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MPLS LDP Graceful Restart
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MPLS LDP Graceful Restart

Table Of Contents

MPLS LDP Graceful Restart

Contents

Restrictions

Information About MPLS LDP Graceful Restart

How MPLS LDP Graceful Restart Works

How a Route Processor Advertises That It Supports MPLS LDP Graceful Restart

What Happens If a Route Processor Does Not Have LDP Graceful Restart

How to Configure MPLS LDP Graceful Restart

Configuring MPLS LDP Graceful Restart

Verifying the Configuration

Configuration Example for MPLS LDP Graceful Restart

Additional References

Related Documents

Standards

MIBs

RFCs

Technical Assistance

Feature Information for MPLS LDP Graceful Restart


MPLS LDP Graceful Restart


When a router is configured with Multiprotocol Label Switching (MPLS) Label Distribution Protocol (LDP) Graceful Restart (GR), it assists a neighboring router that has MPLS LDP Stateful Switchover/Nonstop Forwarding (SSO/NSF) Support and Graceful Restart to recover gracefully from an interruption in service. In this Cisco IOS release, MPLS LDP GR functions strictly in helper mode, which means it can only help other routers that are enabled with MPLS SSO/NSF and GR to recover. If the router with LDP GR fails, its peer routers cannot help it recover.

Notes:

MPLS LDP SSO/NSF Support and Graceful Restart is supported in Cisco IOS Release 12.2(25)S. For brevity, this feature is called LDP SSO/NSF in this document.

The MPLS LDP GR feature described in this document refers to helper mode.

When you enable MPLS LDP GR on a router that peers with an MPLS LDP SSO/NSF-enabled router, the SSO/NSF-enabled router can maintain its forwarding state when the LDP session between them is interrupted. While the SSO/NSF-enabled router recovers, the peer router forwards packets using stale information. This enables the SSO/NSF-enabled router to become operational more quickly.

Finding Support Information for Platforms and Cisco IOS Software Images

Use Cisco Feature Navigator to find information about platform support and Cisco IOS software image support. Access Cisco Feature Navigator at http://www.cisco.com/go/fn. You must have an account on Cisco.com. If you do not have an account or have forgotten your username or password, click Cancel at the login dialog box and follow the instructions that appear.

Contents

Restrictions

Restrictions

How to Configure MPLS LDP Graceful Restart

Configuration Example for MPLS LDP Graceful Restart

Additional References

Restrictions

MPLS LDP GR is supported in strict helper mode.

Tag Distribution Protocol (TDP) sessions are not supported. Only LDP sessions are supported.

MPLS LDP GR cannot be configured on label-controlled ATM (LC-ATM) interfaces.

MPLS LDP SSO/NSF is supported in IOS Release 12.2(25)S. It is not supported in this release.

Information About MPLS LDP Graceful Restart

To configure MPLS LDP GR, you need to understand the following concepts:

How MPLS LDP Graceful Restart Works

How a Route Processor Advertises That It Supports MPLS LDP Graceful Restart

What Happens If a Route Processor Does Not Have LDP Graceful Restart

How MPLS LDP Graceful Restart Works

MPLS LDP GR works in strict helper mode, which means it helps a neighboring route processor that has MPLS LDP SSO/NSF to recover from disruption in service without losing its MPLS forwarding state. The disruption in service could be the result of a TCP or UDP event or the stateful switchover of a route processor. When the neighboring router establishes a new session, the LDP bindings and MPLS forwarding states are recovered.

In the topology shown in Figure 1, the following elements have been configured:

LDP sessions are established between Router 1 and Router 2, as well as between Router 2 and Router 3.

Router 2 has been configured with MPLS LDP SSO/NSF. Routers 1 and 3 have been configured with MPLS LDP GR.

A label switched path (LSP) has been established between Router 1 and Router 3.

Figure 1 Example of a Network Using LDP Graceful Restart

The following process shows how Routers 1 and 3, which have been configured with LDP GR help Router 2, which has been configured with LDP SSO/NSF recover from a disruption in service:

1. Router 1 notices an interruption in service with Router 2. (Router 3 also performs the same actions in this process.)

2. Router 1 marks all the label bindings from Router 2 as stale, but it continues to use the bindings for MPLS forwarding.

Router 1 reestablishes an LDP session with Router 2, but keeps its stale label bindings. If you issue a show mpls ldp neighbor command with the graceful-restart keyword, the command output displays the recovering LDP sessions.

3. Both routers readvertise their label binding information. If Router 1 relearns a label from Router 2 after the session has been established, the stale flags are removed. The show mpls forwarding-table command displays the information in the MPLS forwarding table, including the local label, outgoing label or VC, prefix, label-switched bytes, outgoing interface, and next hop.

You can set various graceful restart timers. See the following commands for more information:

mpls ldp graceful-restart timers neighbor-liveness

mpls ldp graceful-restart timers max-recovery

How a Route Processor Advertises That It Supports MPLS LDP Graceful Restart

A route processor that is configured to perform MPLS LDP GR includes the Fault Tolerant (FT) Type Length Value (TLV) in the LDP initialization message. The route processor sends the LDP initialization message to a neighbor to establish 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 route processor is configured to perform MPLS LDP GR.

The Reconnect Timeout field shows the time (in milliseconds) that the neighbor should wait for a reconnection if the LDP session is lost. In this release, the timer is set to 0, which indicates that if the local router fails, its peers should not wait for it to recover. The timer setting indicates that the local router is working in helper mode.

The Recovery Time field shows the time (in milliseconds) that the neighbor should retain the MPLS forwarding state during a recovery. If a neighbor did not preserve the MPLS forwarding state before the restart of the control plane, the neighbor sets the recovery time to 0.

What Happens If a Route Processor Does Not Have LDP Graceful Restart

If two route processors establish an LDP session and one route processor is not configured for MPLS LDP GR, the two route processors create a normal LDP session but do not have the ability to perform MPLS LDP GR. Both route processors must be configured for MPLS LDP GR.

How to Configure MPLS LDP Graceful Restart

This section contains the following procedures:

Configuring MPLS LDP Graceful Restart (required)

Verifying the Configuration (optional)

Configuring MPLS LDP Graceful Restart

You must enable MPLS LDP GR on all route processors for an LDP session to be preserved during an interruption in service.

MPLS LDP GR is enabled globally. When you enable MPLS LDP GR, it has no effect on existing LDP sessions. New LDP sessions that are established can perform MPLS LDP GR.

SUMMARY STEPS

1. enable

2. configure terminal

3. ip cef [distributed]

4. mpls ldp graceful-restart

5. interface type slot/port

6. mpls ip

7. mpls label protocol {ldp | tdp | both}

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

ip cef [distributed]

Example:

Router(config)# ip cef distributed

Enables Cisco Express Forwarding (CEF).

Step 4 

mpls ldp graceful-restart
Example:
Router(config)# mpls ldp graceful-restart

Enables the router to protect the LDP bindings and MPLS forwarding state during a disruption in service.

Step 5 

interface type slot/port
Example:
Router(config)# interface pos 3/0 

Specifies an interface and enters interface configuration mode.

Step 6 

mpls ip
Example:

Router(config-if)# mpls ip

Configures MPLS hop-by-hop forwarding for an interface.

Step 7 

mpls label protocol {ldp | tdp | both}
Example:

Router(config-if)# mpls label protocol ldp

Configures the use of LDP for an interface. You must use LDP.



Note You can also issue the mpls label protocol ldp command in global configuration mode, which enables LDP on all interfaces configured for MPLS.


Verifying the Configuration

The following commands help verify that MPLS LDP GR has been configured correctly:

show mpls ldp neighbor with the graceful-restart keyword

Displays the Graceful Restart information for LDP sessions.

show mpls ldp graceful-restart

Displays Graceful Restart sessions and session parameters.


Configuration Example for MPLS LDP Graceful Restart

Figure 2 shows a configuration where MPLS LDP GR is enabled on Router 1 and MPLS LDP SSO/NSF is enabled on Routers 2 and 3. In this configuration example, Router 1 creates an LDP session with Router 2. Router 1 also creates a targeted session with Router 3 through a traffic engineering tunnel using Router 2.


Note MPLS LDP SSO/NSF is supported in Cisco IOS Release 12.2(25)S. It is not supported in this release.


Figure 2 MPLS LDP Graceful Restart Configuration Example

Router 1 configured with LDP GR:

boot system slot0:rsp-pv-mz 
hw-module slot 2 image slot0:rsp-pv-mz 
hw-module slot 3 image slot0:rsp-pv-mz  
!
ip subnet-zero
ip cef
mpls label range 16 10000 static 10001 1048575
mpls label protocol ldp
mpls ldp logging neighbor-changes
mpls ldp graceful-restart
mpls traffic-eng tunnels
no mpls traffic-eng auto-bw timers frequency 0
mpls ldp router-id Loopback0 force
!
interface Loopback0
 ip address 20.20.20.20 255.255.255.255
 no ip directed-broadcast
 no ip mroute-cache
!
interface Tunnel1
 ip unnumbered Loopback0
 no ip directed-broadcast
 mpls label protocol ldp
 mpls ip
 tunnel destination 19.19.19.19
 tunnel mode mpls traffic-eng
 tunnel mpls traffic-eng autoroute announce
 tunnel mpls traffic-eng priority 7 7
 tunnel mpls traffic-eng bandwidth  500
 tunnel mpls traffic-eng path-option 1 dynamic
!
interface ATM5/1/0
 no ip address
 no ip directed-broadcast
 atm clock INTERNAL
 no atm enable-ilmi-trap
 no atm ilmi-keepalive
!
interface ATM5/1/0.5 point-to-point
 ip address 12.0.0.2 255.0.0.0
 no ip directed-broadcast
 no atm enable-ilmi-trap
 pvc 6/100 
  encapsulation aal5snap
mpls label protocol ldp
mpls traffic-eng tunnels
mpls ip
ip rsvp bandwidth 1000
!
router ospf 100
 log-adjacency-changes
 redistribute connected
     network 12.0.0.0 0.255.255.255 area 100
 network 20.20.20.20 0.0.0.0 area 100
 mpls traffic-eng router-id Loopback0
 mpls traffic-eng area 100

Router 2 configured with LDP SSO/NSF:

boot system slot0:rsp-pv-mz 
hw-module slot 2 image slot0:rsp-pv-mz 
hw-module slot 3 image slot0:rsp-pv-mz  
!
redundancy
  mode sso
!
ip cef
no ip domain-lookup
mpls label range 17 10000 static 10001 1048575
mpls label protocol ldp
mpls ldp logging neighbor-changes
mpls ldp graceful-restart
mpls traffic-eng tunnels
no mpls traffic-eng auto-bw timers frequency 0
no mpls advertise-labels
mpls ldp router-id Loopback0 force
!
interface Loopback0
 ip address 17.17.17.17 255.255.255.255
 no ip directed-broadcast
!
interface ATM4/0/0
 no ip address
 no ip directed-broadcast
 no ip mroute-cache
 atm clock INTERNAL
 atm sonet stm-1
 no atm enable-ilmi-trap
 no atm ilmi-keepalive
!
interface ATM4/0/0.5 point-to-point
 ip address 12.0.0.1 255.0.0.0
 no ip directed-broadcast
 no atm enable-ilmi-trap
 pvc 6/100 
  encapsulation aal5snap
mpls label protocol ldp
mpls traffic-eng tunnels
mpls ip
ip rsvp bandwidth 1000
!
interface POS5/1/0
 ip address 11.0.0.1 255.0.0.0
 no ip directed-broadcast
 encapsulation ppp
 mpls label protocol ldp
 mpls traffic-eng tunnels
 mpls ip
 no peer neighbor-route
 clock source internal
 ip rsvp bandwidth 1000
!
router ospf 100
 log-adjacency-changes
 redistribute connected
 nsf enforce global
 network 11.0.0.0 0.255.255.255 area 100
 network 12.0.0.0 0.255.255.255 area 100
 network 17.17.17.17 0.0.0.0 area 100
 mpls traffic-eng router-id Loopback0
 mpls traffic-eng area 100
!
ip classless

Router 3 configured with LDP SSO/NSF:

boot system slot0:rsp-pv-mz 
hw-module slot 2 image slot0:rsp-pv-mz 
hw-module slot 3 image slot0:rsp-pv-mz  
!
redundancy
  mode sso
!
ip subnet-zero
ip cef
!
no ip finger
no ip domain-lookup
mpls label protocol ldp
mpls ldp neighbor 11.11.11.11 targeted ldp
mpls ldp logging neighbor-changes
mpls ldp graceful-restart
mpls traffic-eng tunnels
no mpls traffic-eng auto-bw timers frequency 0
mpls ldp discovery directed-hello interval 12
mpls ldp discovery directed-hello holdtime 130
mpls ldp discovery directed-hello accept
mpls ldp router-id Loopback0 force
!
interface Loopback0
 ip address 19.19.19.19 255.255.255.255
 no ip directed-broadcast
!
interface POS1/0
 ip address 11.0.0.2 255.0.0.0
 no ip directed-broadcast
 encapsulation ppp
 mpls label protocol ldp
 mpls traffic-eng tunnels
 mpls ip
 no peer neighbor-route
 clock source internal
 ip rsvp bandwidth 1000
!
router ospf 100
 log-adjacency-changes
 redistribute connected
 nsf enforce global
network 11.0.0.0 0.255.255.255 area 100
network 19.19.19.19 0.0.0.0 area 100
mpls traffic-eng router-id Loopback0
 mpls traffic-eng area 100
!
ip classless

Additional References

Related Documents

Related Topic
Document Title

MPLS Label Distribution Protocol

MPLS Label Distribution Protocol (LDP)


Standards

Standards
Title

None


MIBs

MIBs 1
MIBs Link

MPLS Label Distribution Protocol MIB Version 8 Upgrade

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

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

1 Not all supported MIBs are listed.


RFCs

RFCs 1
Title

RFC 3036

LDP Specification

RFC 3478

Graceful Restart Mechanism for Label Distribution

1 Not all supported RFCs are listed.


Technical Assistance

Description
Link

The Cisco Technical Support & Documentation 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


Feature Information for MPLS LDP Graceful Restart

Table 1 lists the release history for this feature.

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


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


Table 1 Feature Information for MPLS LDP Graceful Restart

Feature Name
Releases
Feature Information

MPLS LDP Graceful Restart

12.0(29)S 12.3(14)T
12.2(33)SRA

MPLS LDP Graceful Restart assists a neighboring router that has MPLS LDP Stateful Switchover/Nonstop Forwarding (SSO/NSF) Support and Graceful Restart to recover gracefully from an interruption in service.

In Cisco IOS Release 12.0(29)S, this feature was introduced.

This feature was integrated into Cisco IOS Release 12.3(14)T.

This feature was integrated into Cisco IOS Release 12.2(33)SRA.

The following sections provide information about this feature:

Information About MPLS LDP Graceful Restart

How to Configure MPLS LDP Graceful Restart

The following commands were introduced or modified:

debug mpls ldp graceful-restart

mpls ldp graceful-restart

mpls ldp graceful-restart timers max-recovery

mpls ldp graceful-restart timers neighbor-liveness

show mpls ip binding

show mpls ldp bindings

show mpls ldp graceful-restart

show mpls ldp neighbor