Cisco Release 12.4 User Manual

MPLS LDP Configuration Guide, Cisco IOS Release 12.4

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C O N T E N T S

MPLS Label Distribution Protocol (LDP)

Finding Feature Information 1 Prerequisites for MPLS LDP 1 Information About MPLS LDP 1

Introduction to MPLS LDP 2 MPLS LDP Functional Overview 2 LDP and TDP Support 2 Introduction to LDP Sessions 3

Directly Connected MPLS LDP Sessions 3 Nondirectly Connected MPLS LDP Sessions 4

Introduction to LDP Label Bindings Label Spaces and LDP Identifiers 4

How to Configure MPLS LDP 5

Enabling Directly Connected LDP Sessions 6 Establishing Nondirectly Connected MPLS LDP Sessions 8 Saving Configurations MPLS Tag Switching Commands 11 Specifying the LDP Router ID 12 Preserving QoS Settings with MPLS LDP Explicit Null 14 Protecting Data Between LDP Peers with MD5 Authentication 18

MPLS LDP Configuration Examples 21

Configuring Directly Connected MPLS LDP Sessions Example 21

Establishing Nondirectly Connected MPLS LDP Sessions Example 23 Additional References 25 Feature Information for MPLS Label Distribution Protocol 26

MPLS LDP Session Protection 31

Finding Feature Information 31 Restrictions for MPLS LDP Session Protection 31 Information About MPLS LDP Session Protection 31

MPLS LDP Session Protection Customizations 32 How to Configure MPLS LDP Session Protection 33

MPLS LDP Configuration Guide, Cisco IOS Release 12.4

iii

Contents

Enabling MPLS LDP Session Protection 33

Verifying MPLS LDP Session Protection 35

Troubleshooting Tips 36 Configuration Examples for MPLS LDP Session Protection 36 Additional References 39 Command Reference 40

MPLS LDP Inbound Label Binding Filtering 41

Finding Feature Information 41 Restrictions 41 Information about MPLS LDP Inbound Label Binding Filtering 41 How to Configure MPLS LDP Inbound Label Binding Filtering 42

Configuring MPLS LDP Inbound Label Binding Filtering 42

Verifying that MPLS LDP Inbound Label Bindings are Filtered 44 Configuration Examples for MPLS LDP Inbound Label Binding Filtering 45 Additional References 46 Feature Information for MPLS LDP Inbound Label Binding Filtering Feature 47 Glossary 48

MPLS LDP Autoconfiguration 51

Finding Feature Information 51 Restrictions for MPLS LDP Autoconfiguration 51 Information About MPLS LDP Autoconfiguration 52

MPLS LDP Autoconfiguration on OSPF and IS-IS Interfaces 52 How to Configure MPLS LDP Autoconfiguration 52

Configuring MPLS LDP Autoconfiguration with OSPF Interfaces 52

Disabling MPLS LDP Autoconfiguration from Selected OSPF Interfaces 54

Verifying MPLS LDP Autoconfiguration with OSPF 55

Configuring MPLS LDP Autoconfiguration with IS-IS Interfaces 57

Disabling MPLS LDP Autoconfiguration from Selected IS-IS Interfaces 59

Verifying MPLS LDP Autoconfiguration with IS-IS 60

Troubleshooting Tips 61

Configuration Examples for MPLS LDP Autoconfiguration 61

MPLS LDP Autoconfiguration with OSPF Example 61

MPLS LDP Autoconfiguration with IS-IS Examples 62 Additional References 62 Feature Information for MPLS LDP Autoconfiguration 63

MPLS LDP Configuration Guide, Cisco IOS Release 12.4

Contents

MPLS LDP Graceful Restart 65

Finding Feature Information 65 Restrictions 65 Information About MPLS LDP Graceful Restart 66

How MPLS LDP Graceful Restart Works 66

How a Route Processor Advertises That It Supports MPLS LDP Graceful Restart 67

What Happens If a Route Processor Does Not Have LDP Graceful Restart 67 How to Configure MPLS LDP Graceful Restart 67

Configuring MPLS LDP Graceful Restart 67

Verifying the Configuration 69 Configuration Example for MPLS LDP Graceful Restart 69 Additional References 72 Feature Information for MPLS LDP Graceful Restart 73

MPLS LDP Configuration Guide, Cisco IOS Release 12.4

Contents

MPLS LDP Configuration Guide, Cisco IOS Release 12.4

MPLS Label Distribution Protocol (LDP)

Multiprotocol Label Switching (MPLS) Label Distribution Protocol (LDP) enables peer label switch routers (LSRs) in an MPLS network to exchange label binding information for supporting hop-by-hop forwarding in an MPLS network. This module explains the concepts related to MPLS LDP and describes how to configure MPLS LDP in a network.

• Finding Feature Information, page 1

• Prerequisites for MPLS LDP, page 1

• Information About MPLS LDP, page 1

• How to Configure MPLS LDP, page 5

• MPLS LDP Configuration Examples, page 21

• Additional References, page 25

• Feature Information for MPLS Label Distribution Protocol, page 26

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.

Prerequisites for MPLS LDP

Label switching on a router requires that Cisco Express Forwarding (CEF) be enabled on that router.

Information About MPLS LDP

• Introduction to MPLS LDP, page 2

• MPLS LDP Functional Overview, page 2

• LDP and TDP Support, page 2

• Introduction to LDP Sessions, page 3

• Introduction to LDP Label Bindings Label Spaces and LDP Identifiers, page 4

MPLS LDP Configuration Guide, Cisco IOS Release 12.4

Information About MPLS LDP

Introduction to MPLS LDP

MPLS LDP provides the means for LSRs to request, distribute, and release label prefix binding information to peer routers in a network. LDP enables LSRs to discover potential peers and to establish LDP sessions with those peers for the purpose of exchanging label binding information.

MPLS LDP enables one LSR to inform another LSR of the label bindings it has made. Once a pair of routers communicate the LDP parameters, they establish a label-switched path (LSP). MPLS LDP enables LSRs to distribute labels along normally routed paths to support MPLS forwarding. This method of label distribution is also called hop-by-hop forwarding. With IP forwarding, when a packet arrives at a router the router looks at the destination address in the IP header, performs a route lookup, and forwards the packet to the next hop. With MPLS forwarding, when a packet arrives at a router the router looks at the incoming label, looks up the label in a table, and then forwards the packet to the next hop. MPLS LDP is useful for applications that require hop-by-hop forwarding, such as MPLS VPNs.

MPLS LDP Functional Overview

Cisco MPLS LDP provides the building blocks for MPLS-enabled applications, such as MPS Virtual Private Networks (VPNs).

Introduction to MPLS LDP

LDP provides a standard methodology for hop-by-hop, or dynamic label, distribution in an MPLS network by assigning labels to routes that have been chosen by the underlying Interior Gateway Protocol (IGP) routing protocols. The resulting labeled paths, called label switch paths (LSPs), forward label traffic across an MPLS backbone to particular destinations. These capabilities enable service providers to implement MPLS-based IP VPNs and IP+ATM services across multivendor MPLS networks.

LDP and TDP Support

LDP supercedes Tag Distribution Protocol (TDP). See the table below for information about LDP and TDP support in Cisco IOS releases.

Use caution when upgrading the image on a router that uses TDP. Ensure that the TDP sessions are established when the new image is loaded. You can accomplish this by issuing the global configuration command mpls label protocol tdp. Issue this command and save it to the startup configuration before loading the new image. Alternatively, you can enter the command and save the running configuration immediately after loading the new image.

Table 1

Train and Release LDP/TDP Support

12.0S Train

LDP and TDP Support

• TDP is enabled by default.

• Cisco IOS Release 12.0(29)S and earlier releases: TDP is supported for LDP features.

• Cisco IOS Release 12.0(30)S and later releases: TDP is not support for LDP features.

MPLS LDP Configuration Guide, Cisco IOS Release 12.4

Introduction to LDP Sessions

Train and Release LDP/TDP Support

Directly Connected MPLS LDP Sessions

12.2S, SB, and SR Trains

12.T/Mainline Trains

Introduction to LDP Sessions

When you enable MPLS LDP, the LSRs send out messages to try to find other LSRs with which they can create LDP sessions. The following sections explain the differences between directly connected LDP sessions and nondirectly connected LDP sessions.

• Directly Connected MPLS LDP Sessions, page 3

• Nondirectly Connected MPLS LDP Sessions, page 4

• LDP is enabled by default.

• Cisco IOS Release 12.2(25)S and earlier releases: TDP is

supported for LDP features.

• Cisco IOS Releases 12.2(27)SBA, 12.2(27)SRA,

12.2(27)SRB and later releases: TDP is not supported for LDP features.

• Cisco IOS Release 12.3(14)T and earlier releases: TDP is

enabled by default.

• Cisco IOS Releases 12.4 and 12.4T and later releases:

LDP is enabled by default.

• Cisco IOS Release 12.3(11)T and earlier releases: TDP is

supported for LDP features.

• Cisco IOS Release 12.3(14)T and later releases: TDP is

not support ed for LDP features.

Directly Connected MPLS LDP Sessions

If an LSR is one hop from its neighbor, it is directly connected to its neighbor. The LSR sends out LDP link Hello messages as User Datagram Protocol (UDP) packets to all the routers on the subnet (multicast). A neighboring LSR may respond to the link Hello message, allowing the two routers to establish an LDP session. This is called basic discovery.

To initiate an LDP session between routers, the routers determine which router will take the active role and which router will take the passive role. The router that takes the active role establishes the LDP TCP connection session and initiates the negotiation of the LDP session parameters. To determine the roles, the two routers compare their transport addresses. The router with the higher IP address takes the active role and establishes the session.

After the LDP TCP connection session is established, the LSRs negotiate the session parameters, including the method of label distribution to be used. Two methods are available:

• Downstream Unsolicited: An LSR advertises label mappings to peers without being asked to.

• Downstream on Demand: An LSR advertises label mappings to a peer only when the peer asks for them.

For information about creating LDP sessions, see the Enabling Directly Connected LDP Sessions, page

MPLS LDP Configuration Guide, Cisco IOS Release 12.4

Nondirectly Connected MPLS LDP Sessions

If the LSR is more than one hop from its neighbor, it is nondirectly connected to its neighbor. For these nondirectly connected neighbors, the LSR sends out a targeted Hello message as a UDP packet, but as a unicast message specifically addressed to that LSR. The nondirectly connected LSR responds to the Hello message and the two routers begin to establish an LDP session. This is called extended discovery.

An MPLS LDP targeted session is a label distribution session between routers that are not directly connected. When you create an MPLS traffic engineering tunnel interface, you need to establish a label distribution session between the tunnel headend and the tailend routers. You establish nondirectly connected MPLS LDP sessions by enabling the transmission of targeted Hello messages.

You can use the mpls ldp neighbor targetedcommand to set up a targeted session when other means of establishing targeted sessions do not apply, such as configuring mpls ipon a traffic engineering (TE) tunnel or configuring Any Transport over MPLS (AToM) virtual circuits (VCs). For example, you can use this command to create a targeted session between directly connected MPLS label switch routers (LSRs) when MPLS label forwarding convergence time is an issue.

The mpls ldp neighbor targetedcommand can improve label convergence time for directly connected neighbor LSRs when the link(s) directly connecting them are down. When the links between the neighbor LSRs are up, both the link and targeted Hellos maintain the LDP session. If the links between the neighbor LSRs go down, the targeted Hellos maintain the session, allowing the LSRs to retain labels learned from each other. When a link directly connecting the LSRs comes back up, the LSRs can immediately reinstall labels for forwarding use without having to reestablish their LDP session and exchange labels.

The exchange of targeted Hello messages between two nondirectly connected neighbors can occur in several ways, including the following:

Introduction to LDP Label Bindings Label Spaces and LDP Identifiers

• Router 1 sends targeted Hello messages carrying a response request to Router 2. Router 2 sends targeted Hello messages in response if its configuration permits. In this situation, Router 1 is considered to be active and Router 2 is considered to be passive.

• Router 1 and Router 2 both send targeted Hello messages to each other. Both routers are considered to be active. Both, one, or neither router can also be passive, if they have been configured to respond to requests for targeted Hello messages from each other.

The default behavior of an LSR is to ignore requests from other LSRs that send targeted Hello messages. You can configure an LSR to respond to requests for targeted Hello messages by issuing the mpls ldp discovery targeted-hello accept command.

The active LSR mandates the protocol that is used for a targeted session. The passive LSR uses the protocol of the received targeted Hello messages.

For information about creating MPLS LDP targeted sessions, see the Establishing Nondirectly Connected

MPLS LDP Sessions, page 8.

Introduction to LDP Label Bindings Label Spaces and LDP Identifiers

An LDP label binding is an association between a destination prefix and a label. The label used in a label binding is allocated from a set of possible labels called a label space.

LDP supports two types of label spaces:

• Interface-specific--An interface-specific label space uses interface resources for labels. For example, label-controlled ATM (LC-ATM) interfaces use virtual path identifiers/virtual circuit identifiers (VPIs/ VCIs) for labels. Depending on its configuration, an LDP platform may support zero, one, or more interface-specific label spaces.

MPLS LDP Configuration Guide, Cisco IOS Release 12.4

MPLS Label Distribution Protocol (LDP)

• Platform-wide--An LDP platform supports a single platform-wide label space for use by interfaces that

can share the same labels. For Cisco platforms, all interface types, except LC-ATM, use the platformwide label space.

LDP uses a 6-byte quantity called an LDP Identifier (or LDP ID) to name label spaces. The LDP ID is made up of the following components:

• The first four bytes, called the LPD router ID, identify the LSR that owns the label space.

• The last two bytes, called the local label space ID, identify the label space within the LSR. For the

platform-wide label space, the last two bytes of the LDP ID are always both 0.

The LDP ID takes the following form: <LDP router ID> : <local label space ID> The following are examples of LPD IDs:

• 172.16.0.0:0

• 192.168.0.0:3

The router determines the LDP router ID as follows, if the mpls ldp router-id command is not executed,

How to Configure MPLS LDP

The router examines the IP addresses of all operational interfaces.

If these IP addresses include loopback interface addresses, the router selects the largest loopback

address as the LDP router ID. Otherwise, the router selects the largest IP address pertaining to an operational interface as the LDP

router ID.

The normal (default) method for determining the LDP router ID may result in a router ID that is not usable in certain situations. For example, the router might select an IP address as the LDP router ID that the routing protocol cannot advertise to a neighboring router. The mpls ldp router-id command allows you to specify the IP address of an interface as the LDP router ID. Make sure the specified interface is operational so that its IP address can be used as the LDP router ID.

When you issue the mpls ldp router-id command without the force keyword, the router select selects the IP address of the specified interface (provided that the interface is operational) the next time it is necessary to select an LDP router ID, which is typically the next time the interface is shut down or the address is configured.

When you issue the mpls ldp router-idcommand with the force keyword, the effect of the mpls ldp router-idcommand depends on the current state of the specified interface:

• If the interface is up (operational) and if its IP address is not currently the LDP router ID, the LDP

router ID changes to the IP address of the interface. This forced change in the LDP router ID tears down any existing LDP sessions, releases label bindings learned via the LDP sessions, and interrupts MPLS forwarding activity associated with the bindings.

•

If the interface is down (not operational) when the mpls ldp router-idinterface force command is issued, when the interface transitions to up, the LDP router ID changes to the IP address of the interface. This forced change in the LDP router ID tears down any existing LDP sessions, releases label bindings learned via the LDP sessions, and interrupts MPLS forwarding activity associated with the bindings.

How to Configure MPLS LDP

• Enabling Directly Connected LDP Sessions, page 6

MPLS LDP Configuration Guide, Cisco IOS Release 12.4

How to Configure MPLS LDP

• Establishing Nondirectly Connected MPLS LDP Sessions, page 8

• Saving Configurations MPLS Tag Switching Commands, page 11

• Specifying the LDP Router ID, page 12

• Preserving QoS Settings with MPLS LDP Explicit Null, page 14

• Protecting Data Between LDP Peers with MD5 Authentication, page 18

Enabling Directly Connected LDP Sessions

This procedure explains how to configure MPLS LDP sessions between two directly connected routers.

SUMMARY STEPS

enable

configure terminal

mpls ip

mpls label protocol {ldp | tdp | both}

Router(config)# interface type number

mpls ip

exit

show mpls interfaces [interface] [detail

10.

show mpls ldp discovery [all | vrf vpn-name] [detail

11.

show mpls ldp neighbor [[vrf vpn-name] [address | interface] [detail] | [all]]

Enabling Directly Connected LDP Sessions

DETAILED STEPS

Command or Action

Step 1

Step 2

Step 3

enable

Example:

Router> enable

configure terminal

Example:

Router# configure terminal

mpls ip

Example:

Router(config)# mpls ip

Purpose

Enables privileged EXEC mode.

• Enter your password if prompted.

Enters global configuration mode.

Configures MPLS hop-by-hop forwarding globally.

•

The mpls ip command is enabled by default; you do not have to specify this command.

• Globally enabling MPLS forwarding does not enable it on the router interfaces. You must enable MPLS forwarding on the interfaces as well as for the router.

MPLS LDP Configuration Guide, Cisco IOS Release 12.4

MPLS Label Distribution Protocol (LDP)

Command or Action Purpose

How to Configure MPLS LDP

Step 4

Step 5

Step 6

Step 7

mpls label protocol {ldp | tdp | both}

Example:

Router(config)# mpls label protocol ldp

Router(config)# interface type number

Example:

Router(config)# interface ethernet3/0

mpls ip

Example:

Router(config-if)# mpls ip

exit

Example:

Router(config-if)# exit

Configures the use of LDP on all interfaces. LDP is the default.

• If you set all interfaces globally to LDP, you can override

specific interfaces with either the tdp or both keyword by specifying the command in interface configuration mode.

Specifies the interface to be configured and enters interface configuration mode.

Configures MPLS hop-by-hop forwarding on the interface.

• You must enable MPLS forwarding on the interfaces as well

as for the router.

Exits interface configuration mode and enters global configuration mode.

Step 8

Step 9

Step 10

exit

Example:

Router(config)# exit

show mpls interfaces [interface] [detail

Example:

Router# show mpls interfaces

show mpls ldp discovery [all | vrf vpn-name] [detail

Example:

Router# show mpls ldp discovery

Exits global configuration mode and enters privileged EXEC mode.

Verifies that the interfaces have been configured to use LDP, TDP, or both.

Verifies that the interface is up and is sending Discovery Hello messages.

MPLS LDP Configuration Guide, Cisco IOS Release 12.4

How to Configure MPLS LDP

Command or Action Purpose

Establishing Nondirectly Connected MPLS LDP Sessions

Step 11

show mpls ldp neighbor [[vrf vpn-name] [address | interface] [detail] | [all]]

Example:

Router# show mpls ldp neighbor

Examples

The following show mpls interfaces command verifies that interfaces Ethernet 1/0 and 1/1 have been configured to use LDP:

Router# show mpls interfaces Interface IP Tunnel BGP Static Operational Ethernet3/0 Yes (ldp) No No No Yes Ethernet3/1 Yes No No No Yes

The following show mpls ldp discovery command verifies that the interface is up and is sending LDP Discovery Hello messages (as opposed to TDP Hello messages):

Router# show mpls ldp discovery Local LDP Identifier:

172.16.12.1:0 Discovery Sources: Interfaces: Ethernet3/0 (ldp): xmit

The following example shows that the LDP session between routers was successfully established:

Displays the status of LDP sessions.

Router# show mpls ldp neighbor Peer LDP Ident: 10.1.1.2:0; Local LDP Ident 10.1.1.1:0 TCP connection: 10.1.1.2.18 - 10.1.1.1.66 State: Oper; Msgs sent/rcvd: 12/11; Downstream Up time: 00:00:10 LDP discovery sources: FastEthernet1/0, Src IP addr: 10.20.10.2 Addresses bound to peer LDP Ident:

10.1.1.2 10.20.20.1 10.20.10.2

For examples on configuring directly connected LDP sessions, see the Configuring Directly Connected

MPLS LDP Sessions Example, page 21.

Establishing Nondirectly Connected MPLS LDP Sessions

This section explains how to configure nondirectly connected MPLS LDP sessions, which enable you to establish an LDP session between routers that are not directly connected.

• MPLS requires CEF.

• You must configure the routers at both ends of the tunnel to be active or enable one router to be

passive with the mpls ldp discovery targeted-hello accept command.

MPLS LDP Configuration Guide, Cisco IOS Release 12.4

MPLS Label Distribution Protocol (LDP)

SUMMARY STEPS

enable

configure terminal

mpls ip

mpls label protocol {ldp | tdp | both}

interface tunnelnumber

tunnel destination ip-address

mpls ip

exit

10.

show mpls ldp discovery [all | vrf vpn-name] [detail

DETAILED STEPS

Command or Action Purpose

How to Configure MPLS LDP

Step 1

Step 2

Step 3

Step 4

enable

Example:

Router> enable

configure terminal

Example:

Router# configure terminal

mpls ip

Example:

Router(config)# mpls ip

mpls label protocol {ldp | tdp | both}

Example:

Router(config)# mpls label protocol ldp

Enables privileged EXEC mode.

• Enter your password if prompted.

Enters global configuration mode.

Configures MPLS hop-by-hop forwarding globally.

•

The mpls ip command is enabled by default; you do not have to specify this command.

• Globally enabling MPLS forwarding does not enable it on the router interfaces. You must enable MPLS forwarding on the interfaces as well as for the router.

Configures the use of LDP on all interfaces. LDP is the default.

• If you set all interfaces globally to LDP, you can override specific interfaces with either the tdp or both keyword by specifying the command in interface configuration mode.

MPLS LDP Configuration Guide, Cisco IOS Release 12.4

How to Configure MPLS LDP

Command or Action Purpose

MPLS Label Distribution Protocol (LDP)

Step 5

Step 6

Step 7

Step 8

interface tunnelnumber

Example:

Router(config)# interface tunnel1

tunnel destination ip-address

Example:

Router(config-if)# tunnel destination

172.16.1.1

mpls ip

Example:

Router(config-if)# mpls ip

exit

Example:

Configures a tunnel interface and enters interface configuration mode.

Assigns an IP address to the tunnel interface.

Configures MPLS hop-by-hop forwarding on the interface.

• You must enable MPLS forwarding on the interfaces as well as for the router.

Exits interface configuration mode and enters global configuration mode.

Step 9

Step 10

Router(config-if)# exit

exit

Example:

Router(config)# exit

show mpls ldp discovery [all | vrf vpn-name] [detail

Example:

Router# show mpls ldp discovery

Example

The following example shows the output of the show mpls ldp discovery command for a nondirectly connected LDP session.

Router# show mpls ldp discovery Local LDP Identifier:

172.16.0.0:0 Discovery Sources:

Exits global configuration mode and enters privileged EXEC mode.

Verifies that the interface is up and is sending Discovery Hello messages.

MPLS LDP Configuration Guide, Cisco IOS Release 12.4

Saving Configurations MPLS Tag Switching Commands

Interfaces: POS2/0 (ldp): xmit/recv LDP Id: 172.31.255.255:0 Tunnel1 (ldp): Targeted -> 192.168.255.255 Targeted Hellos:

172.16.0.0 -> 192.168.255.255 (ldp): active, xmit/recv LDP Id: 192.168.255.255:0

172.16.0.0 -> 192.168.0.0 (tdp): passive, xmit/recv TDP Id: 192.168.0.0:0

This command output indicates that:

• The local LSR (172.16.0.0) sent LDP link Hello messages on interface POS2/0 and discovered neighbor 172.31.255.255.

• The local LSR sent LDP targeted Hello messages associated with interface Tunnel1 to target

192.168.255.255. The LSR was configured to use LDP.

• The local LSR is active for targeted discovery activity with 192.168.255.255; this means that the targeted Hello messages it sends to 192.168.255.255 carry a response request. The local LSR was configured to have an LDP session with the nondirectly connected LSR 192.168.255.255.

• The local LSR is not passive from the discovery activity with 192.168.255.255 for one of the following reasons:

◦ The targeted Hello messages it receives from 192.168.255.255 do not carry a response request. ◦ The local LSR has not been configured to respond to such requests.

• The local LSR sent TDP directed Hello messages to the target LSR 192.168.0.0. This LSR uses TDP because the Hello messages received from the target LSR 192.168.0.0 were TDP directed Hello messages.

• The local LSR is passive in discovery activity with LSR 192.168.0.0. This means that the directed Hello messages it receives from LSR 192.168.0.0 carry a response request and that the local LSR has been configured with the mpls ldp discovery targeted-hello accept command to respond to such requests from LSR 192.168.0.0.

• The local LSR is not active in discovery activity with LSR 192.168.0.0, because no application that requires an LDP session with LSR 192.168.0.0 has been configured on the local LSR.

How to Configure MPLS LDP

For examples of configuring LDP targeted sessions, see the Establishing Nondirectly Connected MPLS

LDP Sessions Example, page 23.

Saving Configurations MPLS Tag Switching Commands

In releases of Cisco IOS software prior to 12.4(2)T, some MPLS commands had both a tag-switching version and an MPLS version. For example, the two commands tag-switching ip and mpls ip were the same. To support backward compatibility, the tag-switching form of the command was written to the saved configuration.

Starting in Cisco IOS Release 12.4(2)T, the MPLS form of the command is written to the saved configuration.

For example, if an ATM interface is configured using the following commands, which have both a tagswitching form and an MPLS form:

Router(config)# interface ATM3/0 Router(config-if)# ip unnumbered Loopback0 router(config-if)# tag-switching ip Router(config-if)# mpls label protocol ldp

After you enter these commands and save this configuration or display the running configuration with the show runningcommand, the commands saved or displayed appear as follows:

interface ATM3/0

MPLS LDP Configuration Guide, Cisco IOS Release 12.4

How to Configure MPLS LDP

ip unnumbered Loopback0 mpls ip mpls label protocol ldp

Specifying the LDP Router ID

The mpls ldp router-id command allows you to establish the IP address of an interface as the LDP router ID.

The following steps describe the normal process for determining the LDP router ID:

The router considers all the IP addresses of all operational interfaces.

If these addresses include loopback interface addresses, the router selects the largest loopback address.

Configuring a loopback address helps ensure a stable LDP ID for the router, because the state of loopback addresses does not change. However, configuring a loopback interface and IP address on each router is not required.

The loopback IP address does not become the router ID of the local LDP ID under the following circumstances:

• ◦ If the loopback interface has been explicitly shut down.

◦

If the mpls ldp router-id command specifies that a different interface should be used as the LDP router ID.

Specifying the LDP Router ID

If you use a loopback interface, make sure that the IP address for the loopback interface is configured with a /32 network mask. In addition, make sure that the routing protocol in use is configured to advertise the corresponding /32 network.

Otherwise, the router selects the largest interface address.

The router might select a router ID that is not usable in certain situations. For example, the router might select an IP address that the routing protocol cannot advertise to a neighboring router.

The router implements the router ID the next time it is necessary to select an LDP router ID. The effect of the command is delayed until the next time it is necessary to select an LDP router ID, which is typically the next time the interface is shut down or the address is deconfigured.

If you use the force keyword with the mpls ldp router-id command, the router ID takes effect more quickly. However, implementing the router ID depends on the current state of the specified interface:

• If the interface is up (operational) and its IP address is not currently the LDP router ID, the LDP router ID is forcibly changed to the IP address of the interface. This forced change in the LDP router ID tears down any existing LDP sessions, releases label bindings learned via the LDP sessions, and interrupts MPLS forwarding activity associated with the bindings.

• If the interface is down, the LDP router ID is forcibly changed to the IP address of the interface when the interface transitions to up. This forced change in the LDP router ID tears down any existing LDP sessions, releases label bindings learned via the LDP sessions, and interrupts MPLS forwarding activity associated with the bindings.

Make sure the specified interface is operational before assigning it as the LDP router ID.

MPLS LDP Configuration Guide, Cisco IOS Release 12.4

MPLS Label Distribution Protocol (LDP)

SUMMARY STEPS

enable

configure terminal

mpls ip

mpls label protocol {ldp | tdp | both}

mpls ldp router-id interface [force]

exit

show mpls ldp discovery [all | detail |vrf vpn-name]

DETAILED STEPS

Command or Action Purpose

How to Configure MPLS LDP

Step 1

Step 2

Step 3

Step 4

enable

Example:

Router> enable

configure terminal

Example:

Router# configure terminal

mpls ip

Example:

Router(config)# mpls ip

mpls label protocol {ldp | tdp | both}

Example:

Router(config)# mpls label protocol ldp

Enables privileged EXEC mode.

• Enter your password if prompted.

Enters global configuration mode.

Configures MPLS hop-by-hop forwarding globally.

•

The mpls ip command is enabled by default; you do not have to specify this command.

• Globally enabling MPLS forwarding does not enable it on the router interfaces. You must enable MPLS forwarding on the interfaces as well as for the router.

Configures the use of LDP on all interfaces. LDP is the default.

• If you set all interfaces globally to LDP, you can override specific interfaces with either the tdp or both keyword by specifying the command in interface configuration mode.

Step 5

mpls ldp router-id interface [force]

Example:

Router(config)# mpls ldp

Specifies the preferred interface for determining the LDP router ID.

router-id pos2/0/0

MPLS LDP Configuration Guide, Cisco IOS Release 12.4

How to Configure MPLS LDP

Command or Action Purpose

Preserving QoS Settings with MPLS LDP Explicit Null

Step 6

Step 7

exit

Example:

Router(config)# exit

show mpls ldp discovery [all | detail |vrf vpnname]

Example:

Router# show mpls ldp discovery

Example

The following example assigns interface pos2/0/0 as the LDP router ID:

Router> enable Router# configure terminal Router(config)# mpls ip Router(config)# mpls label protocol ldp

Router(config)# mpls ldp router-id pos2/0/0 force

The following example displays the LDP router ID (10.15.15.15):

Exits global configuration mode and enters privileged EXEC mode.

Displays the LDP identifier for the local router.

Router# show mpls ldp discovery Local LDP Identifier:

10.15.15.15:0 Discovery Sources: Interfaces: Ethernet4 (ldp): xmit/recv LDP Id: 10.14.14.14:0

Preserving QoS Settings with MPLS LDP Explicit Null

Normally, LDP advertises an Implicit Null label for directly connected routes. The Implicit Null label causes the second last (penultimate) label switched router (LSR) to remove the MPLS header from the packet. In this case, the penultimate LSR and the last LSR do not have access to the quality of service (QoS) values that the packet carried before the MPLS header was removed. To preserve the QoS values, you can configure the LSR to advertise an explicit NULL label (a label value of zero). The LSR at the penultimate hop forwards MPLS packets with a NULL label instead of forwarding IP packets.

Note

An explicit NULL label is not needed when the penultimate hop receives MPLS packets with a label stack that contains at least two labels and penultimate hop popping is performed. In that case, the inner label can still carry the QoS value needed by the penultimate and edge LSR to implement their QoS policy.

When you issue the mpls ldp explicit-null command, Explicit Null is advertised in place of Implicit Null for directly connected prefixes.

MPLS LDP Configuration Guide, Cisco IOS Release 12.4

MPLS Label Distribution Protocol (LDP)

SUMMARY STEPS

enable

configure terminal

mpls ip

mpls label protocol {ldp | tdp | both}

interface type number

mpls ip

exit

mpls ldp explicit-null [for prefix-acl | to peer-acl | for prefix-acl to peer-acl]

exit

10.

show mpls forwarding-table [network {mask | length} | labels label [- label] | interface interface | next-hop address | lsp-tunnel[tunnel-id]] [vrf vpn-name] [detail]

DETAILED STEPS

How to Configure MPLS LDP

Step 1

Step 2

Step 3

Step 4

Command or Action Purpose

enable

Enables privileged EXEC mode.

• Enter your password if prompted.

Example:

Router> enable

configure terminal

Example:

Router# configure terminal

mpls ip

Example:

Enters global configuration mode.

Configures MPLS hop-by-hop forwarding globally.

•

The mpls ip command is enabled by default; you do not have to specify this command.

• Globally enabling MPLS forwarding does not enable it on

Router(config)# mpls ip

the router interfaces. You must enable MPLS forwarding on the interfaces as well as for the router.

mpls label protocol {ldp | tdp | both}

Configures the use of LDP on all interfaces. LDP is the default.

• If you set all interfaces globally to LDP, you can override

Example:

Router(config)# mpls label protocol ldp

specific interfaces with either the tdp or both keyword by specifying the command in interface configuration mode.

MPLS LDP Configuration Guide, Cisco IOS Release 12.4

How to Configure MPLS LDP

Command or Action Purpose

MPLS Label Distribution Protocol (LDP)

Step 5

Step 6

Step 7

Step 8

interface type number

Example:

Router(config)# interface atm2/0

mpls ip

Example:

Router(config-if)# mpls ip

exit

Example:

Router(config-if)# exit

mpls ldp explicit-null [for prefix-acl | to peer-acl | for prefix-acl to peer-acl]

Specifies the interface to be configured and enters interface configuration mode.

Configures MPLS hop-by-hop forwarding on the interface.

• You must enable MPLS forwarding on the interfaces as well as for the router.

Exits interface configuration mode and enters global configuration mode.

Advertises an Explicit Null label in situations where it would normally advertise an Implicit Null label.

Step 9

Step 10

Example:

Router(config)# mpls ldp explicit-null

exit

Example:

Router(config)# exit

show mpls forwarding-table [network {mask | length} | labels label [- label] | interface interface | next-hop address | lsp-tunnel[tunnel-id]] [vrf vpnname] [detail]

Example:

Router# show mpls forwarding-table

Exits global configuration mode and enter privileged EXEC mode.

Verifies that MPLS packets are forwarded with an explicit-null label (value of 0).

MPLS LDP Configuration Guide, Cisco IOS Release 12.4

MPLS Label Distribution Protocol (LDP)

Examples

Enabling explicit-null on an egress LSR causes that LSR to advertise the explicit-null label to all adjacent MPLS routers.

Router# configure terminal Router(config)# mpls ldp explicit-null

If you issue the show mpls forwarding-table command on an adjacent router, the output shows that MPLS packets are forwarded with an explicit-null label (value of 0). In the following example, the second column shows that entries have outgoing labels of 0, where once they were marked “Pop label”.

Router# show mpls forwarding-table Local Outgoing Prefix Bytes label Outgoing Next Hop label label or VC or Tunnel Id switched interface 19 Pop tag 10.12.12.12/32 0 Fa2/1/0 172.16.0.1 22 0 10.14.14.14/32 0 Fa2/0/0 192.168.0.2 23 0 172.24.24.24/32 0 Fa2/0/0 192.168.0.2 24 0 192.168.0.0/8 0 Fa2/0/0 192.168.0.2 25 0 10.15.15.15/32 0 Fa2/0/0 192.168.0.2 26 0 172.16.0.0/8 0 Fa2/0/0 192.168.0.2 27 25 10.16.16.16/32 0 Fa2/0/0 192.168.0.22 28 0 10.34.34.34/32 0 Fa2/0/0 192.168.0.2

Enabling explicit-null and specifying the forkeyword with a standard access control list (ACL) changes all adjacent MPLS routers' tables to swap an explicit-null label for only those entries specified in the accesslist. In the following example, an access-list is created that contains the 10.24.24.24/32 entry. Explicit null is configured and the access list is specified.

How to Configure MPLS LDP

Router# configure terminal Router(config)# mpls label protocol ldp Router(config)# access-list 24 permit host 10.24.24.24 Router(config)# mpls ldp explicit-null for 24

If you issue the show mpls forwarding-table command on an adjacent router, the output shows that the only the outgoing labels for the addresses specified (172.24.24.24/32) change from Pop label to 0. All other Pop label outgoing labels remain the same.

Enabling explicit null and adding the to keyword and an access list enables you to advertise explicit-null labels to only those adjacent routers specified in the access-list.To advertise explicit-null to a particular router, you must specify the router's LDP ID in the access-list.

In the following example, an access-list contains the 10.15.15.15/32 entry, which is the LDP ID of an adjacent MPLS router. The router that is configured with explicit null advertises explicit-null labels only to that adjacent router.

Router# show mpls ldp discovery Local LDP Identifier:

10.15.15.15:0 Discovery Sources: Interfaces: Ethernet4 (ldp): xmit/recv TDP Id: 10.14.14.14:0

MPLS LDP Configuration Guide, Cisco IOS Release 12.4

How to Configure MPLS LDP

Router# configure terminal Router(config)# mpls label protocol ldp Router(config)# access-list 15 permit host 10.15.15.15 Router(config)# mpls ldp explicit-null to 15

If you issue the show mpls forwarding-table command, the output shows that explicit null labels are going only to the router specified in the access list.

Enabling explicit-null with both the forand tokeywords enables you to specify which routes to advertise with explicit-null labels and to which adjacent routers to advertise these explicit-null labels.

Router# show access 15 Standard IP access list 15 permit 10.15.15.15 (7 matches) Router# show access 24 Standard IP access list 24 permit 10.24.24.24 (11 matches) Router# configure terminal Router(config)# mpls label protocol ldp Router(config)# mpls ldp explicit-null for 24 to 15

If you issue the show mpls forwarding-table command on the router called 47K-60-4, the output shows that it receives explicit null labels for 10.24.24.24/32.

Protecting Data Between LDP Peers with MD5 Authentication

Router# show mpls forwarding-table Local Outgoing Prefix Bytes label Outgoing Next Hop label label or VC or Tunnel Id switched interface 17 0 <--- 10.24.24.24/32 0 Et4 172.16.0.1 20 Pop tag 172.16.0.0/8 0 Et4 172.16.0.1 21 20 10.12.12.12/32 0 Et4 172.16.0.1 22 16 10.0.0.0/8 0 Et4 172.16.0.1 23 21 10.13.13.13/32 0 Et4 172.16.0.1 25 Pop tag 10.14.14.14/32 0 Et4 172.16.0.1 27 Pop tag 192.168.0.0/8 0 Et4 172.16.0.1 28 25 10.16.16.16/32 0 Et4 172.16.0.1 29 Pop tag 192.168.34.34/32 0 Et4 172.16.0.1

Protecting Data Between LDP Peers with MD5 Authentication

You can enable authentication between two LDP peers, which verifies each segment sent on the TCP connection between the peers. You must configure authentication on both LDP peers using the same password; otherwise, the peer session is not established.

Authentication uses the Message Digest 5 (MD5) algorithm to verify the integrity of the communication and authenticate the origin of the message.

To enable authentication, issue the mpls ldp neighborcommand with the passwordkeyword. This causes the router to generate an MD5 digest for every segment sent on the TCP connection and check the MD5 digest for every segment received from the TCP connection.

When you configure a password for an LDP neighbor, the router tears down existing LDP sessions and establishes new sessions with the neighbor.

MPLS LDP Configuration Guide, Cisco IOS Release 12.4

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