Cisco Nexus 7000 Series NX-OS MPLS Configuration Guide
Configuring MPLS LDP Graceful Restart
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Table of Contents

Configuring MPLS LDP Graceful Restart

Information About MPLS LDP Graceful Restart

Introduction to MPLS LDP Graceful Restart

What Happens if a Router Does Not Have MPLS LDP Graceful Restart Enabled

How a Router Advertises that it Supports MPLS LDP Graceful Restart

Licensing Requirements for MPLS LDP Graceful Restart

Prerequisites for MPLS LDP Graceful Restart

Default Settings for MPLS LDP Graceful Restart

Configuring MPLS LDP Graceful Restart

Verifying the MPLS LDP Graceful Restart Configuration

Configuration Examples for MPLS LDP Graceful Restart

Additional References for MPLS LDP Graceful Restart

Related Documents

MIBs

Feature History for MPLS LDP Graceful Restart

Information About MPLS LDP Graceful Restart

MPLS LDP graceful restart is an LDP protocol feature that, in the presence of a temporary LDP control plane disruption, preserves Nonstop Forwarding (NSF) support of data plane traffic being carried on label-switched paths (LSPs).

An example of an LDP control plane disruption is when the LDP control plane on a router restarts. With MPLS LDP graceful restart, that router and all of its neighbors preserve their forwarding state so that traffic continues to be forwarded along the LDP LSPs. As the LDP control plane restarts, that router and its neighbors use graceful restart procedures to transition back to normal control plane operation. The result is that the disruption associated with a control plane restart is greatly reduced.

This section includes the following topics:

Introduction to MPLS LDP Graceful Restart

MPLS LDP graceful restart preserves data plane forwarding along LDP LSPs in the presence of temporary LDP control plane disruption. The disruption might be an LDP control plane restart caused by a supervisor switchover or process restart or a TCP or UDP event that disrupts control plane communication between two LDP control planes, even if neither restarts.

The following example describes how MPLS LDP graceful restart operates in the presence of an LDP control plane restart. Note that this interaction has two different roles: the router with the restarting LDP control plane and the neighbor router that detects a loss and recovery of its LDP session to the restarting router. Each carries out MPLS LDP graceful restart procedures appropriate to its role.

The topology shown in Figure 1-1 has the following elements:

  • All three routers are running MPLS LDP, all with graceful restart enabled.
  • An LDP session exists between R2 and R1, and another LDP session exists between R2 and R3.
  • LDP LSPs have been established, including LSPs that connect R1 and R3 and are carrying data traffic. In a network with more routers, multiple LSPs might traverse R1-R2-R3 in both directions.

Figure 1-1 Example of a Network Using LDP Graceful Restart

 

The following sequence shows how the three routers cooperate to provide NSF and avoid a disruption to data traffic:

1. At session establishment, each router reports to its neighbors that it supports graceful restart. Each session endpoint knows that both ends of the session support graceful restart.

2. R2 begins a supervisor switchover. R2’s LDP control plane and all of its LDP TCP connections and LDP UDP hello adjacencies stop operating. R2’s data plane marks all of its label entries as stale but continues to use those entries for forwarding MPLS data traffic.

3. R1 notices a loss of communication with R2. (In this sequence, R3 performs the same actions as R1.) R1 marks all of its label bindings from R2 as stale but continues to use those entries for forwarding data traffic.

4. R1 and R2 reestablish an LDP session. On R1, entering the show mpls ldp neighbor graceful-restart command displays information about the recovering session.

5. R2 reacquires its local label binding information. R2’s data plane typically provides R2’s control plane with the same local label for each prefix that was used prior to the restart.

6. Both routers readvertise their label binding information. If R1 relearns a label from R2, R1 marks the binding as no longer stale. If R2 learns a label from R1 and submits that label to its data plane, the data plane marks the entry as no longer stale.

7. After a certain amount of time has passed, R1’s LDP control plane cleans up all entries that are still marked as stale. Similarly, R2’s data plane removes all entries that are still marked as stale.

Typically, if no other network disruption occurs during this graceful restart operation, all bindings are relearned from the neighbor with the same label values as before the session restart. In this scenario, all saved bindings are marked as not stale during recovery, and no entries need to be cleaned up.

Another scenario of interest is a TCP or UDP communication failure without an LDP control plane restart. In this case, the two LDP control planes at either end of the session detect the communication failure. Each LDP control plane applies the same procedures as R1 used above, and NSF is achieved. R1 carries out the same MPLS LDP graceful restart procedures whether the communication failure with R2 was caused by a restart of R2’s control plane or by a networking issue without a restart of R2’s control plane.

You can set various timers to limit how long the routers wait for an LDP session to be reestablished before restarting the router.

What Happens if a Router Does Not Have MPLS LDP Graceful Restart Enabled

When a router that does not support MPLS LDP graceful restart undergoes a control plane restart, its data plane MPLS forwarding entries are freed.

A neighbor of such a router, detecting a loss of communication, frees all bindings from the restarting router. This behavior occurs whether the neighbor supports MPLS LDP graceful restart or not. A neighbor that supports MPLS LDP graceful restart learns at session establishment that the restarting router is not supporting MPLS LDP graceful restart. The neighbor does not run graceful restart procedures when detecting a loss of communication to the restarting router.

The cleanup actions of both the restarting router and its neighbor cause any data traffic on the old LSPs to be dropped until recovery activities have addressed the situation. Recovery activities include traffic reroute or the reestablishment of MPLS LDP LSPs or both.

How a Router Advertises that it Supports MPLS LDP Graceful Restart

A router that supports MPLS LDP graceful restart announces its capabilities to its neighbors in the fault tolerant (FT) type-length-value (TLV) in the LDP initialization message. The router sends the LDP initialization message to a neighbor as part of establishing an LDP session.

The FT session TLV includes the following information:

  • The Learn from Network (L) flag is set to 1, which indicates that the router is configured to perform MPLS LDP graceful restart.
  • The Reconnect Timeout field shows the time (in milliseconds) that the neighbor should wait for a reconnection if the LDP session is lost. If the timer is set to 0 and the local router fails, its peers do not wait for it to recover.
  • The Recovery Time field shows the time (in milliseconds) that the neighbor should retain the MPLS forwarding state during a recovery currently in progress. For example, if a neighbor restarted and did not preserve the MPLS forwarding state across the restart, the neighbor should set its recovery time to 0.

Note The reconnect time applies to the next communication loss while the recovery time applies to the recovery from the preceding communication loss.


Licensing Requirements for MPLS LDP Graceful Restart

 

Product
License Requirement

Cisco NX-OS

MPLS LDP graceful restart requires an MPLS license. For a complete explanation of the Cisco NX-OS licensing scheme and how to obtain and apply licenses, see the Cisco NX-OS Licensing Guide .

Prerequisites for MPLS LDP Graceful Restart

MPLS LDP graceful restart has the following prerequisites:

  • You must enable MPLS LDP on the device.
  • Ensure that MPLS LDP graceful restart has not been disabled on the device.
  • For Nonstop Forwarding (NSF) to occur, MPLS LDP graceful restart must be present on the restarting router and its peers.

Default Settings for MPLS LDP Graceful Restart

Table 1-1 lists the default settings for MPLS LDP graceful restart parameters.

 

Table 1-1 Default MPLS LDP Graceful Restart Parameters

Parameters
Default

MPLS LDP graceful restart

Enabled

Forwarding state holding time

600 seconds

Max recovery time

120 seconds

Neighbor liveness timer

120 seconds

Configuring MPLS LDP Graceful Restart

The MPLS LDP graceful restart feature is globally disabled or enabled.

You must not disable MPLS LDP graceful restart on the routers in order for forwarding to be preserved during an interruption in service.

When you disable or enable LDP graceful restart, it has no effect on existing LDP sessions. This configuration change applies to new sessions that are established after the change.

Prerequisites

Ensure that you are in the correct VDC (or use the switchto vdc command).

Ensure that MPLS LDP is enabled on the device.

SUMMARY STEPS

1. configure terminal

2. mpls ldp configuration

3. [ no ] graceful-restart [ timers { forwarding-holding seconds | max-recovery seconds | neighbor-liveness seconds } ]

4. (Optional) show mpls ldp graceful-restart

5. (Optional) show mpls ldp neighbor graceful restart

6. (Optional) copy running-config startup-config

DETAILED STEPS

 

Command
Purpose

Step 1

configure terminal

 

Example:

switch# configure terminal

switch(config)#

Enters global configuration mode.

Step 2

mpls ldp configuration

 

Example:

switch(config)# mpls ldp configuration
switch(config-ldp)#

Enters LDP configuration mode.

Step 3

[no] graceful-restart [ timers { forwarding-holding seconds | max-recovery seconds | neighbor-liveness seconds }]

 

Example:
switch(config-ldp)# graceful-restart timers max-recovery 100

Disables or enables the router to protect the LDP bindings and MPLS forwarding state during a disruption in service. MPLS LDP graceful restart is enabled by default.

(Optional) You can configure the following graceful-restart timers if the default values need to be changed:

  • forwarding-holding —Specifies the amount of time that the MPLS forwarding state should be preserved after the control plane restarts. The range is from 30 to 600 seconds.
  • max-recovery —Specifies the amount of time that a router should hold stale label-FEC bindings after an LDP session has been reestablished. The range is from 15 to 600 seconds.
  • neighbor-liveness —Specifies the amount of time that a router should wait for an LDP session to be reestablished. The range is from 5 to 300 seconds.

Step 4

show mpls ldp graceful-restart

 

Example:

switch(config-ldp)# show mpls ldp graceful-restart

(Optional) Displays this router’s LDP graceful-restart configuration.

Step 5

show mpls ldp neighbor graceful-restart

 

Example:

switch(config-ldp)# show mpls ldp neighbor graceful-restart

(Optional) Displays the graceful-restart parameters for this router’s sessions with its LDP neighbors.

Step 6

copy running-config startup-config

 

Example:

switch(config-ldp)# copy running-config startup-config

(Optional) Copies the running configuration to the startup configuration.

Verifying the MPLS LDP Graceful Restart Configuration

To display the MPLS LDP graceful restart configuration, perform one of the following tasks:

 

Command
Purpose

show mpls ldp graceful-restart

Displays the LDP graceful-restart configuration.

show mpls ldp neighbor graceful-restart

Displays the graceful-restart parameters for the router’s sessions with its LDP neighbors.

For detailed information about the fields in the output from these commands, see the Cisco Nexus 7000 Series NX-OS MPLS Command Reference .

Configuration Examples for MPLS LDP Graceful Restart

The following example shows how to enable MPLS LDP graceful restart, if it has been disabled, in order to preserve the LDP session during an interruption in service:

switch# configure terminal
switch(config)# mpls ldp configuration
switch(config-ldp)# graceful-restart
switch(config-ldp)# show mpls ldp graceful-restart neighbor-liveness 200
LDP Graceful Restart is enabled
Neighbor Liveness Timer: 200 seconds
Max Recovery Time: 120 seconds
Forwarding State Holding Time: 600 seconds
Down Neighbor Database (0 records):
Graceful Restart-enabled Sessions:

Additional References for MPLS LDP Graceful Restart

For additional information related to implementing MPLS LDP graceful restart, see the following sections:

Related Documents

Related Topic
Document Title

CLI commands

Cisco Nexus 7000 Series NX-OS MPLS Command Reference

MIBs

MIB
MIBs Link

MPLS-LDP-STD-MIB

To locate and download MIBs, go to the following URL:

http://www.cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml

Feature History for MPLS LDP Graceful Restart

Table 1-2 lists the release history for this feature.

 

Table 1-2 Feature History for MPLS LDP Graceful Restart

Feature Name
Releases
Feature Information

MPLS LDP graceful restart

5.2(1)

This feature was introduced.