How to Implement MPLS LDP on Cisco IOS XR Software 16. Cisco IOS XR MPLS
Configuration Guide for the Cisco CRS Router, Release 4.1. OL-24697-01 iii ...
Cisco IOS XR MPLS Configuration Guide for the Cisco CRS Router, Release 4.1
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Text Part Number: OL-24697-01
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CONTENTS
Preface
Preface xv Changes to This Document xv Obtaining Documentation and Submitting a Service Request xv
CHAPTER 1
Implementing MPLS Label Distribution Protocol 1 Prerequisites for Implementing Cisco MPLS LDP 2 Information About Implementing Cisco MPLS LDP 2 Overview of Label Distribution Protocol 3 Label Switched Paths 3 LDP Control Plane 3 Exchanging Label Bindings 4 LDP Forwarding 5 LDP Graceful Restart 6 Control Plane Failure 7 Phases in Graceful Restart 8 Recovery with Graceful-Restart 9 Label Advertisement Control (Outbound Filtering) 10 Label Acceptance Control (Inbound Filtering) 10 Local Label Allocation Control 11 Session Protection 11 IGP Synchronization 12 IGP Auto-configuration 13 IGP Synchronization Process Restart Delay 13 LDP Nonstop Routing 14 IP LDP Fast Reroute Loop Free Alternate 14 Downstream on Demand 16 How to Implement MPLS LDP 16
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Configuring LDP Discovery Parameters 16 Configuring LDP Discovery Over a Link 18 Configuring LDP Discovery for Active Targeted Hellos 20 Configuring LDP Discovery for Passive Targeted Hellos 22 Configuring Label Advertisement Control (Outbound Filtering) 23 Setting Up LDP Neighbors 25 Setting Up LDP Forwarding 27 Setting Up LDP NSF Using Graceful Restart 29 Configuring Label Acceptance Control (Inbound Filtering) 31 Configuring Local Label Allocation Control 33 Configuring Session Protection 34 Configuring LDP IGP Synchronization: OSPF 35 Configuring LDP IGP Synchronization: ISIS 36 Configuring LDP IGP Synchronization Delay Interval 38 Configuring LDP IGP Synchronization Process Restart Delay 39 Enabling LDP Auto-Configuration for a Specified OSPF Instance 41 Enabling LDP Auto-Configuration in an Area for a Specified OSPF Instance 42 Disabling LDP Auto-Configuration 44 Configuring LDP Nonstop Routing 45 Configuring LDP Downstream on Demand mode 47 Redistributing MPLS LDP Routes into BGP 48 Configuration Examples for Implementing MPLS LDP 49 Configuring LDP with Graceful Restart: Example 50 Configuring LDP Discovery: Example 50 Configuring LDP Link: Example 50 Configuring LDP Discovery for Targeted Hellos: Example 50 Configuring Label Advertisement (Outbound Filtering): Example 51 Configuring LDP Neighbors: Example 51 Configuring LDP Forwarding: Example 52 Configuring LDP Nonstop Forwarding with Graceful Restart: Example 52 Configuring Label Acceptance (Inbound Filtering): Example 52 Configuring Local Label Allocation Control: Example 53 Configuring LDP Session Protection: Example 53 Configuring LDP IGP Synchronization—OSPF: Example 53 Configuring LDP IGP Synchronization—ISIS: Example 54
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Configuring LDP Auto-Configuration: Example 54 Configure IP LDP Fast Reroute Loop Free Alternate: Examples 55 Verify IP LDP Fast Reroute Loop Free Alternate: Example 56 Additional References 58
CHAPTER 2
Implementing RSVP for MPLS-TE and MPLS O-UNI 61 Prerequisites for Implementing RSVP for MPLS-TE and MPLS O-UNI 62 Information About Implementing RSVP for MPLS-TE and MPLS O-UNI 62 Overview of RSVP for MPLS-TE and MPLS O-UNI 62 LSP Setup 63 High Availability 64 Graceful Restart 64 Graceful Restart: Standard and Interface-Based 64 Graceful Restart: Figure 65 ACL-based Prefix Filtering 66 RSVP MIB 67 Information About Implementing RSVP Authentication 67 RSVP Authentication Functions 67 RSVP Authentication Design 67 Global, Interface, and Neighbor Authentication Modes 68 Security Association 69 Key-source Key-chain 71 Guidelines for Window-Size and Out-of-Sequence Messages 71 Caveats for Out-of-Sequence 72 How to Implement RSVP 72 Configuring Traffic Engineering Tunnel Bandwidth 72 Confirming DiffServ-TE Bandwidth 72 Configuring MPLS O-UNI Bandwidth 74 Enabling Graceful Restart 74 Configuring ACL-based Prefix Filtering 75 Configuring ACLs for Prefix Filtering 75 Configuring RSVP Packet Dropping 77 Verifying RSVP Configuration 78 Enabling RSVP Traps 81 How to Implement RSVP Authentication 83
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Configuring Global Configuration Mode RSVP Authentication 83 Enabling RSVP Authentication Using the Keychain in Global Configuration Mode 83 Configuring a Lifetime for RSVP Authentication in Global Configuration Mode 84 Configuring the Window Size for RSVP Authentication in Global Configuration Mode 85 Configuring an Interface for RSVP Authentication 87 Specifying the RSVP Authentication Keychain in Interface Mode 87 Configuring a Lifetime for an Interface for RSVP Authentication 88 Configuring the Window Size for an Interface for RSVP Authentication 90 Configuring RSVP Neighbor Authentication 91 Specifying the Keychain for RSVP Neighbor Authentication 91 Configuring a Lifetime for RSVP Neighbor Authentication 93 Configuring the Window Size for RSVP Neighbor Authentication 94 Verifying the Details of the RSVP Authentication 95 Eliminating Security Associations for RSVP Authentication 95 Configuration Examples for RSVP 95 Bandwidth Configuration (Prestandard): Example 96 Bandwidth Configuration (MAM): Example 96 Bandwidth Configuration (RDM): Example 96 Refresh Reduction and Reliable Messaging Configuration: Examples 97 Refresh Interval and the Number of Refresh Messages Configuration: Example 97 Retransmit Time Used in Reliable Messaging Configuration: Example 97 Acknowledgement Times Configuration: Example 97 Summary Refresh Message Size Configuration: Example 97 Disable Refresh Reduction: Example 98 Configure Graceful Restart: Examples 98 Enable Graceful Restart: Example 98 Enable Interface-Based Graceful Restart: Example 98 Change the Restart-Time: Example 98 Change the Hello Interval: Example 99 Configure ACL-based Prefix Filtering: Example 99 Set DSCP for RSVP Packets: Example 99 Enable RSVP Traps: Example 99 Configuration Examples for RSVP Authentication 100 RSVP Authentication Global Configuration Mode: Example 100
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RSVP Authentication for an Interface: Example 101 RSVP Neighbor Authentication: Example 101 RSVP Authentication by Using All the Modes: Example 101 Additional References 102
CHAPTER 3
Implementing MPLS Forwarding 105 Prerequisites for Implementing Cisco MPLS Forwarding 105 Restrictions for Implementing Cisco MPLS Forwarding 106 Information About Implementing MPLS Forwarding 106 MPLS Forwarding Overview 106 Label Switching Functions 106 Distribution of Label Bindings 107 MFI Control-Plane Services 107 MFI Data-Plane Services 108 Time-to-Live Propagation in Hierarchical MPLS 108 MPLS Maximum Transmission Unit 108 How to Implement MPLS Forwarding 108 Configuring the Time-to-Live Propagation in Hierarchical MPLS 108 Configuring the Size of the Local Label 109 Additional References 111
CHAPTER 4
Implementing MPLS Traffic Engineering 113 Prerequisites for Implementing Cisco MPLS Traffic Engineering 114 Information About Implementing MPLS Traffic Engineering 115 Overview of MPLS Traffic Engineering 115 Benefits of MPLS Traffic Engineering 115 How MPLS-TE Works 115 MPLS Traffic Engineering 117 Backup AutoTunnels 117 Link Protection 117 Node Protection 118 Backup AutoTunnel Assignment 118 Explicit Paths 119 Periodic Backup Promotion 119 Protocol-Based CLI 120
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Differentiated Services Traffic Engineering 120 Prestandard DS-TE Mode 121 IETF DS-TE Mode 121 Bandwidth Constraint Models 121 Maximum Allocation Bandwidth Constraint Model 121 Russian Doll Bandwidth Constraint Model 122 TE Class Mapping 122 Flooding 123 Flooding Triggers 123 Flooding Thresholds 123 Fast Reroute 124 IS-IS IP Fast Reroute Loop-free Alternative 124 MPLS-TE and Fast Reroute over Link Bundles 125 Ignore Intermediate System-to-Intermediate System Overload Bit Setting in MPLS-TE 125 DWDM Transponder Integration 126 GMPLS Benefits 126 GMPLS Support 127 GMPLS Protection and Restoration 127 1:1 LSP Protection 128 Shared Mesh Restoration and M:N Path Protection 128 End-to-end Recovery 128 GMPLS Protection Requirements 128 GMPLS Prerequisites 128 Flexible Name-based Tunnel Constraints 128 MPLS Traffic Engineering Interarea Tunneling 129 Interarea Support 129 Multiarea Support 130 Loose Hop Expansion 131 Loose Hop Reoptimization 131 ABR Node Protection 131 Fast Reroute Node Protection 131 MPLS-TE Forwarding Adjacency 132 MPLS-TE Forwarding Adjacency Benefits 132 MPLS-TE Forwarding Adjacency Restrictions 132 MPLS-TE Forwarding Adjacency Prerequisites 132
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Unequal Load Balancing 133 Path Computation Element 133 Policy-Based Tunnel Selection 134 Policy-Based Tunnel Selection 135 Policy-Based Tunnel Selection Functions 135 PBTS Restrictions 136 Path Protection 136 Pre-requisites for Path Protection 137 Restrictions for Path Protection 137 MPLS-TE Automatic Bandwidth 138 MPLS-TE Automatic Bandwidth Overview 138 Adjustment Threshold 139 Overflow Detection 139 Restrictions for MPLS-TE Automatic Bandwidth 140 Point-to-Multipoint Traffic-Engineering 140 Point-to-Multipoint Traffic-Engineering Overview 140 Point-to-Multipoint RSVP-TE 142 Point-to-Multipoint Fast Reroute 143 Point-to-Multipoint Label Switch Path 143 Path Option for Point-to-Multipoint RSVP-TE 144 MPLS Traffic Engineering Shared Risk Link Groups 145 Explicit Path 146 Fast ReRoute with SRLG Constraints 146 Importance of Protection 148 Delivery of Packets During a Failure 149 Multiple Backup Tunnels Protecting the Same Interface 149 SRLG Limitations 149 Auto-Tunnel Mesh 150 Destination List (Prefix-List) 150 How to Implement Traffic Engineering 151 Building MPLS-TE Topology 151 Creating an MPLS-TE Tunnel 154 Configuring Forwarding over the MPLS-TE Tunnel 156 Protecting MPLS Tunnels with Fast Reroute 158 Enabling an AutoTunnel Backup 163
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Removing an AutoTunnel Backup 164 Establishing MPLS Backup AutoTunnels to Protect Fast Reroutable TE LSPs 165 Establishing Next-Hop Tunnels with Link Protection 167 Configuring a Prestandard DS-TE Tunnel 168 Configuring an IETF DS-TE Tunnel Using RDM 170 Configuring an IETF DS-TE Tunnel Using MAM 173 Configuring MPLS -TE and Fast-Reroute on OSPF 176 Configuring the Ignore Integrated IS-IS Overload Bit Setting in MPLS-TE 177 Configuring GMPLS 179 Configuring IPCC Control Channel Information 179 Configuring Router IDs 179 Configuring OSPF over IPCC 181 Configuring Local and Remote TE Links 183 Configuring Numbered and Unnumbered Links 183 Configuring Local Reservable Bandwidth 185 Configuring Local Switching Capability Descriptors 186 Configuring Persistent Interface Index 188 Enabling LMP Message Exchange 189 Disabling LMP Message Exchange 190 Configuring Remote TE Link Adjacency Information for Numbered Links 192 Configuring Remote TE Link Adjacency Information for Unnumbered Links 194 Configuring Numbered and Unnumbered Optical TE Tunnels 196 Configuring an Optical TE Tunnel Using Dynamic Path Option 196 Configuring an Optical TE Tunnel Using Explicit Path Option 199 Configuring LSP Hierarchy 201 Configuring Border Control Model 201 Configuring Path Protection 202 Configuring an LSP 202 Forcing Reversion of the LSP 205 Configuring Flexible Name-based Tunnel Constraints 206 Assigning Color Names to Numeric Values 206 Associating Affinity-Names with TE Links 207 Associating Affinity Constraints for TE Tunnels 209 Configuring IS-IS to Flood MPLS-TE Link Information 210 Configuring an OSPF Area of MPLS-TE 212
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Configuring Explicit Paths with ABRs Configured as Loose Addresses 213 Configuring MPLS-TE Forwarding Adjacency 215 Configuring Unequal Load Balancing 216 Setting Unequal Load Balancing Parameters 216 Enabling Unequal Load Balancing 217 Configuring a Path Computation Client and Element 219 Configuring a Path Computation Client 219 Configuring a Path Computation Element Address 220 Configuring PCE Parameters 221 Configuring Policy-based Tunnel Selection 224 Configuring the Automatic Bandwidth 226 Configuring the Collection Frequency 226 Forcing the Current Application Period to Expire Immediately 228 Configuring the Automatic Bandwidth Functions 229 Configuring the Shared Risk Link Groups 232 Configuring the SRLG Values of Each Link that has a Shared Risk with Another Link 232 Creating an Explicit Path With Exclude SRLG 234 Using Explicit Path With Exclude SRLG 235 Creating a Link Protection on Backup Tunnel with SRLG Constraint 238 Creating a Node Protection on Backup Tunnel with SRLG Constraint 240 Configuring Point-to-Multipoint TE 244 Enabling Multicast Routing on the Router 244 Configuring the Static Group for the Point-to-Multipoint Interface 246 Configuring Destinations for the Tunnel Interface 248 Disabling Destinations 251 Logging Per Destinations for Point-to-Multipoint 253 Configuring Auto-Tunnel Mesh Tunnel ID 256 Configuring Auto-tunnel Mesh Unused Timeout 257 Configuring Auto-Tunnel Mesh Group 259 Configuring Tunnel Attribute-Set Templates 260 Enabling LDP on Auto-Tunnel Mesh 263 Configuration Examples for Cisco MPLS-TE 264 Configure Fast Reroute and SONET APS: Example 264 Build MPLS-TE Topology and Tunnels: Example 264 Configure IETF DS-TE Tunnels: Example 266
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Configure MPLS-TE and Fast-Reroute on OSPF: Example 266 Configure the Ignore IS-IS Overload Bit Setting in MPLS-TE: Example 266 Configure GMPLS: Example 267 Configure Flexible Name-based Tunnel Constraints: Example 269 Configure an Interarea Tunnel: Example 271 Configure Forwarding Adjacency: Example 271 Configure Unequal Load Balancing: Example 271 Configure PCE: Example 272 Configure Policy-based Tunnel Selection: Example 273 Configure Tunnels for Path Protection: Example 274 Configure Automatic Bandwidth: Example 274 Configure the MPLS-TE Shared Risk Link Groups: Example 274 Configure Point-to-Multipoint TE: Examples 277 Configure Point-to-Multipoint for the Source: Example 277 Configure the Point-to-Multipoint Tunnel: Example 277 Disable a Destination: Example 278 Configure the Point-to-Multipoint Solution: Example 278 Additional References 282
CHAPTER 5
Implementing MPLS Optical User Network Interface Protocol 285 Prerequisites for Implementing GMPLS UNI 286 Information About Implementing MPLS O-UNI 286 How to Implement MPLS O-UNI on Cisco IOS XR Software 288 Setting Up an MPLS O-UNI Connection 288 Tearing Down an MPLS O-UNI Connection 292 Verifying MPLS O-UNI Configuration 294 Configuration Examples for GMPLS UNI 297 MPLS O-UNI Neighbor and Data Link Configuration: Examples 297 O-UNI Router ID Configuration: Example 297 O-UNI-N Neighbor Configuration: Example 297 O-UNI Data Link Configuration: Example 297 O-UNI Connection Establishment: Examples 297 O-UNI Connection Configuration at Active Side: Example 298 O-UNI Connection Configuration at Passive Side: Example 298 O-UNI Connection Tear-Down: Examples 298
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O-UNI Connection Deletion at Active Side: Example 298 O-UNI Connection Deletion at Passive Side: Example 298 Additional References 299
CHAPTER 6
Implementing MPLS OAM 301 Prerequisites for MPLS LSP Ping and Traceroute for P2MP 301 MPLS Network Management with MPLS LSP Ping and MPLS LSP Traceroute 302 Roles of Various Routers 302 P2MP Ping 303 P2MP Traceroute 304 Configure the Ping and Traceroute: Example 304
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Preface The Cisco IOS XR MPLS Configuration Guide for the Cisco CRS-1 Router preface contains these sections: • Changes to This Document, page xv • Obtaining Documentation and Submitting a Service Request, page xv
Changes to This Document Changes to This Document lists the technical changes made to this document since it was first printed. Table 1: Changes to This Document
Revision
Date
Change Summary
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April 2011
Initial release of this document.
Obtaining Documentation and Submitting a Service Request For information on obtaining documentation, using the Cisco Bug Search Tool (BST), submitting a service request, and gathering additional information, see What's New in Cisco Product Documentation, at: http:// www.cisco.com/c/en/us/td/docs/general/whatsnew/whatsnew.html. Subscribe to What's New in Cisco Product Documentation, which lists all new and revised Cisco technical documentation, as an RSS feed and deliver content directly to your desktop using a reader application. The RSS feeds are a free service.
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1
Implementing MPLS Label Distribution Protocol 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). Feature History for Implementing MPLS LDP Release
Modification
Release 2.0
This feature was introduced.
Release 3.2
Support was added for conceptual and configuration information about LDP label advertisement control (Outbound label filtering).
Release 3.3.0
Support was added for these features: • Inbound Label Filtering • Local Label Allocation Control • Session Protection • LDP-IGP Synchronization
Release 3.5.0
Support was added for LDP Auto-configuration.
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Implementing MPLS Label Distribution Protocol Prerequisites for Implementing Cisco MPLS LDP
Release
Modification
Release 3.6.0
Support was added for LDP nonstop routing (NSR).
Release 3.8.0
The feature LDP IGP Synchronization Process Restart Delay was introduced.
Release 4.0.1
Support was added for these features: • IP LDP Fast Reroute Loop Free Alternate • Downstream on Demand
• Prerequisites for Implementing Cisco MPLS LDP, page 2 • Information About Implementing Cisco MPLS LDP, page 2 • How to Implement MPLS LDP, page 16 • Configuration Examples for Implementing MPLS LDP, page 49 • Additional References, page 58
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. • We recommend to use a lower session holdtime bandwidth such as neighbors so that a session down occurs before an adjacency-down on a neighbor. Therefore, the following default values for the hello times are listed: • Holdtime is 15 seconds. • Interval is 5 seconds. For example, the LDP session holdtime can be configured as 30 seconds by using the holdtime command.
Information About Implementing Cisco MPLS LDP To implement MPLS LDP, you should understand these concepts:
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Implementing MPLS Label Distribution Protocol Overview of Label Distribution Protocol
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. This figure shows the control messages exchanged between LDP peers. Figure 1: LDP Control Protocol
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.
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Implementing MPLS Label Distribution Protocol Overview of Label Distribution Protocol
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 Topics Configuring LDP Discovery Parameters, on page 16 Configuring LDP Discovery Over a Link, on page 18 Configuring LDP Link: Example, on page 50 Configuring LDP Discovery for Active Targeted Hellos, on page 20 Configuring LDP Discovery for Passive Targeted Hellos, on page 22 Configuring LDP Discovery for Targeted Hellos: Example, on page 50
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. This figure illustrates the process of label binding exchange for setting up LSPs. Figure 2: Setting Up Label Switched Paths
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Implementing MPLS Label Distribution Protocol Overview of Label Distribution Protocol
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: 1 R4 allocates local label L4 for prefix 10.0.0.0 and advertises it to its neighbors (R3). 2 R3 allocates local label L3 for prefix 10.0.0.0 and advertises it to its neighbors (R1, R2, R4). 3 R1 allocates local label L1 for prefix 10.0.0.0 and advertises it to its neighbors (R2, R3). 4 R2 allocates local label L2 for prefix 10.0.0.0 and advertises it to its neighbors (R1, R3). 5 R1’s label information base (LIB) keeps local and remote labels bindings from its neighbors. 6 R2’s LIB keeps local and remote labels bindings from its neighbors. 7 R3’s LIB keeps local and remote labels bindings from its neighbors. 8 R4’s LIB keeps local and remote labels bindings from its neighbors. Related Topics Setting Up LDP Neighbors, on page 25 Configuring LDP Neighbors: Example, on page 51
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. Once label bindings are learned, the LDP control plane is ready to setup the MPLS forwarding plane as shown in this figure. Figure 3: Forwarding Setup
1 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).
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Implementing MPLS Label Distribution Protocol LDP Graceful Restart
2 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). 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). 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. 5 Incoming IP traffic on ingress LSR R1 gets label-imposed and is forwarded as an MPLS packet with label L3. 6 Incoming IP traffic on ingress LSR R2 gets label-imposed and is forwarded as an MPLS packet with label L3. 7 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. 8 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. 9 IP forwarding takes over and forwards the packet onward. Related Topics Setting Up LDP Forwarding, on page 27 Configuring LDP Forwarding: Example, on page 52
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.
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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
