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 Feature Information
Your software release may not support all the features documented in this module. For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the Feature Information Table at the end of this document.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
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
- 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 the figure below, 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:
- Router 1 notices an interruption in service with Router 2. (Router 3 also performs the same actions in this process.)
- 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.
- 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-tablecommand 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:
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
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.
DETAILED STEPS
 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
The figure below 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 |
Standards
| Standards |
Title |
|---|---|
| None |
-- |
MIBs
| MIBs1 |
MIBs Link |
|---|---|
|
To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL: |
RFCs
| RFCs2 |
Title |
|---|---|
| RFC 3036 |
LDP Specification |
| RFC 3478 |
Graceful Restart Mechanism for Label Distribution |
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. |
Feature Information for MPLS LDP Graceful Restart
The following table provides release information about the feature or features described in this module. This table 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.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
| Table 1 | Feature Information for MPLS LDP Graceful Restart |
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Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be actual addresses and phone numbers. Any examples, command display output, network topology diagrams, and other figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses or phone numbers in illustrative content is unintentional and coincidental.
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