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Contents
- Implementing MPLS Label Distribution Protocol
- Prerequisites for Implementing Cisco MPLS LDP
- Information About Implementing Cisco MPLS LDP
- Overview of Label Distribution Protocol
- Label Switched Paths
- LDP Control Plane
- Exchanging Label Bindings
- LDP Forwarding
- LDP Graceful Restart
- Control Plane Failure
- Phases in Graceful Restart
- Recovery with Graceful-Restart
- Label Advertisement Control (Outbound Filtering)
- Label Acceptance Control (Inbound Filtering)
- Local Label Allocation Control
- Session Protection
- IGP Synchronization
- IGP Auto-configuration
- LDP Nonstop Routing
- How to Implement MPLS LDP
- Configuring LDP Discovery Parameters
- Configuring LDP Discovery Over a Link
- Configuring LDP Discovery for Active Targeted Hellos
- Configuring LDP Discovery for Passive Targeted Hellos
- Configuring Label Advertisement Control (Outbound Filtering)
- Setting Up LDP Neighbors
- Setting Up LDP Forwarding
- Setting Up LDP NSF Using Graceful Restart
- Configuring Label Acceptance Control (Inbound Filtering)
- Configuring Local Label Allocation Control
- Configuring Session Protection
- Configuring LDP IGP Synchronization: OSPF
- Configuring LDP IGP Synchronization: ISIS
- Enabling LDP Auto-Configuration for a Specified OSPF Instance
- Enabling LDP Auto-Configuration in an Area for a Specified OSPF Instance
- Disabling LDP Auto-Configuration
- Configuring LDP Nonstop Routing
- Configuration Examples for Implementing MPLS LDP
- Configuring LDP with Graceful Restart: Example
- Configuring LDP Discovery: Example
- Configuring LDP Link: Example
- Configuring LDP Discovery for Targeted Hellos: Example
- Configuring Label Advertisement (Outbound Filtering): Example
- Configuring LDP Neighbors: Example
- Configuring LDP Forwarding: Example
- Configuring LDP Nonstop Forwarding with Graceful Restart: Example
- Configuring Label Acceptance (Inbound Filtering): Example
- Configuring Local Label Allocation Control: Example
- Configuring LDP Session Protection: Example
- Configuring LDP IGP Synchronization—OSPF: Example
- Configuring LDP IGP Synchronization—ISIS: Example
- Configuring LDP Auto-Configuration: Example
- Additional References
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).
- Prerequisites for Implementing Cisco MPLS LDP
- Information About Implementing Cisco MPLS LDP
- How to Implement MPLS LDP
- Configuration Examples for Implementing MPLS LDP
- Additional References
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.
Information About Implementing Cisco MPLS LDP
To implement MPLS LDP, you should understand these concepts:
- Overview of Label Distribution Protocol
- LDP Graceful Restart
- Label Advertisement Control (Outbound Filtering)
- Label Acceptance Control (Inbound Filtering)
- Local Label Allocation Control
- Session Protection
- IGP Synchronization
- IGP Auto-configuration
- LDP Nonstop Routing
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.
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.
Related Tasks
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:
- R4 allocates local label L4 for prefix 10.0.0.0 and advertises it to its neighbors (R3).
- R3 allocates local label L3 for prefix 10.0.0.0 and advertises it to its neighbors (R1, R2, R4).
- R1 allocates local label L1 for prefix 10.0.0.0 and advertises it to its neighbors (R2, R3).
- R2 allocates local label L2 for prefix 10.0.0.0 and advertises it to its neighbors (R1, R3).
- R1’s label information base (LIB) keeps local and remote labels bindings from its neighbors.
- R2’s LIB keeps local and remote labels bindings from its neighbors.
- R3’s LIB keeps local and remote labels bindings from its neighbors.
- R4’s LIB keeps local and remote labels bindings from its neighbors.
Related Tasks
Related References
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.
- 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).
- 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).
- 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).
- 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.
- Incoming IP traffic on ingress LSR R1 gets label-imposed and is forwarded as an MPLS packet with label L3.
- Incoming IP traffic on ingress LSR R2 gets label-imposed and is forwarded as an MPLS packet with label L3.
- 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.
- 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.
- IP forwarding takes over and forwards the packet onward.
Related Tasks
Related References
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) * 3This 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.
Related Tasks
Related References
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.
- The R4 LSR control plane restarts.
- LIB is lost when the control plane restarts.
- The forwarding states installed by the R4 LDP control plane are immediately deleted.
- Any in-transit packets flowing from R3 to R4 (still labeled with L4) arrive at R4.
- 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.
- The R3 LDP peer detects the failure of the control plane channel and deletes its label bindings from R4.
- The R3 control plane stops using outgoing labels from R4 and deletes the corresponding forwarding state (rewrites), which in turn causes forwarding disruption.
- The established LSPs connected to R4 are terminated at R3, resulting in broken end-to-end LSPs from R1 to R4.
- 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
- 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.
Related Tasks
Related References
Recovery with Graceful-Restart
Figure 5. Recovering with Graceful Restart. This figure illustrates the process of failure recovery using graceful restart.
- The router R4 LSR control plane restarts.
- 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.
- Any in-transit packets from R3 to R4 (still labeled with L4) arrive at R4.
- 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.
- 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.
- At this point there are no forwarding disruptions.
- The peer also starts the neighbor reconnect timer using the reconnect time value.
- 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.
Related Tasks
Related References
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.
Related References
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.
Related Tasks
Related References
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.
Related Tasks
Related References
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.
Note
When LDP session protection is activated (upon link failure), protection is maintained for an unlimited period time.
Related Tasks
Related References
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.
Related Tasks
Related References
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 .
Related Tasks
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
- Configuring LDP Discovery Over a Link
- Configuring LDP Discovery for Active Targeted Hellos
- Configuring LDP Discovery for Passive Targeted Hellos
- Configuring Label Advertisement Control (Outbound Filtering)
- Setting Up LDP Neighbors
- Setting Up LDP Forwarding
- Setting Up LDP NSF Using Graceful Restart
- Configuring Label Acceptance Control (Inbound Filtering)
- Configuring Local Label Allocation Control
- Configuring Session Protection
- Configuring LDP IGP Synchronization: OSPF
- Configuring LDP IGP Synchronization: ISIS
- Enabling LDP Auto-Configuration for a Specified OSPF Instance
- Enabling LDP Auto-Configuration in an Area for a Specified OSPF Instance
- Disabling LDP Auto-Configuration
- Configuring LDP Nonstop Routing
Configuring LDP Discovery Parameters
SUMMARY STEPSPerform 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.
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:
7. (Optional) show mpls ldp parameters
DETAILED STEPSRelated 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 BeginSUMMARY STEPSA 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.
1. configure
2. mpls ldp
3. router-id ip-address
4. interface type interface-path-id
5. Use one of the following commands:
6. (Optional) show mpls ldp discovery
DETAILED STEPSRelated 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 BeginSUMMARY STEPSThese 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.
1. configure
2. mpls ldp
3. router-id ip-address
4. interface type interface-path-id
5. Use one of the following commands:
6. (Optional) show mpls ldp discovery
DETAILED STEPSRelated Concepts
Related References
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 BeginSUMMARY STEPSStable 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.
1. configure
2. mpls ldp
3. router-id ip-address
4. discovery targeted-hello accept
5. Use one of the following commands:
6. (Optional) show mpls ldp discovery
DETAILED STEPSRelated Concepts
Related References
Configuring Label Advertisement Control (Outbound Filtering)
SUMMARY STEPSPerform 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.
1. configure
2. mpls ldp
3. label advertise { disable | for prefix-acl [ to peer-acl ] | interface type interface-path-id }
DETAILED STEPSRelated Concepts
Related References
Setting Up LDP Neighbors
Before You BeginSUMMARY STEPSStable 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.
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:
10. (Optional) show mpls ldp neighbor
DETAILED STEPSRelated References
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 BeginSUMMARY STEPSStable 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.
1. configure
2. mpls ldp
3. explicit-null
4. Use one of the following commands:
5. (Optional) show mpls ldp forwarding
6. (Optional) show mpls forwarding
7. (Optional) ping ip-address
DETAILED STEPSRelated Concepts
Related References
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 BeginSUMMARY STEPSStable 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.
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:
9. (Optional) show mpls ldp parameters
10. (Optional) show mpls ldp neighbor
11. (Optional) show mpls ldp graceful-restart
DETAILED STEPSRelated References
Configuring Label Acceptance Control (Inbound Filtering)
SUMMARY STEPSPerform 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.
1. configure
2. mpls ldp
3. label accept for prefix-acl from ip-address
DETAILED STEPSRelated Concepts
Related References
Configuring Local Label Allocation Control
SUMMARY STEPSPerform 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.
DETAILED STEPSRelated Concepts
Related References
Configuring Session Protection
SUMMARY STEPSPerform this task to configure LDP session protection.
By default, there is no protection is done for link sessions by means of targeted hellos.
1. configure
2. mpls ldp
3. session protection [ for peer-acl ] [ duration seconds ]
DETAILED STEPSRelated Concepts
Related References
Configuring LDP IGP Synchronization: OSPF
SUMMARY STEPSPerform this task to configure LDP IGP Synchronization under OSPF.
Note
By default, there is no synchronization between LDP and IGPs.
DETAILED STEPSRelated Concepts
Related References
Configuring LDP IGP Synchronization: ISIS
SUMMARY STEPSPerform this task to configure LDP IGP Synchronization under ISIS.
Note
By default, there is no synchronization between LDP and ISIS.
1. configure
2. router isis instance-id
3. interface type interface-path-id
4. address-family ipv4 unicast
5. mpls ldp sync
DETAILED STEPSRelated Concepts
Related References
Enabling LDP Auto-Configuration for a Specified OSPF Instance
SUMMARY STEPSPerform 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.
1. configure
2. router ospf process-name
3. mpls ldp auto-config
4. area area-id
5. interface type interface-path-id
DETAILED STEPSRelated Concepts
Related Tasks
Related References
Enabling LDP Auto-Configuration in an Area for a Specified OSPF Instance
SUMMARY STEPSPerform 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.
1. configure
2. router ospf process-name
3. area area-id
4. mpls ldp auto-config
5. interface type interface-path-id
DETAILED STEPS
Command or Action Purpose
Step 1 configure
Example:RP/0/RSP0/CPU0:router# configureEnters 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-configEnables 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:
Example:RP/0/RSP0/CPU0:router(config-ospf-ar-if)# endor
RP/0/RSP0/CPU0:router(config-ospf-ar-if)# commitSaves 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
Related Tasks
Related References
Disabling LDP Auto-Configuration
SUMMARY STEPSPerform 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.
1. configure
2. mpls ldp
3. interface type interface-path-id
4. igp auto-config disable
DETAILED STEPS
Command or Action Purpose
Step 1 configure
Example:RP/0/RSP0/CPU0:router# configureEnters 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 0/6/0/0Enters 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 disableDisables auto-configuration on the specified interface.
Step 5 Use one of the following commands:
Example:RP/0/RSP0/CPU0:router(config-ldp-if)# endor
RP/0/RSP0/CPU0:router(config-ldp-if)# commitSaves 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
Related References
Configuring LDP Nonstop Routing
SUMMARY STEPSPerform this task to configure LDP NSR.
Note
By default, NSR is globally-enabled on all LDP sessions except AToM.
1. configure
2. mpls ldp
3. nsr
4. Use one of the following commands:
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# configureEnters global configuration mode.
Step 2 mpls ldp
Example:RP/0/RSP0/CPU0:router(config)# mpls ldpEnters the MPLS LDP configuration mode.
Step 3 nsr
Example:RP/0/RSP0/CPU0:router(config-ldp)# nsrEnables LDP nonstop routing.
Step 4 Use one of the following commands:
Example:RP/0/RSP0/CPU0:router(config-ldp)# endor
RP/0/RSP0/CPU0:router(config-ldp)# commitSaves 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 statisticsDisplays MPLS LDP NSR statistics.
Step 6 show mpls ldp nsr summary
Example:RP/0/RSP0/CPU0:router# show mpls ldp nsr summaryDisplays MPLS LDP NSR summarized information.
Step 7 show mpls ldp nsr pending
Example:RP/0/RSP0/CPU0:router# show mpls ldp nsr pendingDisplays MPLS LDP NSR pending information.
Related Concepts
Configuration Examples for Implementing MPLS LDP
- Configuring LDP with Graceful Restart: Example
- Configuring LDP Discovery: Example
- Configuring LDP Link: Example
- Configuring LDP Discovery for Targeted Hellos: Example
- Configuring Label Advertisement (Outbound Filtering): Example
- Configuring LDP Neighbors: Example
- Configuring LDP Forwarding: Example
- Configuring LDP Nonstop Forwarding with Graceful Restart: Example
- Configuring Label Acceptance (Inbound Filtering): Example
- Configuring Local Label Allocation Control: Example
- Configuring LDP Session Protection: Example
- Configuring LDP IGP Synchronization—OSPF: Example
- Configuring LDP IGP Synchronization—ISIS: Example
- Configuring LDP Auto-Configuration: Example
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 bindingRelated 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 forwardingRelated Tasks
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 1/1/1/1The 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 1/1/1/1Related 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
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
1 For L3VPN Inter-AS option C, LDP may also be required to assign local labels for some BGP prefixes.
