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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.
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.
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 Label Distribution Protocol (LDP) sessions. Therefore, Any Transport over MPLS (AToM) sessions are not supported.
The Tag Distribution Protocol (TDP) is not supported. You must specify that the default label distribution protocol is LDP for a device or for an interface.
Packet loss can occur because the actions of the Interior Gateway Protocol (IGP) and the Label Distribution Protocol (LDP) are not synchronized. Packet loss can occur in the following situations:
When an IGP adjacency is established, the device 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 device 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 Open Shortest Path First (OSPF) or Intermediate System-to-Intermediate System (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 no 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.
When the MPLS LDP IGP Synchronization feature is enabled on an interface, the Label Distribution Protocol (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 Interior Gateway Protocol (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.
The MPLS LDP IGP Synchronization feature provide the option to configure a delay time for Multiprotocol Label Switching (MPLS) Label Distribution Protocol (LDP) and Interior Gateway Protocol (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 Open Shortest Path First (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.
The MPLS LDP IGP Synchronization feature is not supported during the startup period if the Interior Gateway Protocol (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.
LDP Graceful Restart protects traffic when a Label Distribution Protocol (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.
| Command or Action | Purpose | |
|---|---|---|
|
Step 1 |
enable Example: |
Enables privileged EXEC mode.
|
|
Step 2 |
configure terminal Example: |
Enters global configuration mode. |
|
Step 3 |
mpls ip Example: |
Globally enables hop-by-hop forwarding. |
|
Step 4 |
mpls label protocol ldp Example: |
Specifies the Label Distribution Protocol (LDP) as the default protocol. |
|
Step 5 |
interface type number Example: |
Specifies the interface to configure, and enters interface configuration mode. |
|
Step 6 |
ip address prefix mask Example: |
Assigns an IP address to the interface. |
|
Step 7 |
mpls ip Example: |
Enables hop-by-hop forwarding on the interface. |
|
Step 8 |
exit Example: |
Returns to global configuration mode. |
|
Step 9 |
router ospf process-id Example: |
Enables Open Shortest Path First (OSPF) routing, and enters router configuration mode. |
|
Step 10 |
network ip-address wildcard-mask area area-id Example: |
Specifies the interface on which OSPF runs and defines the area ID for that interface. |
|
Step 11 |
mpls ldp sync Example: |
Enables the Multiprotocol Label Switching (MPLS) Interior Gateway Protocol (IGP) synchronization for interfaces belonging for an OSPF or an Intermediate System-to-Intermediate System (IS-IS) process. |
|
Step 12 |
end Example: |
Returns to privileged EXEC mode. |
When you issue the mpls ldp sync command, all of the interfaces that belong to an Open Shortest Path First (OSPF) process are enabled for Label Distribution Protocol (LDP) Interior Gateway Protocol (IGP) synchronization. To remove LDP IGP synchronization from some interfaces, use the no 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.
| Command or Action | Purpose | |
|---|---|---|
|
Step 1 |
enable Example: |
Enables privileged EXEC mode.
|
|
Step 2 |
configure terminal Example: |
Enters global configuration mode. |
|
Step 3 |
interface type number Example: |
Specifies the interface to configure, and enters interface configuration mode. |
|
Step 4 |
no mpls ldp igp sync Example: |
Disables MPLS LDP IGP synchronization for that interface. |
|
Step 5 |
end Example: |
Returns to privileged EXEC mode. |
After you configure the interfaces for the Label Distribution Protocol (LDP), Open Shortest Path First (OSPF), and LDP Interior Gateway Protocol (IGP) synchronization, verify that the configuration is working correctly by using the show mpls ldp igp sync and show ip ospf mpls ldp interface commands.
|
Step 1 |
enable Enables privileged EXEC mode. Enter your password if prompted. Example: |
|
Step 2 |
show mpls ldp igp sync Shows that the Multiprotocol Label Switching (MPLS) LDP IGP synchronization is configured correctly because LDP is configured and the SYNC status shows that synchronization is enabled. Example:If MPLS LDP IGP synchronization is not enabled on an interface, the output appears as follows: Example: |
|
Step 3 |
show ip ospf mpls ldp interface Shows that the interfaces are properly configured. Example: |
|
Step 4 |
exit Returns to user EXEC mode. Example: |
| Command or Action | Purpose | |
|---|---|---|
|
Step 1 |
enable Example: |
Enables privileged EXEC mode.
|
|
Step 2 |
configure terminal Example: |
Enters global configuration mode. |
|
Step 3 |
mpls ip Example: |
Globally enables hop-by-hop forwarding. |
|
Step 4 |
mpls label protocol ldp Example: |
Specifies the Label Distribution Protocol (LDP) as the default label distribution protocol. |
|
Step 5 |
router isis process-name Example: |
Enables the Intermediate System-to-Intermediate System (IS-IS) protocol on the device, specifies an IS-IS process, and enters router configuration mode. |
|
Step 6 |
mpls ldp sync Example: |
Enables Multiprotocol Label Switching (MPLS) LDP Interior Gateway Protocol (IGP) synchronization on interfaces belonging to an IS-IS process. |
|
Step 7 |
interface type number Example: |
Specifies the interface to configure, and enters interface configuration mode. |
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Step 8 |
ip address prefix mask Example: |
Assigns an IP address to the interface. |
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Step 9 |
ip router isis process-name Example: |
Enables IS-IS. |
|
Step 10 |
end Example: |
Returns to privileged EXEC mode. |
| Command or Action | Purpose | |
|---|---|---|
|
Step 1 |
enable Example: |
Enables privileged EXEC mode.
|
|
Step 2 |
configure terminal Example: |
Enters global configuration mode. |
|
Step 3 |
interface type number Example: |
Specifies the interface to configure, and enters interface configuration mode. |
|
Step 4 |
ip address prefix mask Example: |
Assigns an IP address to the interface. |
|
Step 5 |
ip router isis Example: |
Enables the Intermediate System-to-Intermediate System (IS-IS) protocol for IP on the interface. |
|
Step 6 |
exit Example: |
Returns to global configuration mode. |
|
Step 7 |
router isis Example: |
Enters router configuration mode, and enables an IS-IS process on the device. |
|
Step 8 |
mpls ldp sync Example: |
Enables Label Distribution Protocol (LDP) Interior Gateway Protocol (IGP) synchronization for interfaces belonging to an IS-IS process. |
|
Step 9 |
end Example: |
Returns to privileged EXEC mode. |
When you issue the mpls ldp sync command, all of the interfaces that belong to an Intermediate System-to-Intermediate System (IS-IS) process are enabled for Label Distribution Protocol (LDP) Interior Gateway Protocol (IGP) synchronization. To remove LDP IGP synchronization from some interfaces, use the no 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.
| Command or Action | Purpose | |
|---|---|---|
|
Step 1 |
enable Example: |
Enables privileged EXEC mode.
|
|
Step 2 |
configure terminal Example: |
Enters global configuration mode. |
|
Step 3 |
interface type number Example: |
Specifies the interface to configure, and enters interface configuration mode. |
|
Step 4 |
no mpls ldp igp sync Example: |
Disables Multiprotocol Label Switching (MPLS) LDP IGP synchronization for that interface. |
|
Step 5 |
end Example: |
Returns to privileged EXEC mode. |
Use the debug mpls ldp igp sync command to display events related to MPLS LDP IGP synchronization.
The following task shows how to enable the Label Distribution Protocol (LDP) for Open Shortest Path First (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.
Device# configure terminal
Device(config)# interface POS0/0/0
Device(config-if)# ip address 10.0.0.1
Device(config-if)# mpls ip
!
Device(config)# interface POS0/1/0
Device(config-if)# ip address 10.0.1.1
Device(config-if)# mpls ip
!
Device(config)# interface POS1/1/0
Device(config-if)# ip address 10.1.1.1
Device(config-if)# mpls ip
!
Device(config)# interface POS1/0/0
Device(config-if)# ip address 10.1.0.1
Device(config-if)# mpls ip
!
Device(config)# router ospf 1
Device(config-router)# network 10.0.0.0 0.0.255.255 area 3
Device(config-router)# network 10.1.0.0 0.0.255.255 area 3
Device(config-router)# mpls ldp sync
Device(config-router)# exit
Device(config)# interface POS1/0/0
Device(config-if)# no mpls ldp igp sync
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:
Device# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Device(config)# interface POS0/2/0
Device(config-if)# ip router isis
Device(config-if)# exit
Device(config)# router isis
Device(config-router)# mpls ldp sync
Device(config-router)# exit
.
.
.
Device(config)# interface POS0/3/0
Device(config-if)# ip router isis
Device(config-if)# exit
Device(config)# router isis
Device(config-router)# mpls ldp sync
Device(config-router)# exit
Device(config) exit
Device#|
Related Topic |
Document Title |
|---|---|
|
Cisco IOS commands |
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|
MPLS LDP commands |
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LDP autoconfiguration |
“MPLS LDP Autoconfiguration” module in the MPLS Label Distribution Protocol Configuration Guide |
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Standard/RFC |
Title |
|---|---|
|
RFC 3037 |
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RFC 5036 |
|
MIBs |
MIBs Link |
|---|---|
|
MPLS LDP MIB |
To locate and download MIBs for selected platforms, Cisco software releases, and feature sets, use Cisco MIB Locator found at the following URL: |
|
Description |
Link |
|---|---|
|
The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password. |
The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
|
Feature Name |
Releases |
Feature Information |
|---|---|---|
|
MPLS LDP IGP Synchronization |
The MPLS LDP IGP Synchronization feature ensures that LDP is fully established before the IGP path is used for switching. |
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