS processes:

•Configuring MPLS LDP-IGP Synchronization on All IS-IS Interfaces

•Configuring MPLS LDP-IGP Synchronization on an IS-IS Interface

Configuring MPLS LDP-IGP Synchronization on All IS-IS Interfaces

Perform the following task to configure the MPLS LDP-IGP Synchronization feature on all interfaces running IS-IS processes.

SUMMARY STEPS

1. enable

2. configure terminal

3. mpls ip

4. mpls label protocol ldp

5. router isis process-name

6. mpls ldp sync

7. interface type slot/subslot/port[.subinterface-number]

8. ip address prefix mask

9. ip router isis process-name

10. exit

11. exit

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

•Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

mpls ip

Example:

Router(config)# mpls ip

Globally enables hop-by-hop forwarding.

Step 4 

mpls label protocol ldp

Example:

Router(config)# mpls label protocol ldp

Specifies LDP as the default label distribution protocol.

Step 5 

router isis process-name

Example:

Router(config)# router isis ISIS

Enables the IS-IS protocol on the router, specifies an IS-IS process, and enters router configuration mode.

Step 6 

mpls ldp sync

Example:

Router(config-router)# mpls ldp sync

Enables MPLS LDP-IGP synchronization on interfaces belonging to an IS-IS process.

Step 7 

interface type slot/subslot/port[.subinterface-number]

Example:

Router(config-router)# interface POS0/3/0

Specifies the interface to configure and enters interface configuration mode.

Step 8 

ip address prefix mask

Example:

Router(config-if)# ip address 10.25.25.11 255.255.255.0

Assigns an IP address to the interface.

Step 9 

ip router isis process-name

Example:

Router(config-if)# ip router isis ISIS

Enables IS-IS.

Step 10 

exit

Example:

Router(config-if)# exit

Exits interface configuration mode and returns to global configuration mode.

Step 11 

exit

Example:

Router(config)# exit

Exits global configuration mode and returns to privileged EXEC mode.

Configuring MPLS LDP-IGP Synchronization on an IS-IS Interface

This section contains the steps for configuring the MPLS LDP-IGP Synchronization feature on an interface that is running an IS-IS process.

SUMMARY STEPS

1. enable

2. configure terminal

3. interface type slot/subslot/port[.subinterface-number]

4. ip address prefix mask

5. ip router isis

6. exit

7. router isis

8. mpls ldp sync

9. exit

10. exit

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

•Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

interface type slot/subslot/port[.subinterface-number]

Example:

Router(config)# interface POS0/2/0

Specifies the interface to configure and enters interface configuration mode.

Step 4 

ip address prefix mask

Example:

Router(config-if)# ip address 10.50.72.4 255.0.0.0

Assigns an IP address to the interface.

Step 5 

ip router isis

Example:

Router(config-if)# ip router isis

Enables the IS-IS protocol for IP on the interface.

Step 6 

Exit

Example:

Router(config-if)# exit

Exits to global configuration mode.

Step 7 

router isis

Example:

Router(config)# router isis

Enters router configuration mode and enables an IS-IS process on the router.

Step 8 

mpls ldp sync

Example:

Router(config-router)# mpls ldp sync

Enables LDP-IGP synchronization for interfaces belonging to an IS-IS process.

Step 9 

exit

Example:

Router(config-router)# exit

Exits router configuration mode and returns to global configuration mode.

Step 10 

exit

Example:

Router(config)# exit

Exits global configuration mode and returns to privileged EXEC mode.

Disabling MPLS LDP-IGP Synchronization from Some IS-IS Interfaces

When you issue the mpls ldp sync command, all of the interfaces that belong to an IS-IS process are enabled for LDP-IGP synchronization. To remove LDP-IGP synchronization from some interfaces, use the no form of the mpls ldp igp sync command on those interfaces.

Perform the following task to disable LDP-IGP synchronization from some IS-IS interfaces after they are configured with LDP-IGP synchronization through the mpls ldp sync command.

SUMMARY STEPS

1. enable

2. configure terminal

3. interface type slot/subslot/port[.subinterface-number]

4. no mpls ldp igp sync

5. exit

6. exit

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

•Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

interface type slot/subslot/port[.subinterface-number]

Example:

Router(config)# interface POS0/3/0

Specifies the interface to configure and enters interface configuration mode.

Step 4 

no mpls ldp igp sync

Example:

Router(config-if)# no mpls ldp igp sync

Disables MPLS LDP-IGP synchronization for that interface.

Step 5 

exit

Example:

Router(config-if)# exit

Exits interface configuration mode and returns to global configuration mode.

Step 6 

exit

Example:

Router(config)# exit

Exits global configuration mode and returns to privileged EXEC mode.

Troubleshooting Tips

Use the debug mpls ldp igp sync command to display events related to MPLS LDP-IGP synchronization.

Configuration Examples for MPLS LDP-IGP Synchronization

The following sections show examples for the MPLS LDP-IGP synchronization feature with OSPF and IS-IS processes:

•MPLS LDP-IGP Synchronization with OSPF: Examples

•MPLS LDP-IGP Synchronization with IS-IS: Examples

MPLS LDP-IGP Synchronization with OSPF: Examples

The following task shows how to enable LDP for OSPF process 1. The mpls ldp sync and the OSPF network commands enable LDP on interfaces POS0/0/0, POS0/1/0, and POS1/1/0, respectively. The no mpls ldp igp sync command on interface POS1/0/0 prevents LDP from being enabled on interface POS1/0/0, even though OSPF is enabled for that interface. 

Router# configure terminal 

Router(config)# interface POS0/0/0 

Router(config-if)# ip address 10.0.0.1 

Router(config-if)# mpls ip 

!

Router(config)# interface POS0/1/0 

Router(config-if)# ip address 10.0.1.1 

Router(config-if)# mpls ip 

!

Router(config)# interface POS1/1/0 

Router(config-if)# ip address 10.1.1.1 

Router(config-if)# mpls ip 

!

Router(config)# interface POS1/0/0 

Router(config-if)# ip address 10.1.0.1 

Router(config-if)# mpls ip 

!

Router(config)# router ospf 1 

Router(config-router)# network 10.0.0.0 0.0.255.255 area 3 

Router(config-router)# network 10.1.0.0 0.0.255.255 area 3 

Router(config-router)# mpls ldp sync 

Router(config-router)# exit 

Router(config)# interface POS1/0/0 

Router(config-if)# no mpls ldp igp sync

MPLS LDP-IGP Synchronization with IS-IS: Examples

The following examples show the configuration commands you can use to configure MPLS LDP-IGP synchronization on interfaces POS0/2 /0 and POS0/3/0, which are running IS-IS processes: 

Router# configure terminal 


Enter configuration commands, one per line.  End with CNTL/Z.

Router(config)# interface POS0/2/0 

Router(config-if)# ip router isis 

Router(config-if)# exit


Router(config)# router isis 

Router(config-router)# mpls ldp sync 

Router(config-router)# exit

.

.

.

Router(config)# interface POS0/3/0

Router(config-if)# ip router isis

Router(config-if)# exit


Router(config)# router isis

Router(config-router)# mpls ldp sync

Router(config-router)# exit

Router(config) exit

Router#

Additional References

The following sections provide references related to the MPLS LDP-IGP Synchronization feature.

Related Documents

Related Topic
Document Title

MPLS LDP

MPLS Label Distribution Protocol

MPLS LDP Autoconfiguration

MPLS LDP Autoconfiguration

MPLS LDP commands

Cisco IOS Multiprotocol Label Switching Command Reference


Standards

Standards
Title

No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature.

—


MIBs

MIBs
MIBs Link

MPLS LDP MIB

To locate and download MIBs for selected platforms, Cisco IOS XE software releases, and feature sets, use Cisco MIB Locator found at the following URL:

http://www.cisco.com/go/mibs


RFCs

RFCs
Title

RFC 3036

LDP Specification

RFC 3037

LDP Applicability


Technical Assistance

Description
Link

The Cisco Support website provides extensive online resources, including documentation and tools for troubleshooting and resolving technical issues with Cisco products and technologies.

To receive security and technical information about your products, you can subscribe to various services, such as the Product Alert Tool (accessed from Field Notices), the Cisco Technical Services Newsletter, and Really Simple Syndication (RSS) Feeds.

Access to most tools on the Cisco Support website requires a Cisco.com user ID and password.

http://www.cisco.com/techsupport


Feature Information for MPLS LDP-IGP Synchronization

Table 1 lists the features in this module and provides links to specific configuration information.

Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which Cisco IOS XE software images support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Note Table 1 lists only the Cisco IOS XE software release that introduced support for a given feature in a given Cisco IOS XE software release train. Unless noted otherwise, subsequent releases of that Cisco IOS XE software release train also support that feature.

Table 1 Feature Information for MPLS LDP-IGP Synchronization

Feature Name
Releases
Feature Information

MPLS LDP-IGP Synchronization

Cisco IOS XE Release 2.1

The MPLS LDP-IGP Synchronization feature ensures that the Label Distribution Protocol (LDP) is fully established before the Interior Gateway Protocol (IGP) path is used for switching.

In Cisco  IOS XE Release 2.1, this feature was integrated on the Cisco ASR 1000 Series Aggregation Services Routers.

The following sections provide information about this feature:

•How MPLS LDP-IGP Synchronization Works

•MPLS LDP-IGP Synchronization with Peers

•MPLS LDP-IGP Synchronization Delay Timer

•MPLS LDP-IGP Synchronization Incompatibility with IGP Nonstop Forwarding

•MPLS LDP-IGP Synchronization Compatibility with LDP Graceful Restart

•Configuring MPLS LDP-IGP Synchronization with OSPF Interfaces

•Disabling MPLS LDP-IGP Synchronization from Some OSPF Interfaces

•Verifying MPLS LDP-IGP Synchronization with OSPF

•Configuring MPLS LDP-IGP Synchronization with IS-IS Interfaces

•Disabling MPLS LDP-IGP Synchronization from Some IS-IS Interfaces

•Troubleshooting Tips

The following commands were modified: debug mpls ldp igp sync, mpls ldp igp sync, mpls ldp igp sync holddown, mpls ldp sync, show ip ospf mpls ldp interface, show isis mpls ldp, and show mpls ldp igp sync.


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