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MPLS LDP Graceful Restart

Contents

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

Restrictions

Verifying the Configuration

Configuration Example for MPLS LDP Graceful Restart

Additional References

Related Documents

Standards

MIBs

RFCs

Technical Assistance

Command Reference


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.

Feature History for MPLS LDP Graceful Restart

Release
Modification

12.0(29)S

The MPLS LDP Graceful Restart feature (in helper mode) was introduced.

12.3(14)T

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

12.2(33)SRA

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


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

•Information About MPLS LDP Graceful Restart

•How to Configure MPLS LDP Graceful Restart

•Configuration Example for MPLS LDP Graceful Restart

•Additional References

•Command Reference

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.

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.

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

The following sections provide references related to MPLS LDP GR.

Related Documents

Related Topic
Document Title

MPLS Label Distribution Protocol

MPLS Label Distribution Protocol (LDP)


Standards

Standards
Title

None

—


MIBs

MIBs1
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

RFCs1
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


Command Reference

This section documents new and modified commands. All other commands used with this feature are documented in the Cisco IOScommand reference publications.

•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

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