Cisco ASR 9000 Series Aggregation Services Router MPLS Configuration Guide, Release 4.0
Implementing MPLS Label Distribution Protocol
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Implementing MPLS Label Distribution Protocol

Contents

Implementing MPLS Label Distribution Protocol

This module describes how to implement MPLS Label Distribution Protocol on Cisco ASR 9000 Series Aggregation Services Routers.

The Multiprotocol Label Switching (MPLS) is a standards-based solution driven by the Internet Engineering Task Force (IETF) that was devised to convert the Internet and IP backbones from best-effort networks into business-class transport mediums.

MPLS, with its label switching capabilities, eliminates the need for an IP route look-up and creates a virtual circuit (VC) switching function, allowing enterprises the same performance on their IP-based network services as with those delivered over traditional networks such as Frame Relay or ATM.

Label Distribution Protocol (LDP) performs label distribution in MPLS environments. LDP provides the following capabilities:

  • LDP performs hop-by-hop or dynamic path setup; it does not provide end-to-end switching services.
  • LDP assigns labels to routes using the underlying Interior Gateway Protocols (IGP) routing protocols.
  • LDP provides constraint-based routing using LDP extensions for traffic engineering.

Finally, LDP is deployed in the core of the network and is one of the key protocols used in MPLS-based Layer 2 and Layer 3 virtual private networks (VPNs).

Feature History for Implementing MPLS LDP

Release

Modification

Release 3.7.2

This feature was introduced.

   

Prerequisites for Implementing Cisco MPLS LDP

These prerequisites are required to implement MPLS LDP:

  • You must be in a user group associated with a task group that includes the proper task IDs. The command reference guides include the task IDs required for each command. If you suspect user group assignment is preventing you from using a command, contact your AAA administrator for assistance.
  • You must be running Cisco IOS XR software.
  • You must install a composite mini-image and the MPLS package.
  • You must activate IGP.
  • We recommend to use a lower session holdtime bandwidth such as neighbors so that a session down occurs before an adjacency-down on a neighbor. Therefore, the following default values for the hello times are listed:
    • Holdtime is 15 seconds.
    • Interval is 5 seconds.
    For example, the LDP session holdtime can be configured as 30 seconds by using the holdtime command.

Information About Implementing Cisco MPLS LDP

To implement MPLS LDP, you should understand these concepts:

Overview of Label Distribution Protocol

LDP performs label distribution in MPLS environments. LDP uses hop-by-hop or dynamic path setup, but does not provide end-to-end switching services. Labels are assigned to routes that are chosen by the underlying IGP routing protocols. The Label Switched Paths (LSPs) that result from the routes, forward labeled traffic across the MPLS backbone to adjacent nodes.

Label Switched Paths

LSPs are created in the network through MPLS. They can be created statically, by RSVP traffic engineering (TE), or by LDP. LSPs created by LDP perform hop-by-hop path setup instead of an end-to-end path.

LDP Control Plane

The control plane enables label switched routers (LSRs) to discover their potential peer routers and to establish LDP sessions with those peers to exchange label binding information.

Figure 1. LDP Control Protocol. This figure shows the control messages exchanged between LDP peers.



LDP uses the hello discovery mechanism to discover its neighbor or peer on the network. When LDP is enabled on an interface, it sends hello messages to a link-local multicast address, and joins a specific multicast group to receive hellos from other LSRs present on the given link. When LSRs on a given link receive hellos, their neighbors are discovered and the LDP session (using TCP) is established.


Note


Hellos are not only used to discover and trigger LDP sessions; they are also required to maintain LDP sessions. If a certain number of hellos from a given peer are missed in sequence, LDP sessions are brought down until the peer is discovered again.


LDP also supports non-link neighbors that could be multiple hops away on the network, using the targeted hello mechanism. In these cases, hellos are sent on a directed, unicast address.

The first message in the session establishment phase is the initialization message, which is used to negotiate session parameters. After session establishment, LDP sends a list of all its interface addresses to its peers in an address message. Whenever a new address becomes available or unavailable, the peers are notified regarding such changes via ADDRESS or ADDRESS_WITHDRAW messages respectively.

When MPLS LDP learns an IGP prefix it allocates a label locally as the inbound label. The local binding between the prefix label is conveyed to its peers via LABEL_MAPPING message. If the binding breaks and becomes unavailable, a LABEL_WITHDRAW message is sent to all its peers, which responds with LABEL_RELEASE messages.

The local label binding and remote label binding received from its peer(s) is used to setup forwarding entries. Using routing information from the IGP protocol and the forwarding information base (FIB), the next active hop is selected. Label binding is learned from the next hop peer, and is used as the outbound label while setting up the forwarding plane.

The LDP session is also kept alive using the LDP keepalive mechanism, where an LSR sends a keepalive message periodically to its peers. If no messages are received and a certain number of keepalive messages are missed from a peer, the session is declared dead, and brought down immediately.

Exchanging Label Bindings

LDP creates LSPs to perform the hop-by-hop path setup so that MPLS packets can be transferred between the nodes on the MPLS network.

Figure 2. Setting Up Label Switched Paths . This figure illustrates the process of label binding exchange for setting up LSPs.



For a given network (10.0.0.0), hop-by-hop LSPs are set up between each of the adjacent routers (or, nodes) and each node allocates a local label and passes it to its neighbor as a binding:

  1. R4 allocates local label L4 for prefix 10.0.0.0 and advertises it to its neighbors (R3).
  2. R3 allocates local label L3 for prefix 10.0.0.0 and advertises it to its neighbors (R1, R2, R4).
  3. R1 allocates local label L1 for prefix 10.0.0.0 and advertises it to its neighbors (R2, R3).
  4. R2 allocates local label L2 for prefix 10.0.0.0 and advertises it to its neighbors (R1, R3).
  5. R1’s label information base (LIB) keeps local and remote labels bindings from its neighbors.
  6. R2’s LIB keeps local and remote labels bindings from its neighbors.
  7. R3’s LIB keeps local and remote labels bindings from its neighbors.
  8. R4’s LIB keeps local and remote labels bindings from its neighbors.
Related Tasks

LDP Forwarding

Once label bindings are learned, the LDP control plane is ready to setup the MPLS forwarding plane as shown in the following figure.

Figure 3. Forwarding Setup. Once label bindings are learned, the LDP control plane is ready to setup the MPLS forwarding plane as shown in this figure.



  1. Because R3 is next hop for 10.0.0.0 as notified by the FIB, R1 selects label binding from R3 and installs forwarding entry (Layer 1, Layer 3).
  2. Because R3 is next hop for 10.0.0.0 (as notified by FIB), R2 selects label binding from R3 and installs forwarding entry (Layer 2, Layer 3).
  3. Because R4 is next hop for 10.0.0.0 (as notified by FIB), R3 selects label binding from R4 and installs forwarding entry (Layer 3, Layer 4).
  4. Because next hop for 10.0.0.0 (as notified by FIB) is beyond R4, R4 uses NO-LABEL as the outbound and installs the forwarding entry (Layer 4); the outbound packet is forwarded IP-only.
  5. Incoming IP traffic on ingress LSR R1 gets label-imposed and is forwarded as an MPLS packet with label L3.
  6. Incoming IP traffic on ingress LSR R2 gets label-imposed and is forwarded as an MPLS packet with label L3.
  7. R3 receives an MPLS packet with label L3, looks up in the MPLS label forwarding table and switches this packet as an MPLS packet with label L4.
  8. R4 receives an MPLS packet with label L4, looks up in the MPLS label forwarding table and finds that it should be Unlabeled, pops the top label, and passes it to the IP forwarding plane.
  9. IP forwarding takes over and forwards the packet onward.

LDP Graceful Restart

LDP (Label Distribution Protocol) graceful restart provides a control plane mechanism to ensure high availability and allows detection and recovery from failure conditions while preserving Nonstop Forwarding (NSF) services. Graceful restart is a way to recover from signaling and control plane failures without impacting forwarding.

Without LDP graceful restart, when an established session fails, the corresponding forwarding states are cleaned immediately from the restarting and peer nodes. In this case LDP forwarding restarts from the beginning, causing a potential loss of data and connectivity.

The LDP graceful restart capability is negotiated between two peers during session initialization time, in FT SESSION TLV. In this typed length value (TLV), each peer advertises the following information to its peers:

Reconnect time

Advertises the maximum time that other peer will wait for this LSR to reconnect after control channel failure.

Recovery time

Advertises the maximum time that the other peer has on its side to reinstate or refresh its states with this LSR. This time is used only during session reestablishment after earlier session failure.

FT flag

Specifies whether a restart could restore the preserved (local) node state for this flag.

Once the graceful restart session parameters are conveyed and the session is up and running, graceful restart procedures are activated.

When configuring the LDP graceful restart process in a network with multiple links, targeted LDP hello adjacencies with the same neighbor, or both, make sure that graceful restart is activated on the session before any hello adjacency times out in case of neighbor control plane failures. One way of achieving this is by configuring a lower session hold time between neighbors such that session timeout occurs before hello adjacency timeout. It is recommended to set LDP session hold time using the following formula:

Session Holdtime <= (Hello holdtime - Hello interval) * 3

This means that for default values of 15 seconds and 5 seconds for link Hello holdtime and interval respectively, session hold time should be set to 30 seconds at most.

For more information about LDP commands, see the Implementing MPLS Label Distribution Protocol module of the Cisco ASR 9000 Series Aggregation Services Router MPLS Configuration Guide.

Control Plane Failure

When a control plane failure occurs, connectivity can be affected. The forwarding states installed by the router control planes are lost, and the in-transit packets could be dropped, thus breaking NSF.

Figure 4. Control Plane Failure. This figure illustrates a control plane failure and shows the process and results of a control plane failure leading to loss of connectivity.



  1. The R4 LSR control plane restarts.
  2. LIB is lost when the control plane restarts.
  3. The forwarding states installed by the R4 LDP control plane are immediately deleted.
  4. Any in-transit packets flowing from R3 to R4 (still labeled with L4) arrive at R4.
  5. The MPLS forwarding plane at R4 performs a lookup on local label L4 which fails. Because of this failure, the packet is dropped and NSF is not met.
  6. The R3 LDP peer detects the failure of the control plane channel and deletes its label bindings from R4.
  7. The R3 control plane stops using outgoing labels from R4 and deletes the corresponding forwarding state (rewrites), which in turn causes forwarding disruption.
  8. The established LSPs connected to R4 are terminated at R3, resulting in broken end-to-end LSPs from R1 to R4.
  9. The established LSPs connected to R4 are terminated at R3, resulting in broken LSPs end-to-end from R2 to R4.

Phases in Graceful Restart

The graceful restart mechanism is divided into different phases:

Control communication failure detection
Control communication failure is detected when the system detects either:
  • Missed LDP hello discovery messages
  • Missed LDP keepalive protocol messages
  • Detection of Transmission Control Protocol (TCP) disconnection a with a peer
Forwarding state maintenance during failure

Persistent forwarding states at each LSR are achieved through persistent storage (checkpoint) by the LDP control plane. While the control plane is in the process of recovering, the forwarding plane keeps the forwarding states, but marks them as stale. Similarly, the peer control plane also keeps (and marks as stale) the installed forwarding rewrites associated with the node that is restarting. The combination of local node forwarding and remote node forwarding plane states ensures NSF and no disruption in the traffic.

Control state recovery

Recovery occurs when the session is reestablished and label bindings are exchanged again. This process allows the peer nodes to synchronize and to refresh stale forwarding states.

Recovery with Graceful-Restart

Figure 5. Recovering with Graceful Restart. This figure illustrates the process of failure recovery using graceful restart.



  1. The router R4 LSR control plane restarts.
  2. With the control plane restart, LIB is gone but forwarding states installed by R4’s LDP control plane are not immediately deleted but are marked as stale.
  3. Any in-transit packets from R3 to R4 (still labeled with L4) arrive at R4.
  4. The MPLS forwarding plane at R4 performs a successful lookup for the local label L4 as forwarding is still intact. The packet is forwarded accordingly.
  5. The router R3 LDP peer detects the failure of the control plane and channel and deletes the label bindings from R4. The peer, however, does not delete the corresponding forwarding states but marks them as stale.
  6. At this point there are no forwarding disruptions.
  7. The peer also starts the neighbor reconnect timer using the reconnect time value.
  8. The established LSPs going toward the router R4 are still intact, and there are no broken LSPs.

When the LDP control plane recovers, the restarting LSR starts its forwarding state hold timer and restores its forwarding state from the checkpointed data. This action reinstates the forwarding state and entries and marks them as old.

The restarting LSR reconnects to its peer, indicated in the FT Session TLV, that it either was or was not able to restore its state successfully. If it was able to restore the state, the bindings are resynchronized.

The peer LSR stops the neighbor reconnect timer (started by the restarting LSR), when the restarting peer connects and starts the neighbor recovery timer. The peer LSR checks the FT Session TLV if the restarting peer was able to restore its state successfully. It reinstates the corresponding forwarding state entries and receives binding from the restarting peer. When the recovery timer expires, any forwarding state that is still marked as stale is deleted.

If the restarting LSR fails to recover (restart), the restarting LSR forwarding state and entries will eventually timeout and is deleted, while neighbor-related forwarding states or entries are removed by the Peer LSR on expiration of the reconnect or recovery timers.

Label Advertisement Control (Outbound Filtering)

By default, LDP advertises labels for all the prefixes to all its neighbors. When this is not desirable (for scalability and security reasons), you can configure LDP to perform outbound filtering for local label advertisement for one or more prefixes to one more peers. This feature is known as LDP outbound label filtering, or local label advertisement control.

Label Acceptance Control (Inbound Filtering)

By default, LDP accepts labels (as remote bindings) for all prefixes from all peers. LDP operates in liberal label retention mode, which instructs LDP to keep remote bindings from all peers for a given prefix. For security reasons, or to conserve memory, you can override this behavior by configuring label binding acceptance for set of prefixes from a given peer.

The ability to filter remote bindings for a defined set of prefixes is also referred to as LDP inbound label filtering.


Note


Inbound filtering can also be implemented using an outbound filtering policy; however, you may not be able to implement this system if an LDP peer resides under a different administration domain. When both inbound and outbound filtering options are available, we recommend that you use outbound label filtering.


Local Label Allocation Control

By default, LDP allocates local labels for all prefixes that are not Border Gateway Protocol (BGP) prefixes1. This is acceptable when LDP is used for applications other than Layer 3 virtual private networks (L3VPN) core transport. When LDP is used to set up transport LSPs for L3VPN traffic in the core, it is not efficient or even necessary to allocate and advertise local labels for, potentially, thousands of IGP prefixes. In such a case, LDP is typically required to allocate and advertise local label for loopback /32 addresses for PE routers. This is accomplished using LDP local label allocation control, where an access list can be used to limit allocation of local labels to a set of prefixes. Limiting local label allocation provides several benefits, including reduced memory usage requirements, fewer local forwarding updates, and fewer network and peer updates.


Tip


You can configure label allocation using an IP access list to specify a set of prefixes that local labels can allocate and advertise.


Session Protection

When a link comes up, IP converges earlier and much faster than MPLS LDP and may result in MPLS traffic loss until MPLS convergence. If a link flaps, the LDP session will also flap due to loss of link discovery. LDP session protection minimizes traffic loss, provides faster convergence, and protects existing LDP (link) sessions by means of “parallel” source of targeted discovery hello. An LDP session is kept alive and neighbor label bindings are maintained when links are down. Upon reestablishment of primary link adjacencies, MPLS convergence is expedited as LDP need not relearn the neighbor label bindings.

LDP session protection lets you configure LDP to automatically protect sessions with all or a given set of peers (as specified by peer-acl). When configured, LDP initiates backup targeted hellos automatically for neighbors for which primary link adjacencies already exist. These backup targeted hellos maintain LDP sessions when primary link adjacencies go down.

The Session Protection figure illustrates LDP session protection between neighbors R1 and R3. The primary link adjacency between R1 and R3 is directly connected link and the backup; targeted adjacency is maintained between R1 and R3. If the direct link fails, LDP link adjacency is destroyed, but the session is kept up and running using targeted hello adjacency (through R2). When the direct link comes back up, there is no change in the LDP session state and LDP can converge quickly and begin forwarding MPLS traffic.

Figure 6. Session Protection




Note


When LDP session protection is activated (upon link failure), protection is maintained for an unlimited period time.


IGP Synchronization

Lack of synchronization between LDP and IGP can cause MPLS traffic loss. Upon link up, for example, IGP can advertise and use a link before LDP convergence has occurred; or, a link may continue to be used in IGP after an LDP session goes down.

LDP IGP synchronization synchronizes LDP and IGP so that IGP advertises links with regular metrics only when MPLS LDP is converged on that link. LDP considers a link converged when at least one LDP session is up and running on the link for which LDP has sent its applicable label bindings and received at least one label binding from the peer. LDP communicates this information to IGP upon link up or session down events and IGP acts accordingly, depending on sync state.

In the event of an LDP graceful restart session disconnect, a session is treated as converged as long as the graceful restart neighbor is timed out. Additionally, upon local LDP restart, a checkpointed recovered LDP graceful restart session is used and treated as converged and is given an opportunity to connect and resynchronize.

Under certain circumstances, it might be required to delay declaration of resynchronization to a configurable interval. LDP provides a configuration option to delay declaring synchronization up for up to 60 seconds. LDP communicates this information to IGP upon linkup or session down events.


Note


The configuration for LDP IGP synchronization resides in respective IGPs (OSPF and IS-IS) and there is no LDP-specific configuration for enabling of this feature. However, there is a specific LDP configuration for IGP sync delay timer.


IGP Auto-configuration

To enable LDP on a large number of interfaces, IGP auto-configuration lets you automatically configure LDP on all interfaces associated with a specified IGP interface; for example, when LDP is used for transport in the core network. However, there needs to be one IGP set up to enable LDP auto-configuration.

Typically, LDP assigns and advertises labels for IGP routes and must often be enabled on all active interfaces by an IGP. Without IGP auto-configuration, you must define the set of interfaces under LDP, a procedure that is time-intensive and error-prone.


Note


LDP auto-configuration is supported for IPv4 unicast family in the default VRF. The IGP is responsible for verifying and applying the configuration.


You can also disable auto-configuration on a per-interface basis. This permits LDP to enable all IGP interfaces except those that are explicitly disabled and prevents LDP from enabling an interface when LDP auto-configuration is configured under IGP.

LDP Nonstop Routing

LDP nonstop routing (NSR) functionality makes failures, such as Route Processor (RP) or Distributed Route Processor (DRP) failover, invisible to routing peers with minimal to no disruption of convergence performance. By default, NSR is globally enabled on all LDP sessions except AToM.

A disruption in service may include any of these events:

  • Route processor (RP) or distributed route processor (DRP) failover
  • LDP process restart
  • In-service system upgrade (ISSU)
  • Minimum disruption restart (MDR)

Note


Unlike graceful restart functionality, LDP NSR does not require protocol extensions and does not force software upgrades on other routers in the network, nor does LDP NSR require peer routers to support NSR.

L2VPN configuration is not supported on NSR.


Process failures of active TCP or LDP results in session loss and, as a result, NSR cannot be provided unless RP switchover is configured as a recovery action. For more information about how to configure switchover as a recovery action for NSR, see the Configuring Transports module in Cisco ASR 9000 Series Aggregation Services Router IP Addresses and Services Configuration Guide .

How to Implement MPLS LDP

A typical MPLS LDP deployment requires coordination among several global neighbor routers. Various configuration tasks are required to implement MPLS LDP :

Configuring LDP Discovery Parameters

Perform this task to configure LDP discovery parameters (which may be crucial for LDP operations).


Note


The LDP discovery mechanism is used to discover or locate neighbor nodes.


SUMMARY STEPS

    1.    configure

    2.    mpls ldp

    3.    router-id { type number | ip-address }

    4.    discovery { hello | targeted-hello } holdtime seconds

    5.    discovery { hello | targeted-hello } interval seconds

    6.    Use one of the following commands:

    • end
    • commit

    7.    (Optional) show mpls ldp parameters


DETAILED STEPS
      Command or Action Purpose
    Step 1 configure


    Example:
    RP/0/RSP0/CPU0:router# configure 
     

    Enters global configuration mode.

     
    Step 2 mpls ldp


    Example:
    RP/0/RSP0/CPU0:router(config)# mpls ldp
    
     

    Enters MPLS LDP configuration mode.

     
    Step 3 router-id { type number | ip-address }


    Example:
    RP/0/RSP0/CPU0:router(config-ldp)# router-id loopback 1
    
     

    Specifies the router ID of the local node.

    • In Cisco IOS XR software, the router ID is specified as an interface name or IP address. By default, LDP uses the global router ID (configured by the global router ID process).
     
    Step 4 discovery { hello | targeted-hello } holdtime seconds


    Example:
    RP/0/RSP0/CPU0:router(config-ldp)# discovery hello holdtime 30
      RP/0/RSP0/CPU0:router(config-ldp)# discovery targeted-hello holdtime 180
      
     

    Specifies the time that a discovered neighbor is kept without receipt of any subsequent hello messages. The default value for the seconds argument is 15 seconds for link hello and 90 seconds for targeted hello messages.

     
    Step 5 discovery { hello | targeted-hello } interval seconds


    Example:
    RP/0/RSP0/CPU0:router(config-ldp)# discovery hello interval 15 
      RP/0/RSP0/CPU0:router(config-ldp)# discovery targeted-hello interval 20
      
     

    Selects the period of time between the transmission of consecutive hello messages. The default value for the seconds argument is 5 seconds for link hello messages and 10 seconds for targeted hello messages.

     
    Step 6 Use one of the following commands:
    • end
    • commit


    Example:
    
    RP/0/RSP0/CPU0:router
    (config-ldp)# end

    or

    
    RP/0/RSP0/CPU0:router
    (config-ldp)# commit
     

    Saves configuration changes.

    • When you issue the end command, the system prompts you to commit changes:
      Uncommitted changes found, commit them before exiting(yes/no/cancel)?
      [cancel]:
      • Entering yes saves configuration changes to the running configuration file, exits the configuration session, and returns the router to EXEC mode.
      • Entering no exits the configuration session and returns the router to EXEC mode without committing the configuration changes.
      • Entering cancel leaves the router in the current configuration session without exiting or committing the configuration changes.
    • Use the commit command to save the configuration changes to the running configuration file and remain within the configuration session.
     
    Step 7 show mpls ldp parameters


    Example:
    
    RP/0/RSP0/CPU0:router
    show mpls ldp parameters
    
     
    (Optional)

    Displays all the current MPLS LDP parameters.

     
    Related Concepts

    Configuring LDP Discovery Over a Link

    Perform this task to configure LDP discovery over a link.


    Note


    There is no need to enable LDP globally.


    Before You Begin

    A stable router ID is required at either end of the link to ensure the link discovery (and session setup) is successful. If you do not assign a router ID to the routers, the system will default to the global router ID. Default router IDs are subject to change and may cause an unstable discovery.

    SUMMARY STEPS

      1.    configure

      2.    mpls ldp

      3.    router-id ip-address

      4.    interface type interface-path-id

      5.    Use one of the following commands:

      • end
      • commit

      6.    (Optional) show mpls ldp discovery


    DETAILED STEPS
        Command or Action Purpose
      Step 1 configure


      Example:
      RP/0/RSP0/CPU0:router# configure
      
       

      Enters global configuration mode.

       
      Step 2 mpls ldp


      Example:
      RP/0/RSP0/CPU0:router(config)# mpls ldp
      
       

      Enters MPLS LDP configuration mode.

       
      Step 3 router-id ip-address


      Example:
      RP/0/RSP0/CPU0:router(config-ldp)# router-id loopback 1
      
       

      Specifies the router ID of the local node.

      • In Cisco IOS XR software, the router ID is specified as an interface name or IP address. By default, LDP uses the global router ID (configured by the global router ID process).
       
      Step 4 interface type interface-path-id


      Example:
      RP/0/RSP0/CPU0:router(config-ldp)# interface tunnel-te 12001
      RP/0/RSP0/CPU0:router(config-ldp-if)#
      
       

      Enters interface configuration mode for the LDP protocol. Interface type must be Tunnel-TE.

       
      Step 5 Use one of the following commands:
      • end
      • commit


      Example:
      RP/0/RSP0/CPU0:router(config-ldp-if)# end

      or

      RP/0/RSP0/CPU0:router(config-ldp-if)# commit
       

      Saves configuration changes.

      • When you issue the end command, the system prompts you to commit changes:
        Uncommitted changes found, commit them before exiting(yes/no/cancel)?
        [cancel]:
        • Entering yes saves configuration changes to the running configuration file, exits the configuration session, and returns the router to EXEC mode.
        • Entering no exits the configuration session and returns the router to EXEC mode without committing the configuration changes.
        • Entering cancel leaves the router in the current configuration session without exiting or committing the configuration changes.
      • Use the commit command to save the configuration changes to the running configuration file and remain within the configuration session.
       
      Step 6 show mpls ldp discovery


      Example:
      RP/0/RSP0/CPU0:routershow mpls ldp discovery
      
       
      (Optional)

      Displays the status of the LDP discovery process. This command, without an interface filter, generates a list of interfaces over which the LDP discovery process is running. The output information contains the state of the link (xmt/rcv hellos), local LDP identifier, the discovered peer’s LDP identifier, and holdtime values.

       
      Related Concepts
      Related References

      Configuring LDP Discovery for Active Targeted Hellos

      Perform this task to configure LDP discovery for active targeted hellos.


      Note


      The active side for targeted hellos initiates the unicast hello toward a specific destination.


      Before You Begin

      These prerequisites are required to configure LDP discovery for active targeted hellos:

      • Stable router ID is required at either end of the targeted session. If you do not assign a router ID to the routers, the system will default to the global router ID. Please note that default router IDs are subject to change and may cause an unstable discovery.
      • One or more MPLS Traffic Engineering tunnels are established between non-directly connected LSRs.
      SUMMARY STEPS

        1.    configure

        2.    mpls ldp

        3.    router-id ip-address

        4.    interface type interface-path-id

        5.    Use one of the following commands:

        • end
        • commit

        6.    (Optional) show mpls ldp discovery


      DETAILED STEPS
          Command or Action Purpose
        Step 1 configure


        Example:
        RP/0/RSP0/CPU0:router# configure 
         

        Enters global configuration mode.

         
        Step 2 mpls ldp


        Example:
        RP/0/RSP0/CPU0:router(config)# mpls ldp
        
         

        Enters MPLS LDP configuration mode.

         
        Step 3 router-id ip-address


        Example:
        RP/0/RSP0/CPU0:router(config-ldp)# router-id loopback 1
        
         

        Specifies the router ID of the local node.

        In Cisco IOS XR software, the router ID is specified as an interface name or IP address. By default, LDP uses the global router ID (configured by global router ID process).

         
        Step 4 interface type interface-path-id


        Example:
        RP/0/RSP0/CPU0:router(config-ldp)# interface tunnel-te 12001
        
         

        Enters interface configuration mode for the LDP protocol.

         
        Step 5 Use one of the following commands:
        • end
        • commit


        Example:
        RP/0/RSP0/CPU0:router(config-ldp)# end

        or

        RP/0/RSP0/CPU0:router(config-ldp)# commit
         

        Saves configuration changes.

        • When you issue the end command, the system prompts you to commit changes:
          Uncommitted changes found, commit them before exiting(yes/no/cancel)?
          [cancel]:
          • Entering yes saves configuration changes to the running configuration file, exits the configuration session, and returns the router to EXEC mode.
          • Entering no exits the configuration session and returns the router to EXEC mode without committing the configuration changes.
          • Entering cancel leaves the router in the current configuration session without exiting or committing the configuration changes.
        • Use the commit command to save the configuration changes to the running configuration file and remain within the configuration session.
         
        Step 6 show mpls ldp discovery


        Example:
        RP/0/RSP0/CPU0:routershow mpls ldp discovery
        
         
        (Optional)

        Displays the status of the LDP discovery process. This command, without an interface filter, generates a list of interfaces over which the LDP discovery process is running. The output information contains the state of the link (xmt/rcv hellos), local LDP identifier, the discovered peer’s LDP identifier, and holdtime values.

         
        Related Concepts

        Configuring LDP Discovery for Passive Targeted Hellos

        Perform this task to configure LDP discovery for passive targeted hellos.

        A passive side for targeted hello is the destination router (tunnel tail), which passively waits for an incoming hello message. Because targeted hellos are unicast, the passive side waits for an incoming hello message to respond with hello toward its discovered neighbor.

        Before You Begin

        Stable router ID is required at either end of the link to ensure that the link discovery (and session setup) is successful. If you do not assign a router ID to the routers, the system defaults to the global router ID. Default router IDs are subject to change and may cause an unstable discovery.

        SUMMARY STEPS

          1.    configure

          2.    mpls ldp

          3.    router-id ip-address

          4.    discovery targeted-hello accept

          5.    Use one of the following commands:

          • end
          • commit

          6.    (Optional) show mpls ldp discovery


        DETAILED STEPS
            Command or Action Purpose
          Step 1 configure


          Example:
          RP/0/RSP0/CPU0:router# configure
          
           

          Enters global configuration mode.

           
          Step 2 mpls ldp


          Example:
          RP/0/RSP0/CPU0:router(config)# mpls ldp
          
           

          Enters MPLS LDP configuration mode.

           
          Step 3 router-id ip-address


          Example:
          RP/0/RSP0/CPU0:router(config-ldp)# router-id loopback 1
          
           

          Specifies the router ID of the local node.

          • In Cisco IOS XR software, the router ID is specified as an interface name or IP address. By default, LDP uses the global router ID (configured by global router ID process).
           
          Step 4 discovery targeted-hello accept


          Example:
          RP/0/RSP0/CPU0:router(config-ldp)# discovery targeted-hello accept
          
           

          Directs the system to accept targeted hello messages from any source and activates passive mode on the LSR for targeted hello acceptance.

          • This command is executed on the receiver node (with respect to a given MPLS TE tunnel).
          • You can control the targeted-hello acceptance using the discovery targeted-hello accept command.
           
          Step 5 Use one of the following commands:
          • end
          • commit


          Example:
          RP/0/RSP0/CPU0:router(config-ldp)# end

          or

          RP/0/RSP0/CPU0:router(config-ldp)# commit
           

          Saves configuration changes.

          • When you issue the end command, the system prompts you to commit changes:
            Uncommitted changes found, commit them before exiting(yes/no/cancel)?
            [cancel]:
            • Entering yes saves configuration changes to the running configuration file, exits the configuration session, and returns the router to EXEC mode.
            • Entering no exits the configuration session and returns the router to EXEC mode without committing the configuration changes.
            • Entering cancel leaves the router in the current configuration session without exiting or committing the configuration changes.
          • Use the commit command to save the configuration changes to the running configuration file and remain within the configuration session.
           
          Step 6 show mpls ldp discovery


          Example:
          RP/0/RSP0/CPU0:routershow mpls ldp discovery
          
           
          (Optional)

          Displays the status of the LDP discovery process. This command, without an interface filter, generates a list of interfaces over which the LDP discovery process is running. The output information contains the state of the link (xmt/rcv hellos), local LDP identifier, the discovered peer’s LDP identifier, and holdtime values.

           
          Related Concepts

          Configuring Label Advertisement Control (Outbound Filtering)

          Perform this task to configure label advertisement (outbound filtering).

          By default, a label switched router (LSR) advertises all incoming label prefixes to each neighboring router. You can control the exchange of label binding information using the mpls ldp label advertise command. Using the optional keywords, you can advertise selective prefixes to all neighbors, advertise selective prefixes to defined neighbors, or disable label advertisement to all peers for all prefixes.


          Note


          Prefixes and peers advertised selectively are defined in the access list.


          Before You Begin

          Before configuring label advertisement, enable LDP and configure an access list.

          SUMMARY STEPS

            1.    configure

            2.    mpls ldp

            3.    label advertise { disable | for prefix-acl [ to peer-acl ] | interface type interface-path-id }

            4.    Use one of the following commands:

            • end
            • commit


          DETAILED STEPS
              Command or Action Purpose
            Step 1 configure


            Example:
            RP/0/RSP0/CPU0:router# configure
            
             

            Enters global configuration mode.

             
            Step 2 mpls ldp


            Example:
            RP/0/RSP0/CPU0:router(config)# mpls ldp
            
             

            Enters MPLS LDP configuration mode.

             
            Step 3 label advertise { disable | for prefix-acl [ to peer-acl ] | interface type interface-path-id }


            Example:
            RP/0/RSP0/CPU0:router(config-ldp)# label advertise interface POS 0/1/0/0
            RP/0/RSP0/CPU0:router(config-ldp)# for pfx_acl1 to peer_acl1
            
             
            Configures label advertisement by specifying one of the following options:
            disable

            Disables label advertisement to all peers for all prefixes (if there are no other conflicting rules).

            interface

            Specifies an interface for label advertisement of an interface address.

            for prefix-acl
            to peer-acl

            Specifies neighbors to advertise and receive label advertisements.

             
            Step 4 Use one of the following commands:
            • end
            • commit


            Example:
            RP/0/RSP0/CPU0:router(config-ldp)# end

            or

            RP/0/RSP0/CPU0:router(config-ldp)# commit
             

            Saves configuration changes.

            • When you issue the end command, the system prompts you to commit changes:
              Uncommitted changes found, commit them before exiting(yes/no/cancel)?
              [cancel]:
              • Entering yes saves configuration changes to the running configuration file, exits the configuration session, and returns the router to EXEC mode.
              • Entering no exits the configuration session and returns the router to EXEC mode without committing the configuration changes.
              • Entering cancel leaves the router in the current configuration session without exiting or committing the configuration changes.
            • Use the commit command to save the configuration changes to the running configuration file and remain within the configuration session.
             

            Setting Up LDP Neighbors

            Perform this task to set up LDP neighbors.

            Before You Begin

            Stable router ID is required at either end of the link to ensure the link discovery (and session setup) is successful. If you do not assign a router ID to the routers, the system will default to the global router ID. Default router IDs are subject to change and may cause an unstable discovery.

            SUMMARY STEPS

              1.    configure

              2.    mpls ldp

              3.    interface type interface-path-id

              4.    discovery transport-address [ ip-address | interface ]

              5.    exit

              6.    holdtime seconds

              7.    neighbor ip-address password [ encryption ] password

              8.    backoff initial maximum

              9.    Use one of the following commands:

              • end
              • commit

              10.    (Optional) show mpls ldp neighbor


            DETAILED STEPS
                Command or Action Purpose
              Step 1 configure


              Example:
              RP/0/RSP0/CPU0:router# configure 
               

              Enters global configuration mode.

               
              Step 2 mpls ldp


              Example:
              RP/0/RSP0/CPU0:router(config)# mpls ldp
              
               

              Enters MPLS LDP configuration mode.

               
              Step 3 interface type interface-path-id


              Example:
              RP/0/RSP0/CPU0:router(config-ldp)# interface POS 0/1/0/0
              
               

              Enters interface configuration mode for the LDP protocol.

               
              Step 4 discovery transport-address [ ip-address | interface ]


              Example:
              RP/0/RSP0/CPU0:router(config-ldp-if)# discovery transport-address 192.168.1.42
              

              or

              RP/0/RSP0/CPU0:router(config-ldp)# discovery transport-address interface
              
               

              Provides an alternative transport address for a TCP connection.

              • Default transport address advertised by an LSR (for TCP connections) to its peer is the router ID.
              • Transport address configuration is applied for a given LDP-enabled interface.
              • If the interface version of the command is used, the configured IP address of the interface is passed to its neighbors as the transport address.
               
              Step 5 exit


              Example:
              RP/0/RSP0/CPU0:router(config-ldp-if)# exit
              
               

              Exits the current configuration mode.

               
              Step 6 holdtime seconds


              Example:
              RP/0/RSP0/CPU0:router(config-ldp)# holdtime 30
              
               

              Changes the time for which an LDP session is maintained in the absence of LDP messages from the peer.

              • Outgoing keepalive interval is adjusted accordingly (to make three keepalives in a given holdtime) with a change in session holdtime value.
              • Session holdtime is also exchanged when the session is established.
              • In this example holdtime is set to 30 seconds, which causes the peer session to timeout in 30 seconds, as well as transmitting outgoing keepalive messages toward the peer every 10 seconds.
               
              Step 7 neighbor ip-address password [ encryption ] password


              Example:
              RP/0/RSP0/CPU0:router(config-ldp)# neighbor 192.168.2.44 password secretpasswd
              
               

              Configures password authentication (using the TCP MD5 option) for a given neighbor.

               
              Step 8 backoff initial maximum


              Example:
              RP/0/RSP0/CPU0:router(config-ldp)# backoff 10 20
              
               

              Configures the parameters for the LDP backoff mechanism. The LDP backoff mechanism prevents two incompatibly configured LSRs from engaging in an unthrottled sequence of session setup failures. If a session setup attempt fails due to such incompatibility, each LSR delays its next attempt (backs off), increasing the delay exponentially with each successive failure until the maximum backoff delay is reached.

               
              Step 9 Use one of the following commands:
              • end
              • commit


              Example:
              RP/0/RSP0/CPU0:router(config-ldp)# end

              or

              RP/0/RSP0/CPU0:router(config-ldp)# commit
               

              Saves configuration changes.

              • When you issue the end command, the system prompts you to commit changes:
                Uncommitted changes found, commit them before exiting(yes/no/cancel)?
                [cancel]:
                • Entering yes saves configuration changes to the running configuration file, exits the configuration session, and returns the router to EXEC mode.
                • Entering no exits the configuration session and returns the router to EXEC mode without committing the configuration changes.
                • Entering cancel leaves the router in the current configuration session without exiting or committing the configuration changes.
              • Use the commit command to save the configuration changes to the running configuration file and remain within the configuration session.
               
              Step 10 show mpls ldp neighbor


              Example:
              RP/0/RSP0/CPU0:routershow mpls ldp neighbor
              
               
              (Optional)

              Displays the status of the LDP session with its neighbors. This command can be run with various filters as well as with the brief option.

               

              Setting Up LDP Forwarding

              Perform this task to set up LDP forwarding.

              By default, the LDP control plane implements the penultimate hop popping (PHOP) mechanism. The PHOP mechanism requires that label switched routers use the implicit-null label as a local label for the given Forwarding Equivalence Class (FEC) for which LSR is the penultimate hop. Although PHOP has certain advantages, it may be required to extend LSP up to the ultimate hop under certain circumstances (for example, to propagate MPL QoS). This is done using a special local label (explicit-null) advertised to the peers after which the peers use this label when forwarding traffic toward the ultimate hop (egress LSR).

              Before You Begin

              Stable router ID is required at either end of the link to ensure the link discovery (and session setup) is successful. If you do not assign a router ID to the routers, the system will default to the global router ID. Default router IDs are subject to change and may cause an unstable discovery.

              SUMMARY STEPS

                1.    configure

                2.    mpls ldp

                3.    explicit-null

                4.    Use one of the following commands:

                • end
                • commit

                5.    (Optional) show mpls ldp forwarding

                6.    (Optional) show mpls forwarding

                7.    (Optional) ping ip-address


              DETAILED STEPS
                  Command or Action Purpose
                Step 1 configure


                Example:
                RP/0/RSP0/CPU0:router# configure 
                 

                Enters global configuration mode.

                 
                Step 2 mpls ldp


                Example:
                RP/0/RSP0/CPU0:router(config)# mpls ldp
                
                 

                Enters MPLS LDP configuration mode.

                 
                Step 3 explicit-null


                Example:
                RP/0/RSP0/CPU0:router(config-ldp)# explicit-null
                
                 

                Causes a router to advertise an explicit null label in situations where it normally advertises an implicit null label (for example, to enable an ultimate-hop disposition instead of PHOP).

                 
                Step 4 Use one of the following commands:
                • end
                • commit


                Example:
                RP/0/RSP0/CPU0:router(config-ldp)# end

                or

                
                RP/0/RSP0/CPU0:router(config-ldp)# commit
                 

                Saves configuration changes.

                • When you issue the end command, the system prompts you to commit changes:
                  Uncommitted changes found, commit them before exiting(yes/no/cancel)?
                  [cancel]:
                  • Entering yes saves configuration changes to the running configuration file, exits the configuration session, and returns the router to EXEC mode.
                  • Entering no exits the configuration session and returns the router to EXEC mode without committing the configuration changes.
                  • Entering cancel leaves the router in the current configuration session without exiting or committing the configuration changes.
                • Use the commit command to save the configuration changes to the running configuration file and remain within the configuration session.
                 
                Step 5 show mpls ldp forwarding


                Example:
                RP/0/RSP0/CPU0:routershow mpls ldp forwarding
                
                 
                (Optional)

                Displays the MPLS LDP view of installed forwarding states (rewrites).

                 
                Step 6 show mpls forwarding


                Example:
                RP/0/RSP0/CPU0:routershow mpls forwarding
                
                 
                (Optional)

                Displays a global view of all MPLS installed forwarding states (rewrites) by various applications (LDP, TE, and static).

                 
                Step 7 ping ip-address


                Example:
                RP/0/RSP0/CPU0:routerping 192.168.2.55
                
                 
                (Optional)

                Checks for connectivity to a particular IP address (going through MPLS LSP as shown in the show mpls forwarding command).

                 
                Related Concepts

                Setting Up LDP NSF Using Graceful Restart

                Perform this task to set up NSF using LDP graceful restart.

                LDP graceful restart is a way to enable NSF for LDP. The correct way to set up NSF using LDP graceful restart is to bring up LDP neighbors (link or targeted) with additional configuration related to graceful restart.

                Before You Begin

                Stable router ID is required at either end of the link to ensure the link discovery (and session setup) is successful. If you do not assign a router ID to the routers, the system will default to the global router ID. Default router IDs are subject to change and may cause an unstable discovery.

                SUMMARY STEPS

                  1.    configure

                  2.    mpls ldp

                  3.    interface type interface-path-id

                  4.    exit

                  5.    graceful-restart

                  6.    graceful-restart forwarding-state-holdtime seconds

                  7.    graceful-restart reconnect-timeout seconds

                  8.    Use one of the following commands:

                  • end
                  • commit

                  9.    (Optional) show mpls ldp parameters

                  10.    (Optional) show mpls ldp neighbor

                  11.    (Optional) show mpls ldp graceful-restart


                DETAILED STEPS
                    Command or Action Purpose
                  Step 1 configure


                  Example:
                  RP/0/RSP0/CPU0:router# configure
                  
                   

                  Enters global configuration mode.

                   
                  Step 2 mpls ldp


                  Example:
                  RP/0/RSP0/CPU0:router(config)# mpls ldp
                  
                   

                  Enters MPLS LDP configuration mode.

                   
                  Step 3 interface type interface-path-id


                  Example:
                  RP/0/RSP0/CPU0:router(config-ldp)# interface POS 0/1/0/0
                  RP/0/RSP0/CPU0:router(config-ldp-if)# 
                  
                   

                  Enters interface configuration mode for the LDP protocol.

                   
                  Step 4 exit


                  Example:
                  RP/0/RSP0/CPU0:router(config-ldp-if)# exit
                  
                   

                  Exits the current configuration mode.

                   
                  Step 5 graceful-restart


                  Example:
                  RP/0/RSP0/CPU0:router(config-ldp)# graceful-restart
                  
                   

                  Enables the LDP graceful restart feature.

                   
                  Step 6 graceful-restart forwarding-state-holdtime seconds


                  Example:
                  RP/0/RSP0/CPU0:router(config-ldp)# graceful-restart forwarding-state-holdtime 180
                  
                   

                  Specifies the length of time that forwarding can keep LDP-installed forwarding states and rewrites, and specifies wh en the LDP control plane restarts.

                  • After restart of the control plane, when the forwarding state holdtime expires, any previously installed LDP forwarding state or rewrite that is not yet refreshed is deleted from the forwarding.
                  • Recovery time sent after restart is computed as the current remaining value of the forwarding state hold timer.
                   
                  Step 7 graceful-restart reconnect-timeout seconds


                  Example:
                  RP/0/RSP0/CPU0:router(config-ldp)# graceful-restart reconnect-timeout 169
                  
                   

                  Specifies the length of time a neighbor waits before restarting the node to reconnect before declaring an earlier graceful restart session as down. This command is used to start a timer on the peer (upon a neighbor restart). This timer is referred to as Neighbor Liveness timer.

                   
                  Step 8 Use one of the following commands:
                  • end
                  • commit


                  Example:
                  RP/0/RSP0/CPU0:router(config-ldp)# end

                  or

                  RP/0/RSP0/CPU0:router(config-ldp)# commit
                   

                  Saves configuration changes.

                  • When you issue the end command, the system prompts you to commit changes:
                    Uncommitted changes found, commit them before exiting(yes/no/cancel)?
                    [cancel]:
                    • Entering yes saves configuration changes to the running configuration file, exits the configuration session, and returns the router to EXEC mode.
                    • Entering no exits the configuration session and returns the router to EXEC mode without committing the configuration changes.
                    • Entering cancel leaves the router in the current configuration session without exiting or committing the configuration changes.
                  • Use the commit command to save the configuration changes to the running configuration file and remain within the configuration session.
                   
                  Step 9 show mpls ldp parameters


                  Example:
                  RP/0/RSP0/CPU0:routershow mpls ldp parameters
                  
                   
                  (Optional)

                  Displays all the current MPLS LDP parameters.

                   
                  Step 10 show mpls ldp neighbor


                  Example:
                  RP/0/RSP0/CPU0:routershow mpls ldp neighbor
                  
                   
                  (Optional)

                  Displays the status of the LDP session with its neighbors. This command can be run with various filters as well as with the brief option.

                   
                  Step 11 show mpls ldp graceful-restart


                  Example:
                  RP/0/RSP0/CPU0:routershow mpls ldp graceful-restart
                  
                   
                  (Optional)

                  Displays the status of the LDP graceful restart feature. The output of this command not only shows states of different graceful restart timers, but also a list of graceful restart neighbors, their state, and reconnect count.

                   

                  Configuring Label Acceptance Control (Inbound Filtering)

                  Perform this task to configure LDP inbound label filtering.


                  Note


                  By default, there is no inbound label filtering performed by LDP and thus an LSR accepts (and retains) all remote label bindings from all peers.


                  SUMMARY STEPS

                    1.    configure

                    2.    mpls ldp

                    3.    label accept for prefix-acl from ip-address

                    4.    Use one of the following commands:

                    • end
                    • commit


                  DETAILED STEPS
                      Command or Action Purpose
                    Step 1 configure


                    Example:
                    RP/0/RSP0/CPU0:router# configure
                    
                     

                    Enters the configuration mode.

                     
                    Step 2 mpls ldp


                    Example:
                    RP/0/RSP0/CPU0:router(config)# mpls ldp
                    
                     

                    Enters the MPLS LDP configuration mode.

                     
                    Step 3 label accept for prefix-acl from ip-address


                    Example:
                    RP/0/RSP0/CPU0:router(config-ldp)# label accept for pfx_acl_1 from 192.168.1.1  
                    RP/0/RSP0/CPU0:router(config-ldp)# label accept for pfx_acl_2 from 192.168.2.2
                    
                     

                    Configures inbound label acceptance for prefixes specified by prefix-acl from neighbor (as specified by its IP address).

                     
                    Step 4 Use one of the following commands:
                    • end
                    • commit


                    Example:
                    RP/0/RSP0/CPU0:router(config-ldp)# end

                    or

                    RP/0/RSP0/CPU0:router(config-ldp)# commit
                     

                    Saves configuration changes.

                    • When you issue the end command, the system prompts you to commit changes:
                      Uncommitted changes found, commit them before exiting(yes/no/cancel)?
                      [cancel]:
                      • Entering yes saves configuration changes to the running configuration file, exits the configuration session, and returns the router to EXEC mode.
                      • Entering no exits the configuration session and returns the router to EXEC mode without committing the configuration changes.
                      • Entering cancel leaves the router in the current configuration session without exiting or committing the configuration changes.
                    • Use the commit command to save the configuration changes to the running configuration file and remain within the configuration session.
                     

                    Configuring Local Label Allocation Control

                    Perform this task to configure label allocation control.


                    Note


                    By default, local label allocation control is disabled and all non-BGP prefixes are assigned local labels.


                    SUMMARY STEPS

                      1.    configure

                      2.    mpls ldp

                      3.    label allocate for prefix-acl

                      4.    Use one of the following commands:

                      • end
                      • commit


                    DETAILED STEPS
                        Command or Action Purpose
                      Step 1 configure


                      Example:
                      RP/0/RSP0/CPU0:router# configure
                      
                       

                      Enters the configuration mode.

                       
                      Step 2 mpls ldp


                      Example:
                      RP/0/RSP0/CPU0:router(config)# mpls ldp
                      
                       

                      Enters the MPLS LDP configuration mode.

                       
                      Step 3 label allocate for prefix-acl


                      Example:
                      RP/0/RSP0/CPU0:router(config-ldp)# label allocate for pfx_acl_1
                      
                       

                      Configures label allocation control for prefixes as specified by prefix-acl.

                       
                      Step 4 Use one of the following commands:
                      • end
                      • commit


                      Example:
                      RP/0/RSP0/CPU0:router(config-ldp)# end

                      or

                      RP/0/RSP0/CPU0:router(config-ldp)# commit
                       

                      Saves configuration changes.

                      • When you issue the end command, the system prompts you to commit changes:
                        Uncommitted changes found, commit them before exiting(yes/no/cancel)?
                        [cancel]:
                        • Entering yes saves configuration changes to the running configuration file, exits the configuration session, and returns the router to EXEC mode.
                        • Entering no exits the configuration session and returns the router to EXEC mode without committing the configuration changes.
                        • Entering cancel leaves the router in the current configuration session without exiting or committing the configuration changes.
                      • Use the commit command to save the configuration changes to the running configuration file and remain within the configuration session.
                       

                      Configuring Session Protection

                      Perform this task to configure LDP session protection.

                      By default, there is no protection is done for link sessions by means of targeted hellos.

                      SUMMARY STEPS

                        1.    configure

                        2.    mpls ldp

                        3.    session protection [ for peer-acl ] [ duration seconds ]

                        4.    Use one of the following commands:

                        • end
                        • commit


                      DETAILED STEPS
                          Command or Action Purpose
                        Step 1 configure


                        Example:
                        RP/0/RSP0/CPU0:router# configure
                        
                         

                        Enters the configuration mode.

                         
                        Step 2 mpls ldp


                        Example:
                        RP/0/RSP0/CPU0:router(config)# mpls ldp
                        
                         

                        Enters the MPLS LDP configuration mode.

                         
                        Step 3 session protection [ for peer-acl ] [ duration seconds ]


                        Example:
                        RP/0/RSP0/CPU0:router(config-ldp)# session protection for peer_acl_1 duration 60
                        
                         

                        Configures LDP session protection for peers specified by peer-acl with a maximum duration, in seconds.

                         
                        Step 4 Use one of the following commands:
                        • end
                        • commit


                        Example:
                        RP/0/RSP0/CPU0:router(config-ldp)# end

                        or

                        RP/0/RSP0/CPU0:router(config-ldp)# commit
                         

                        Saves configuration changes.

                        • When you issue the end command, the system prompts you to commit changes:
                          Uncommitted changes found, commit them before exiting(yes/no/cancel)?
                          [cancel]:
                          • Entering yes saves configuration changes to the running configuration file, exits the configuration session, and returns the router to EXEC mode.
                          • Entering no exits the configuration session and returns the router to EXEC mode without committing the configuration changes.
                          • Entering cancel leaves the router in the current configuration session without exiting or committing the configuration changes.
                        • Use the commit command to save the configuration changes to the running configuration file and remain within the configuration session.
                         
                        Related Concepts

                        Configuring LDP IGP Synchronization: OSPF

                        Perform this task to configure LDP IGP Synchronization under OSPF.


                        Note


                        By default, there is no synchronization between LDP and IGPs.


                        SUMMARY STEPS

                          1.    configure

                          2.    router ospf process-name

                          3.    Use one of the following commands:

                          • mpls ldp sync
                          • area area-id mpls ldp sync
                          • area area-id interface name mpls ldp sync

                          4.    Use one of the following commands:

                          • end
                          • commit


                        DETAILED STEPS
                            Command or Action Purpose
                          Step 1 configure


                          Example:
                          RP/0/RSP0/CPU0:router# configure
                          
                           

                          Enters global configuration mode.

                           
                          Step 2 router ospf process-name


                          Example:
                          RP/0/RSP0/CPU0:router(config)# router ospf 100
                          
                           

                          Identifies the OSPF routing process and enters OSPF configuration mode.

                           
                          Step 3 Use one of the following commands:
                          • mpls ldp sync
                          • area area-id mpls ldp sync
                          • area area-id interface name mpls ldp sync


                          Example:
                          RP/0/RSP0/CPU0:router(config-ospf)# mpls ldp sync
                          
                           

                          Enables LDP IGP synchronization on an interface.

                           
                          Step 4 Use one of the following commands:
                          • end
                          • commit


                          Example:
                          RP/0/RSP0/CPU0:router(config-ospf)# end

                          or

                          RP/0/RSP0/CPU0:router(config-ospf)# commit
                           

                          Saves configuration changes.

                          • When you issue the end command, the system prompts you to commit changes:
                            Uncommitted changes found, commit them before exiting(yes/no/cancel)?
                            [cancel]:
                            • Entering yes saves configuration changes to the running configuration file, exits the configuration session, and returns the router to EXEC mode.
                            • Entering no exits the configuration session and returns the router to EXEC mode without committing the configuration changes.
                            • Entering cancel leaves the router in the current configuration session without exiting or committing the configuration changes.
                          • Use the commit command to save the configuration changes to the running configuration file and remain within the configuration session.
                           
                          Related Concepts

                          Configuring LDP IGP Synchronization: ISIS

                          Perform this task to configure LDP IGP Synchronization under ISIS.


                          Note


                          By default, there is no synchronization between LDP and ISIS.


                          SUMMARY STEPS

                            1.    configure

                            2.    router isis instance-id

                            3.    interface type interface-path-id

                            4.    address-family ipv4 unicast

                            5.    mpls ldp sync

                            6.    Use one of the following commands:

                            • end
                            • commit


                          DETAILED STEPS
                              Command or Action Purpose
                            Step 1 configure


                            Example:
                            RP/0/RSP0/CPU0:router# configure
                            
                             

                            Enters global configuration mode.

                             
                            Step 2 router isis instance-id


                            Example:
                            RP/0/RSP0/CPU0:router(config)# router isis 100
                            RP/0/RSP0/CPU0:router(config-isis)#
                            
                             

                            Enables the Intermediate System-to-Intermediate System (IS-IS) routing protocol and defines an IS-IS instance.

                             
                            Step 3 interface type interface-path-id


                            Example:
                            RP/0/RSP0/CPU0:router(config-isis)# interface POS 0/2/0/0
                            RP/0/RSP0/CPU0:router(config-isis-if)#
                            
                             

                            Configures the IS-IS protocol on an interface and enters ISIS interface configuration mode.

                             
                            Step 4 address-family ipv4 unicast


                            Example:
                            RP/0/RSP0/CPU0:router(config-isis-if)# address-family ipv4 unicast
                            RP/0/RSP0/CPU0:router(config-isis-if-af)#
                            
                             

                            Enters address family configuration mode for configuring IS-IS routing for a standard IP Version 4 (IPv4) address prefix.

                             
                            Step 5 mpls ldp sync


                            Example:
                            RP/0/RSP0/CPU0:router(config-isis-if-af)# mpls ldp sync
                            
                             

                            Enables LDP IGP synchronization.

                             
                            Step 6 Use one of the following commands:
                            • end
                            • commit


                            Example:
                            RP/0/RSP0/CPU0:router(config-isis-if-af)# end

                            or

                            RP/0/RSP0/CPU0:router(config-isis-if-af)# commit
                             

                            Saves configuration changes.

                            • When you issue the end command, the system prompts you to commit changes:
                              Uncommitted changes found, commit them before exiting(yes/no/cancel)?
                              [cancel]:
                              • Entering yes saves configuration changes to the running configuration file, exits the configuration session, and returns the router to EXEC mode.
                              • Entering no exits the configuration session and returns the router to EXEC mode without committing the configuration changes.
                              • Entering cancel leaves the router in the current configuration session without exiting or committing the configuration changes.
                            • Use the commit command to save the configuration changes to the running configuration file and remain within the configuration session.
                             
                            Related Concepts

                            Enabling LDP Auto-Configuration for a Specified OSPF Instance

                            Perform this task to enable IGP auto-configuration globally for a specified OSPF process name.

                            You can disable auto-configuration on a per-interface basis. This lets LDP enable all IGP interfaces except those that are explicitly disabled.


                            Note


                            This feature is supported for IPv4 unicast family in default VRF only.


                            SUMMARY STEPS

                              1.    configure

                              2.    router ospf process-name

                              3.    mpls ldp auto-config

                              4.    area area-id

                              5.    interface type interface-path-id

                              6.    Use one of the following commands:

                              • end
                              • commit


                            DETAILED STEPS
                                Command or Action Purpose
                              Step 1 configure


                              Example:
                              RP/0/RSP0/CPU0:router# configure
                              
                               

                              Enters global configuration mode.

                               
                              Step 2 router ospf process-name


                              Example:
                              RP/0/RSP0/CPU0:router(config)# router ospf 190
                              RP/0/RSP0/CPU0:router(config-ospf)#
                              
                               

                              Enters a uniquely identifiable OSPF routing process. The process name is any alphanumeric string no longer than 40 characters without spaces.

                               
                              Step 3 mpls ldp auto-config


                              Example:
                              RP/0/RSP0/CPU0:router(config-ospf)# mpls ldp auto-config
                              
                               

                              Enables LDP auto-configuration.

                               
                              Step 4 area area-id


                              Example:
                              RP/0/RSP0/CPU0:router(config-ospf)# area 8
                              
                               

                              Configures an OSPF area and identifier.

                              area-id

                              Either a decimal value or an IP address.

                               
                              Step 5 interface type interface-path-id


                              Example:
                              RP/0/RSP0/CPU0:router(config-ospf-ar)# interface pos 0/6/0/0
                              
                               

                              Enables LDP auto-configuration on the specified interface.

                              Note   

                              LDP configurable limit for maximum number of interfaces does not apply to IGP auto-configuration interfaces.

                               
                              Step 6 Use one of the following commands:
                              • end
                              • commit


                              Example:
                              RP/0/RSP0/CPU0:router(config-ospf-ar-if)# end

                              or

                              RP/0/RSP0/CPU0:router(config-ospf-ar-if)# commit
                               

                              Saves configuration changes.

                              • When you issue the end command, the system prompts you to commit changes:
                                Uncommitted changes found, commit them before exiting(yes/no/cancel)?
                                [cancel]:
                                • Entering yes saves configuration changes to the running configuration file, exits the configuration session, and returns the router to EXEC mode.
                                • Entering no exits the configuration session and returns the router to EXEC mode without committing the configuration changes.
                                • Entering cancel leaves the router in the current configuration session without exiting or committing the configuration changes.
                              • Use the commit command to save the configuration changes to the running configuration file and remain within the configuration session.
                               
                              Related Concepts

                              Enabling LDP Auto-Configuration in an Area for a Specified OSPF Instance

                              Perform this task to enable IGP auto-configuration in a defined area with a specified OSPF process name.

                              You can disable auto-configuration on a per-interface basis. This lets LDP enable all IGP interfaces except those that are explicitly disabled.


                              Note


                              This feature is supported for IPv4 unicast family in default VRF only.


                              SUMMARY STEPS

                                1.    configure

                                2.    router ospf process-name

                                3.    area area-id

                                4.    mpls ldp auto-config

                                5.    interface type interface-path-id

                                6.    Use one of the following commands:

                                • end
                                • commit


                              DETAILED STEPS
                                  Command or Action Purpose
                                Step 1 configure


                                Example:
                                RP/0/RSP0/CPU0:router# configure
                                
                                 

                                Enters global configuration mode.

                                 
                                Step 2 router ospf process-name


                                Example:
                                RP/0/RSP0/CPU0:router(config)# router ospf 100
                                RP/0/RSP0/CPU0:router(config-ospf)# 
                                
                                 

                                Enters a uniquely identifiable OSPF routing process. The process name is any alphanumeric string no longer than 40 characters without spaces.

                                 
                                Step 3 area area-id


                                Example:
                                RP/0/RSP0/CPU0:router(config-ospf)# area 8
                                RP/0/RSP0/CPU0:router(config-ospf-ar)# 
                                
                                 

                                Configures an OSPF area and identifier.

                                area-id

                                Either a decimal value or an IP address.

                                 
                                Step 4 mpls ldp auto-config


                                Example:
                                RP/0/RSP0/CPU0:router(config-ospf-ar)# mpls ldp auto-config
                                
                                 

                                Enables LDP auto-configuration.

                                 
                                Step 5 interface type interface-path-id


                                Example:
                                RP/0/RSP0/CPU0:router(config-ospf-ar)# interface pos 0/6/0/0  
                                RP/0/RSP0/CPU0:router(config-ospf-ar-if)
                                
                                 

                                Enables LDP auto-configuration on the specified interface. The LDP configurable limit for maximum number of interfaces does not apply to IGP auto-config interfaces.

                                 
                                Step 6 Use one of the following commands:
                                • end
                                • commit


                                Example:
                                RP/0/RSP0/CPU0:router(config-ospf-ar-if)# end

                                or

                                RP/0/RSP0/CPU0:router(config-ospf-ar-if)# commit
                                 

                                Saves configuration changes.

                                • When you issue the end command, the system prompts you to commit changes:
                                  Uncommitted changes found, commit them before exiting(yes/no/cancel)?
                                  [cancel]:
                                  • Entering yes saves configuration changes to the running configuration file, exits the configuration session, and returns the router to EXEC mode.
                                  • Entering no exits the configuration session and returns the router to EXEC mode without committing the configuration changes.
                                  • Entering cancel leaves the router in the current configuration session without exiting or committing the configuration changes.
                                • Use the commit command to save the configuration changes to the running configuration file and remain within the configuration session.
                                 
                                Related Concepts

                                Disabling LDP Auto-Configuration

                                Perform this task to disable IGP auto-configuration.

                                You can disable auto-configuration on a per-interface basis. This lets LDP enable all IGP interfaces except those that are explicitly disabled.

                                SUMMARY STEPS

                                  1.    configure

                                  2.    mpls ldp

                                  3.    interface type interface-path-id

                                  4.    igp auto-config disable

                                  5.    Use one of the following commands:

                                  • end
                                  • commit


                                DETAILED STEPS
                                    Command or Action Purpose
                                  Step 1 configure


                                  Example:
                                  RP/0/RSP0/CPU0:router# configure
                                  
                                   

                                  Enters global configuration mode.

                                   
                                  Step 2 mpls ldp


                                  Example:
                                  RP/0/RSP0/CPU0:router(config)# mpls ldp
                                  RP/0/RSP0/CPU0:router(config-ldp)# 
                                  
                                   

                                  Enters the MPLS LDP configuration mode.

                                   
                                  Step 3 interface type interface-path-id


                                  Example:
                                  RP/0/RSP0/CPU0:router(config-ldp)# interface pos 0/6/0/0
                                  
                                   

                                  Enters interface configuration mode and configures an interface.

                                   
                                  Step 4 igp auto-config disable


                                  Example:
                                  RP/0/RSP0/CPU0:router(config-ldp-if)# igp auto-config disable
                                  
                                   

                                  Disables auto-configuration on the specified interface.

                                   
                                  Step 5 Use one of the following commands:
                                  • end
                                  • commit


                                  Example:
                                  RP/0/RSP0/CPU0:router(config-ldp-if)# end

                                  or

                                  RP/0/RSP0/CPU0:router(config-ldp-if)# commit
                                   

                                  Saves configuration changes.

                                  • When you issue the end command, the system prompts you to commit changes:
                                    Uncommitted changes found, commit them before exiting(yes/no/cancel)?
                                    [cancel]:
                                    • Entering yes saves configuration changes to the running configuration file, exits the configuration session, and returns the router to EXEC mode.
                                    • Entering no exits the configuration session and returns the router to EXEC mode without committing the configuration changes.
                                    • Entering cancel leaves the router in the current configuration session without exiting or committing the configuration changes.
                                  • Use the commit command to save the configuration changes to the running configuration file and remain within the configuration session.
                                   
                                  Related Concepts

                                  Configuring LDP Nonstop Routing

                                  Perform this task to configure LDP NSR.


                                  Note


                                  By default, NSR is globally-enabled on all LDP sessions except AToM.


                                  SUMMARY STEPS

                                    1.    configure

                                    2.    mpls ldp

                                    3.    nsr

                                    4.    Use one of the following commands:

                                    • end
                                    • commit

                                    5.    show mpls ldp nsr statistics

                                    6.    show mpls ldp nsr summary

                                    7.    show mpls ldp nsr pending


                                  DETAILED STEPS
                                      Command or Action Purpose
                                    Step 1 configure


                                    Example:
                                    RP/0/RSP0/CPU0:router# configure
                                    
                                     

                                    Enters global configuration mode.

                                     
                                    Step 2 mpls ldp


                                    Example:
                                    RP/0/RSP0/CPU0:router(config)# mpls ldp
                                    
                                     

                                    Enters the MPLS LDP configuration mode.

                                     
                                    Step 3 nsr


                                    Example:
                                    RP/0/RSP0/CPU0:router(config-ldp)# nsr
                                    
                                     

                                    Enables LDP nonstop routing.

                                     
                                    Step 4 Use one of the following commands:
                                    • end
                                    • commit


                                    Example:
                                    RP/0/RSP0/CPU0:router(config-ldp)# end

                                    or

                                    RP/0/RSP0/CPU0:router(config-ldp)# commit
                                     

                                    Saves configuration changes.

                                    • When you issue the end command, the system prompts you to commit changes:
                                      Uncommitted changes found, commit them before exiting(yes/no/cancel)?
                                      [cancel]:
                                      • Entering yes saves configuration changes to the running configuration file, exits the configuration session, and returns the router to EXEC mode.
                                      • Entering no exits the configuration session and returns the router to EXEC mode without committing the configuration changes.
                                      • Entering cancel leaves the router in the current configuration session without exiting or committing the configuration changes.
                                    • Use the commit command to save the configuration changes to the running configuration file and remain within the configuration session.
                                     
                                    Step 5 show mpls ldp nsr statistics


                                    Example:
                                    RP/0/RSP0/CPU0:router# show mpls ldp nsr statistics
                                    
                                     

                                    Displays MPLS LDP NSR statistics.

                                     
                                    Step 6 show mpls ldp nsr summary


                                    Example:
                                    RP/0/RSP0/CPU0:router# show mpls ldp nsr summary
                                    
                                     

                                    Displays MPLS LDP NSR summarized information.

                                     
                                    Step 7 show mpls ldp nsr pending


                                    Example:
                                    RP/0/RSP0/CPU0:router# show mpls ldp nsr pending
                                    
                                     

                                    Displays MPLS LDP NSR pending information.

                                     
                                    Related Concepts

                                    Configuration Examples for Implementing MPLS LDP

                                    These configuration examples are provided to implement LDP:

                                    Configuring LDP with Graceful Restart: Example

                                    The example shows how to enable LDP with graceful restart on the POS interface 0/2/0/0.

                                      mpls ldp
                                       graceful-restart
                                       interface pos0/2/0/0
                                      !
                                      

                                    Configuring LDP Discovery: Example

                                    The example shows how to configure LDP discovery parameters.

                                      mpls ldp
                                       router-id loopback0
                                       discovery hello holdtime 15
                                       discovery hello interval 5
                                      !
                                      
                                      show mpls ldp parameters
                                      show mpls ldp discovery
                                      

                                    Configuring LDP Link: Example

                                    The example shows how to configure LDP link parameters.

                                      mpls ldp
                                       interface pos 0/1/0/0
                                       !
                                      !
                                      
                                      show mpls ldp discovery
                                      
                                    Related Concepts

                                    Configuring LDP Discovery for Targeted Hellos: Example

                                    The examples show how to configure LDP Discovery to accept targeted hello messages.

                                    Active (tunnel head)

                                      mpls ldp
                                       router-id loopback0
                                       interface tunnel-te 12001
                                       !
                                      !
                                      

                                    Passive (tunnel tail)

                                      mpls ldp
                                       router-id loopback0
                                       discovery targeted-hello accept
                                      !
                                      
                                    Related Concepts

                                    Configuring Label Advertisement (Outbound Filtering): Example

                                    The example shows how to configure LDP label advertisement control.

                                      
                                      mpls ldp 
                                          label
                                              advertise
                                                  disable
                                                  for pfx_acl_1 to peer_acl_1
                                                  for pfx_acl_2 to peer_acl_2
                                                  for pfx_acl_3
                                                  interface POS 0/1/0/0
                                                  interface POS 0/2/0/0
                                              !
                                           !
                                      !
                                      ipv4 access-list pfx_acl_1
                                          10 permit ip host 1.0.0.0 any
                                      !
                                      ipv4 access-list pfx_acl_2
                                          10 permit ip host 2.0.0.0 any
                                      !
                                      ipv4 access-list peer_acl_1
                                          10 permit ip host 1.1.1.1 any
                                          20 permit ip host 1.1.1.2 any
                                      !
                                      ipv4 access-list peer_acl_2
                                          10 permit ip host 2.2.2.2 any
                                      !
                                      
                                      show mpls ldp binding
                                      

                                    Configuring LDP Neighbors: Example

                                    The example shows how to disable label advertisement.

                                      mpls ldp
                                          router-id Loopback0
                                          neighbor 1.1.1.1 password encrypted 110A1016141E
                                          neighbor 2.2.2.2 implicit-withdraw
                                      !
                                      
                                    Related Tasks

                                    Configuring LDP Forwarding: Example

                                    The example shows how to configure LDP forwarding.

                                      mpls ldp
                                       explicit-null
                                      !
                                      
                                      show mpls ldp forwarding
                                      show mpls forwarding
                                      
                                    Related Concepts

                                    Configuring LDP Nonstop Forwarding with Graceful Restart: Example

                                    The example shows how to configure LDP nonstop forwarding with graceful restart.

                                      mpls ldp 
                                      log
                                      graceful-restart
                                      !
                                       graceful-restart
                                       graceful-restart forwarding state-holdtime 180
                                       graceful-restart reconnect-timeout 15
                                       interface pos0/1/0/0
                                      !
                                      
                                      show mpls ldp graceful-restart
                                      show mpls ldp neighbor gr
                                      show mpls ldp forwarding
                                      show mpls forwarding
                                      

                                    Configuring Label Acceptance (Inbound Filtering): Example

                                    The example shows how to configure inbound label filtering.

                                      mpls ldp
                                       label 
                                      accept
                                       for pfx_acl_2 from 192.168.2.2
                                      !
                                       !
                                      !
                                      

                                    Configuring Local Label Allocation Control: Example

                                    The example shows how to configure local label allocation control.

                                      mpls ldp
                                       label 
                                      allocate for pfx_acl_1
                                       ! 
                                      !
                                      

                                    Configuring LDP Session Protection: Example

                                    The example shows how to configure session protection.

                                      mpls ldp
                                       session protection duration 60 for peer_acl_1
                                    
                                      !
                                      
                                    Related Concepts

                                    Configuring LDP IGP Synchronization—OSPF: Example

                                    The example shows how to configure LDP IGP synchronization for OSPF.

                                      router ospf 100
                                      mpls ldp sync
                                      !
                                      mpls ldp
                                       igp sync delay 30
                                      !
                                      
                                    Related Concepts

                                    Configuring LDP IGP Synchronization—ISIS: Example

                                    The example shows how to configure LDP IGP synchronization.

                                      router isis 100
                                       interface POS 0/2/0/0
                                      address-family ipv4 unicast
                                      mpls ldp sync
                                      !
                                       !
                                      !
                                      mpls ldp
                                       igp sync delay 30
                                      !
                                      
                                    Related Concepts

                                    Configuring LDP Auto-Configuration: Example

                                    The example shows how to configure the IGP auto-configuration feature globally for a specific OSPF interface ID.

                                      router ospf 100
                                       mpls ldp auto-config
                                       area 0 
                                        interface pos 1/1/1/1
                                      
                                      

                                    The example shows how to configure the IGP auto-configuration feature on a given area for a given OSPF interface ID.

                                      router ospf 100
                                       area 0
                                        mpls ldp auto-config
                                        interface pos 1/1/1/1
                                      
                                    Related Concepts

                                    Additional References

                                    For additional information related to Implementing MPLS Label Distribution Protocol, refer to the following references:

                                    Related Documents

                                    Related Topic

                                    Document Title

                                    LDP commands on Cisco ASR 9000 Series Router

                                    MPLS Label Distribution Protocol Commands on Cisco ASR 9000 Series Router module in the Cisco ASR 9000 Series Aggregation Services Router MPLS Command Reference

                                    Getting started material

                                    Cisco ASR 9000 Series Aggregation Services Router  Getting Started Guide

                                    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

                                    To locate and download MIBs using Cisco IOS XR software, use the Cisco MIB Locator found at the following URL and choose a platform under the Cisco Access Products menu: http:/​/​cisco.com/​public/​sw-center/​netmgmt/​cmtk/​mibs.shtml

                                    RFCs

                                    RFCs

                                    Title

                                    RFC 3031

                                    Multiprotocol Label Switching Architecture

                                    RFC 3036

                                    LDP Specification

                                    RFC 3037

                                    LDP Applicability

                                    RFC 3478

                                    Graceful Restart Mechanism for Label Distribution Protocol

                                    RFC 3815

                                    Definitions of Managed Objects for MPLS LDP

                                    RFC 5036

                                    Label Distribution and Management

                                    Downstream on Demand Label Advertisement

                                    RFC 5286

                                    Basic Specification for IP Fast Reroute: Loop-Free Alternates

                                    Technical Assistance

                                    Description

                                    Link

                                    The Cisco Technical Support website contains thousands of pages of searchable technical content, including links to products, technologies, solutions, technical tips, and tools. Registered Cisco.com users can log in from this page to access even more content.

                                    http:/​/​www.cisco.com/​techsupport

                                    1 For L3VPN Inter-AS option C, LDP may also be required to assign local labels for some BGP prefixes.