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LDP Overview

LDP Introduction

The Label Distribution Protocol (LDP) is a protocol for distributing labels in non-traffic-engineered applications. LDP allows routers to establish label-switched paths (LSPs) through a network by mapping network-layer routing information directly to data link layer-switched paths.

These LSPs might have an endpoint at a directly attached neighbor (comparable to IP hop-by-hop forwarding), or at a network egress node, enabling switching through all intermediary nodes. LSPs established by LDP can also traverse traffic-engineered LSPs created by RSVP.

LDP associates a forwarding equivalence class (FEC) with each LSP it creates. The FEC associated with an LSP specifies which packets are mapped to that LSP. LSPs are extended through a network as each router chooses the label advertised by the next hop for the FEC and splices it to the label it advertises to all other routers. This process forms a tree of LSPs that converge on the egress router.

Understanding the LDP Signaling Protocol

LDP is a signaling protocol that runs on a device configured for MPLS support. The successful configuration of both MPLS and LDP initiates the exchange of TCP packets across the LDP interfaces. The packets establish TCP-based LDP sessions for the exchange of MPLS information within the network. Enabling both MPLS and LDP on the appropriate interfaces is sufficient to establish LSPs.

LDP is a simple, fast-acting signaling protocol that automatically establishes LSP adjacencies within an MPLS network. Routers then share LSP updates such as hello packets and LSP advertisements across the adjacencies. Because LDP runs on top of an IGP such as IS-IS or OSPF, you must configure LDP and the IGP on the same set of interfaces. After both are configured, LDP begins transmitting and receiving LDP messages through all LDP-enabled interfaces. Because of LDP's simplicity, it cannot perform the true traffic engineering which RSVP can perform. LDP does not support bandwidth reservation or traffic constraints.

When you configure LDP on a label-switching router (LSR), the router begins sending LDP discovery messages out all LDP-enabled interfaces. When an adjacent LSR receives LDP discovery messages, it establishes an underlying TCP session. An LDP session is then created on top of the TCP session. The TCP three-way handshake ensures that the LDP session has bidirectional connectivity. After they establish the LDP session, the LDP neighbors maintain, and terminate, the session by exchanging messages. LDP advertisement messages allow LSRs to exchange label information to determine the next hops within a particular LSP. Any topology changes, such as a router failure, generate LDP notifications that can terminate the LDP session or generate additional LDP advertisements to propagate an LSP change.

Starting in Junos OS Release 20.3R1, support for MPLS to provide LDP signaling protocol configuration with the control plane functionality.

Example: Configuring LDP-Signaled LSPs

This example shows how to create and configure LDP instances within an MPLS network.

Requirements

Before you begin:

  • Configure network interfaces. See Interfaces User Guide for Security Devices.

  • Configure an IGP across your network. (The LDP configuration is added to the existing IGP configuration and included in the MPLS configuration.)

  • Configure a network to use LDP for LSP establishment by enabling MPLS on all transit interfaces in the MPLS network.

    Note:

    Because LDP runs on top of an IGP such as IS-IS or OSPF, you must configure LDP and the IGP on the same set of interfaces.

Overview

To configure LDP-signaled LSPs, you must enable the MPLS family on all transit interfaces in the MPLS network and include all the transit interfaces under the [protocols mpls] and [protocols ldp] hierarchy levels.

In this example, you enable the MPLS family and create an LDP instance on all the transit interfaces. Additionally, you enable the MPLS process on all the transit interfaces in the MPLS network. In this example, you configure a sample network as shown in Figure 1.

Figure 1: Typical LDP-Signaled LSPTypical LDP-Signaled LSP

Configuration

Procedure

CLI Quick Configuration

To quickly configure this example, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, copy and paste the commands into the CLI at the [edit] hierarchy level and then enter commit from configuration mode.

For Router R1, perform the following:

For Router R2, perform the following:

For Router R3, perform the following:

Step-by-Step Procedure

To enable LDP instances within an MPLS network:

  1. Enable the MPLS family on the transit interface on Router R1.

  2. Enable the MPLS process on the transit interface.

  3. Create the LDP instance on the transit interface.

Results

Confirm your configuration by entering the show command from configuration mode. If the output does not display the intended configuration, repeat the configuration instructions in this example to correct it.

For brevity, this show output includes only the configuration that is relevant to this example. Any other configuration on the system has been replaced with ellipses (...).

If you are done configuring the device, enter the commit command from the configuration mode to activate the configuration.

Results

Junos OS LDP Protocol Implementation

The Junos OS implementation of LDP supports LDP version 1. The Junos OS supports a simple mechanism for tunneling between routers in an interior gateway protocol (IGP), to eliminate the required distribution of external routes within the core. The Junos OS allows an MPLS tunnel next hop to all egress routers in the network, with only an IGP running in the core to distribute routes to egress routers. Edge routers run BGP but do not distribute external routes to the core. Instead, the recursive route lookup at the edge resolves to an LSP switched to the egress router. No external routes are necessary on the transit LDP routers.

LDP Operation

You must configure LDP for each interface on which you want LDP to run. LDP creates LSP trees rooted at each egress router for the router ID address that is the subsequent BGP next hop. The ingress point is at every router running LDP. This process provides an inet.3 route to every egress router. If BGP is running, it will attempt to resolve next hops by using the inet.3 table first, which binds most, if not all, of the BGP routes to MPLS tunnel next hops.

Two adjacent routers running LDP become neighbors. If the two routers are connected by more than one interface, they become neighbors on each interface. When LDP routers become neighbors, they establish an LDP session to exchange label information. If per-router labels are in use on both routers, only one LDP session is established between them, even if they are neighbors on multiple interfaces. For this reason, an LDP session is not related to a particular interface.

LDP operates in conjunction with a unicast routing protocol. LDP installs LSPs only when both LDP and the routing protocol are enabled. For this reason, you must enable both LDP and the routing protocol on the same set of interfaces. If this is not done, LSPs might not be established between each egress router and all ingress routers, which might result in loss of BGP-routed traffic.

You can apply policy filters to labels received from and distributed to other routers through LDP. Policy filters provide you with a mechanism to control the establishment of LSPs.

For LDP to run on an interface, MPLS must be enabled on a logical interface on that interface. For more information, see the Logical Interfaces.

LDP Message Types

LDP uses the message types described in the following sections to establish and remove mappings and to report errors. All LDP messages have a common structure that uses a type, length, and value (TLV) encoding scheme.

Discovery Messages

Discovery messages announce and maintain the presence of a router in a network. Routers indicate their presence in a network by sending hello messages periodically. Hello messages are transmitted as UDP packets to the LDP port at the group multicast address for all routers on the subnet.

LDP uses the following discovery procedures:

  • Basic discovery—A router periodically sends LDP link hello messages through an interface. LDP link hello messages are sent as UDP packets addressed to the LDP discovery port. Receipt of an LDP link hello message on an interface identifies an adjacency with the LDP peer router.

  • Extended discovery—LDP sessions between routers not directly connected are supported by LDP extended discovery. A router periodically sends LDP targeted hello messages to a specific address. Targeted hello messages are sent as UDP packets addressed to the LDP discovery port at the specific address. The targeted router decides whether to respond to or ignore the targeted hello message. A targeted router that chooses to respond does so by periodically sending targeted hello messages to the initiating router.

Session Messages

Session messages establish, maintain, and terminate sessions between LDP peers. When a router establishes a session with another router learned through the hello message, it uses the LDP initialization procedure over TCP transport. When the initialization procedure is completed successfully, the two routers are LDP peers and can exchange advertisement messages.

Advertisement Messages

Advertisement messages create, change, and delete label mappings for forwarding equivalence classes (FECs). Requesting a label or advertising a label mapping to a peer is a decision made by the local router. In general, the router requests a label mapping from a neighboring router when it needs one and advertises a label mapping to a neighboring router when it wants the neighbor to use a label.

Notification Messages

Notification messages provide advisory information and signal error information. LDP sends notification messages to report errors and other events of interest. There are two kinds of LDP notification messages:

  • Error notifications, which signal fatal errors. If a router receives an error notification from a peer for an LDP session, it terminates the LDP session by closing the TCP transport connection for the session and discarding all label mappings learned through the session.

  • Advisory notifications, which pass information to a router about the LDP session or the status of some previous message received from the peer.

Tunneling LDP LSPs in RSVP LSPs

You can tunnel LDP LSPs over RSVP LSPs. The following sections describe how tunneling of LDP LSPs in RSVP LSPs works:

Tunneling LDP LSPs in RSVP LSPs Overview

If you are using RSVP for traffic engineering, you can run LDP simultaneously to eliminate the distribution of external routes in the core. The LSPs established by LDP are tunneled through the LSPs established by RSVP. LDP effectively treats the traffic-engineered LSPs as single hops.

When you configure the router to run LDP across RSVP-established LSPs, LDP automatically establishes sessions with the router at the other end of the LSP. LDP control packets are routed hop-by-hop, rather than carried through the LSP. This routing allows you to use simplex (one-way) traffic-engineered LSPs. Traffic in the opposite direction flows through LDP-established LSPs that follow unicast routing rather than through traffic-engineered tunnels.

If you configure LDP over RSVP LSPs, you can still configure multiple OSPF areas and IS-IS levels in the traffic engineered core and in the surrounding LDP cloud.

Beginning with Junos OS Release 15.1, multi-instance support is extended to LDP over RSVP tunneling for a virtual router routing instance. This allows splitting of a single routing and MPLS domain into multiple domains so that each domain can be scaled independently. BGP labeled unicast can be used to stitch these domains for service forwarding equivalence classes (FECs). Each domain uses intra-domain LDP-over-RSVP LSP for MPLS forwarding.

Note:

With the introduction of the multi-instance support for LDP-over-RSVP LSPs, you cannot enable MPLS on an interface that is already assigned to another routing instance. Adding an interface that is part of another routing instance at the [edit protocols mpls] hierarchy level, throws a configuration error at the time of commit.

Benefits of Tunneling LDP LSPs in RSVP LSPs

Tunneling LDP LSPs in RSVP LSPs provides the following benefits:

  • Provides convergence of different traffic types such as IPv4, IPv6, unicast, and multicast across Layer 2 and Layer 3 VPNs.

  • Enables flexible access connectivity options that can accommodate multiple topologies, different protocols, and multiple administrative boundaries.

  • Enables secure interworking among multiple providers.

  • Enables provision of differentiated services on a per customer basis because RSVP-TE supports traffic engineering, bandwidth guarantees, and link and node redundancy capabilities.

  • Reduces the number of LSPs required in the core, which reduces the resource requirements of the protocols and routers as well as reducing convergence time.

  • Provides cost-efficient rollouts with minimal network disruption because the LSPs are built using point-to-point TE tunnels to directly attached neighbors. These TE tunnels only go to the next hop, not end to end. Then when LDP is run over those tunnels, the sessions are built to the directly connected neighbor. When there is a change in the network, such as adding a new node, the directly connected neighbors of the new node have RSVP and LDP sessions. Thus, the RSVP LSPs are only to the next hop, and LDP takes care of advertising labels for the new addresses.

Tunneling LDP over SR-TE

Learn about the benefits and get an overview of tunneling LDP over SR-TE.

Benefits of Tunneling LDP over SR-TE

  • Enables seamless integration of LDP over SR-TE in the core network.

  • Provides flexible connectivity options to accommodate multiple topologies, protocols, and domains.

  • Enables interoperability between LDP and SR capable devices.

  • Leverages SR-TE load sharing capabilities.

  • Provides faster restoration of network connectivity using Topology Independent Loop-Free Alternate (TI-LFA) within the SR-TE domain. SR using TI-LFA routes the traffic instantly to a backup or an alternate path if the primary path fails or becomes unavailable.

Tunneling LDP over SR-TE Overview

It’s common for service providers to use the LDP signaling protocol with MPLS transport at the edges of their networks. LDP offers the advantage of being simple, but LDP lacks traffic engineering (TE) and sophisticated path repair capabilities that are often desired in the network’s core. Many service providers are migrating from RSVP to segment routing traffic engineering (SR-TE) in the core. SR-TE is also referred to as source routing in packet networks (SPRING).

It’s possible that the routers running LDP at the edge may not support SR capabilities. The service provider may wish to continue using LDP on these routers to avoid the need for an upgrade. In such scenarios, the LDP over SR-TE tunneling feature provides the ability to integrate routers that are not SR capable (running LDP) with routers that are SR capable (running SR-TE).

The LDP LSPs are tunneled through the SR-TE network, enabling interworking of LDP LSPs with SR-TE LSPs. For example, if you have LDP domains on the provider edge network and SR-TE in the core network, then you can connect the LDP domains over SR-TE, as shown in Figure 2.

Tunneling LDP over SR-TE supports co-existence of both LDP LSPs and SR-TE LSPs.

Figure 2: Interconnect LDP Domains over SR-TE in the Core NetworkInterconnect LDP Domains over SR-TE in the Core Network

You can also tunnel LDP over SR-TE between LDP domains connected to inter-region core networks. For example, if you have multiple regional LDP domains connected to the inter-region SR-TE core networks, you can tunnel LDP across the inter-region SR-TE core network, as shown in Figure 3.

Figure 3: LDP over SR-TE between Inter-region Core NetworksLDP over SR-TE between Inter-region Core Networks

In Figure 3, you have three regional networks (A, B, and C) running LDP. These regional LDP domains are connected to their respective regional core networks running SR-TE. The regional SR-TE core networks are further interconnected to other regional SR-TE core networks (inter-region core network). You can tunnel LDP over these inter-region SR-TE core networks and deploy services, such as Layer 3 VPNs, seamlessly. This scenario could be used in a mobile backhaul network, where the core aggregation layer runs LDP tunneled over SR-TE while the access layer runs LDP only.

To enable LDP tunneling over SR-TE, you need to configure the following statements:

  • ldp-tunneling at the [edit protocols source-packet-routing source-routing-path source-routing-path-name] hierarchy level to enable LDP tunneling over SR-TE.

  • spring-te at the [edit protocols isis traffic-engineering tunnel-source-protocol] hierarchy level selects LDP over SR-TE LSPs as the tunnel source protocol.

You can configure more than one tunnel source protocol for IGPs to create shortcut routes. When more than one tunnel source protocol is configured and if the tunnels from more than one protocol are available to a destination, the tunnel with the most preferred route is established. For example, if the core network has both RSVP LSPs and SR-TE LSPs and LDP tunneling is enabled for both RSVP and SR-TE LSPs, then the tunnel-source-protocol configuration selects the tunnel based on the preference value. The tunnel with the lowest preference value is most preferred. You can override this route preference with a specific protocol for all destinations by configuring the preference value, as shown in the following example:

In this example, you can see the preference value configured for the SR-TE tunnel source protocol is 2 and the preference value for RSVP tunnel source protocol is 5. In this case, the SR-TE tunnel are preferred because they have the lowest preference value as compared to RSVP tunnel source protocol.

Note:

It is not mandatory to configure the tunnel source protocol preference value. If more than one tunnel source protocol has the same preference value, then the tunnel is established based on the preferred route to the destination.

The targeted LDP session is established and is triggered when the SR-TE LSP comes up. The LSP session remains established until the LDP tunneling (ldp-tunneling) configuration is removed, or the SR-TE LSP is removed from the configuration.

Note:

Junos OS currently does not support LDP over colored SR-TE LSPs.

Example: Tunneling LDP over SR-TE

Use this example to learn how to tunnel LDP LSPs over SR-TE in your core network.

Requirements

This example uses the following hardware and software components:

  • MX Series routers as CE, PE, and core routers.

  • Junos OS Release 20.3R1 or later running on all devices.

Overview

The following topology (Figure 4) shows two LDP domains (LDP Domain A and LDP Domain B) connected to the SR-TE core network, which extends the LSP session over the core by tunneling them over SR-TE.

Topology

Figure 4: Tunneling LDP over SR-TE in the Core NetworkTunneling LDP over SR-TE in the Core Network
Table 1: Describes the domains, routers, and connections in the Topology

Domain

Devices

Router ID/lo0 address

Connection Details

LDP Domain A

CE1

192.168.11.11

Connected to PE1 through interface ge-0/0/1, assigned IP address 192.168.1.1/24.

PE1

192.168.22.22

Connected to CE1 through interface ge-0/0/1, assigned IP address 192.168.1.2/24.

Connected to R1 through interface ge-0/0/3, assigned IP address 192.168.2.1/24.

SR-TE Domain(core network)

R1

192.168.33.33

Connected to R2 through interface ge-0/0/1, assigned IP address 192.168.3.1/24.

Connected to R3 through interface ge-0/0/2, assigned IP address 192.168.4.1/24.

Connected to PE1 through interface ge-0/0/3, assigned IP address 192.168.2.2/24.

R2

192.168.44.44

Connected to R1 through interface ge-0/0/1, assigned IP address 192.168.3.2/24.

Connected to R4 through interface ge-0/0/2, assigned IP address 192.168.6.2/24.

Connected to PE2 through interface ge-0/0/3, assigned IP address 192.168.7.2/24.

R3

192.168.55.55

Connected to R4 through interface ge-0/0/1, assigned IP address 192.168.5.1/24.

Connected to R1 through interface ge-0/0/2, assigned IP address 192.168.4.2/24.

R4

192.168.66.66

Connected to R3 through interface ge-0/0/1, assigned IP address 192.168.5.2/24.

Connected to R2 through interface ge-0/0/2, assigned IP address 192.168.6.1/24.

LDP Domain B

PE2

192.168.77.77

Connected to CE2 through interface ge-0/0/2, assigned IP address 192.168.8.1/24.

Connected to R2 through interface ge-0/0/3, assigned IP address 192.168.7.2/24.

CE2

192.168.88.88

Connected to PE2 through interface ge-0/0/2, assigned IP address 192.168.8.2/24.

Configuration

To tunnel LDP LSP over SR-TE in your core network, perform these tasks:

CLI Quick Configuration

To quickly configure this example, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, copy and paste the commands into the CLI at the [edit] hierarchy level, and then enter commit from configuration mode.

Device CE1

Device PE1

Device R1

Device R2

Device R3

Device R4

Device PE2

Device CE2

Configuring CE1

Step-by-Step Procedure

The following example requires you to navigate various levels in the configuration hierarchy. For information about navigating the CLI, see Using the CLI Editor in Configuration Mode in the CLI User Guide.

To configure device CE1:

  1. Configure the network services mode as Enhanced IP. Enhanced IP sets the router's network services to enhanced Internet Protocol and uses enhanced mode capabilities.

    After you configure the enhanced-ip statement and commit the configuration, the following warning message appears prompting you to reboot the router:

    The reboot brings up the FPCs on the router.

  2. Configure the interfaces to enable IP, MPLS, and ISO transport.

  3. Configure the loopback interface to enable tunnel endpoints and service endpoints.

  4. Configure routing options to identify the router in the domain.

  5. Enable OSPF protocols on the interface.

Results

From configuration mode, confirm your configuration by entering the show chassis, show interfaces, show routing-options, and show protocols commands. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

Configuring PE1

Step-by-Step Procedure

The following example requires you to navigate various levels in the configuration hierarchy. For information about navigating the CLI, see Using the CLI Editor in Configuration Mode in the CLI User Guide.

To configure device PE1:

  1. Configure the network services mode as Enhanced IP. Enhanced IP sets the router's network services to enhanced Internet Protocol and uses enhanced mode capabilities.

    After you configure the enhanced-ip statement and commit the configuration, the following warning message appears prompting you to reboot the router:

    The reboot brings up the FPCs on the router.

  2. Configure the interfaces to enable IP and MPLS transport.

  3. Configure the loopback interface to provide an LSP tunnel and BGP peering endpoint.

  4. Configure policy options to export BGP routes to the CE router, which runs the OSPF protocol in this example.

  5. Configure a Layer 3 VPN routing instance to support the OSPF-based CE1 device.

  6. Configure routing options to identify the router in the domain.

  7. Configuring ISIS and LDP on the interfaces connected to the core network.

  8. Configure BGP between the provider edge routers.

Results

From configuration mode, confirm your configuration by entering the show chassis, show interfaces, show policy-options, show routing-instances,show routing-options, and show protocols commands. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

Configuring R1 Device

Step-by-Step Procedure

The following example requires you to navigate various levels in the configuration hierarchy. For information about navigating the CLI, see Using the CLI Editor in Configuration Mode in the CLI User Guide.

To configure device R1:

  1. Configure the network services mode as Enhanced IP. Enhanced IP sets the router's network services to enhanced Internet Protocol and uses enhanced mode capabilities.

    After you configure the enhanced-ip statement and commit the configuration, the following warning message appears prompting you to reboot the router:

    The reboot brings up the FPCs on the router.

  2. Configure the interfaces to enable IP and MPLS transport.

  3. Configure the loopback interface to enable tunnel endpoints and service endpoints.

  4. Configure routing options to identify the router in the domain.

  5. Configure ISIS adjacency SIDs on the interfaces and allocate SRGB labels to enable segment routing. The labels in the entire SRGB are available for ISIS. Prefix SIDs (and Node SIDs) are indexed from the SRGB.

  6. Configure TI-LFA to enable protection against link and node failures. SR using TI-LFA provides faster restoration of network connectivity by routing the traffic instantly to a backup or an alternate path if the primary path fails or becomes unavailable.

  7. Configure ISIS traffic engineering parameters.

  8. Enable LDP tunneling over SR-TE.

  9. Configure MPLS and LDP protocols on the interfaces in the LDP domain to exchange labels in the LDP domain.

  10. Enable targeted LDP session between the edge routers in the LDP domain.

  11. Configure a segment list to route the traffic to a specific path.

  12. Configure SR-TE LSP to the remote edge routers to enable LDP tunneling over SR-TE.

Results

From configuration mode, confirm your configuration by entering the show chassis, show interfaces, show routing-options, and show protocols commands. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

Configuring R2 Device

Step-by-Step Procedure

The following example requires you to navigate various levels in the configuration hierarchy. For information about navigating the CLI, see Using the CLI Editor in Configuration Mode in the CLI User Guide.

To configure device R2:

  1. Configure the network services mode as Enhanced IP. Enhanced IP sets the router's network services to enhanced Internet Protocol and uses enhanced mode capabilities.

    After you configure the enhanced-ip statement and commit the configuration, the following warning message appears prompting you to reboot the router:

    The reboot brings up the FPCs on the router.

  2. Configure the interfaces to enable IP and MPLS transport.

  3. Configure the loopback interface to provide an LSP tunnel and BGP peering endpoint.

  4. Configure routing options to identify the router in the domain.

  5. Configure ISIS adjacency SIDs on the interfaces and allocate SRGB labels to enable segment routing. The labels in the entire SRGB are available for ISIS. Prefix SIDs (and Node SIDs) are indexed from the SRGB.

  6. Configure TI-LFA to enable protection against link and node failures. SR using TI-LFA provides faster restoration of network connectivity by routing the traffic instantly to a backup or an alternate path if the primary path fails or becomes unavailable.

  7. Configure ISIS traffic engineering parameters.

  8. Enable LDP tunneling over SR-TE.

  9. Configure MPLS and LDP protocols on the interfaces in the LDP domain to exchange labels in the LDP domain.

  10. Configure a segment list to route the traffic to a specific path.

  11. Configure SR-TE LSP to the remote edge routers to enable LDP tunneling over SR-TE.

Results

From configuration mode, confirm your configuration by entering the show chassis, show interfaces, show routing-options, and show protocols commands. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

Configuring R3 Device

Step-by-Step Procedure

The following example requires you to navigate various levels in the configuration hierarchy. For information about navigating the CLI, see Using the CLI Editor in Configuration Mode in the CLI User Guide.

To configure device R3:

  1. Configure the network services mode as Enhanced IP. Enhanced IP sets the router's network services to enhanced Internet Protocol and uses enhanced mode capabilities.

    After you configure the enhanced-ip statement and commit the configuration, the following warning message appears prompting you to reboot the router:

    The reboot brings up the FPCs on the router.

  2. Configure the interfaces to enable IP and MPLS transport.

  3. Configure the loopback interface to provide an LSP tunnel and BGP peering endpoint.

  4. Configure routing options to identify the router in the domain.

  5. Configure ISIS adjacency SIDs on the interfaces and allocate SRGB labels to enable segment routing. The labels in the entire SRGB are available for ISIS. Prefix SIDs (and Node SIDs) are indexed from the SRGB.

  6. Configure TI-LFA to enable protection against link and node failures. SR using TI-LFA provides faster restoration of network connectivity by routing the traffic instantly to a backup or an alternate path if the primary path fails or becomes unavailable.

  7. Configure ISIS traffic engineering parameters.

  8. Configure MPLS protocols on the interfaces.

Results

From configuration mode, confirm your configuration by entering the show interfaces, show interfaces, show routing-options, and show protocols commands. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

Configuring R4 Device

Step-by-Step Procedure

The following example requires you to navigate various levels in the configuration hierarchy. For information about navigating the CLI, see Using the CLI Editor in Configuration Mode in the CLI User Guide.

To configure device R4:

  1. Configure the network services mode as Enhanced IP. Enhanced IP sets the router's network services to enhanced Internet Protocol and uses enhanced mode capabilities.

    After you configure the enhanced-ip statement and commit the configuration, the following warning message appears prompting you to reboot the router:

    The reboot brings up the FPCs on the router.

  2. Configure the interfaces to enable IP and MPLS transport.

  3. Configure the loopback interface to provide an LSP tunnel and BGP peering endpoint.

  4. Configure routing options to identify the router in the domain.

  5. Configure ISIS adjacency SIDs on the interfaces and allocate SRGB labels to enable segment routing. The labels in the entire SRGB are available for ISIS. Prefix SIDs (and Node SIDs) are indexed from the SRGB.

  6. Configure TI-LFA to enable protection against link and node failures. SR using TI-LFA provides faster restoration of network connectivity by routing the traffic instantly to a backup or an alternate path if the primary path fails or becomes unavailable.

  7. Configure ISIS traffic engineering parameters.

  8. Configure MPLS protocols on the interfaces.

Results

From configuration mode, confirm your configuration by entering the show interfaces, show interfaces, show routing-options, and show protocols commands. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

Configuring PE2

Step-by-Step Procedure

The following example requires you to navigate various levels in the configuration hierarchy. For information about navigating the CLI, see Using the CLI Editor in Configuration Mode in the CLI User Guide.

To configure device PE2:

  1. Configure the network services mode as Enhanced IP. Enhanced IP sets the router's network services to enhanced Internet Protocol and uses enhanced mode capabilities.

    After you configure the enhanced-ip statement and commit the configuration, the following warning message appears prompting you to reboot the router:

    The reboot brings up the FPCs on the router.

  2. Configure the interfaces to enable IP and MPLS transport.

  3. Configure the loopback interface to provide an LSP tunnel and BGP peering endpoint.

  4. Configure policy options to export BGP routes to the CE router, which runs the OSPF protocol in this example.

  5. Configure a Layer 3 VPN routing instance to support the OSPF-based CE2 device.

  6. Configure routing options to identify the router in the domain.

  7. Configuring ISIS, LDP, and MPLS protocols on the interfaces connected to the core network.

  8. Configure BGP between the provider edge routers.

Results

From configuration mode, confirm your configuration by entering the show chassis, show interfaces, show policy-options, show routing-instances,show routing-options, and show protocols commands. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

Configuring CE2

Step-by-Step Procedure

The following example requires you to navigate various levels in the configuration hierarchy. For information about navigating the CLI, see Using the CLI Editor in Configuration Mode in the CLI User Guide.

To configure device CE2:

  1. Configure the network services mode as Enhanced IP. Enhanced IP sets the router's network services to enhanced Internet Protocol and uses enhanced mode capabilities.

    After you configure the enhanced-ip statement and commit the configuration, the following warning message appears prompting you to reboot the router:

    The reboot brings up the FPCs on the router.

  2. Configure the interfaces to enable IP and MPLS transport.

  3. Configure the loopback interface to provide an LSP tunnel and BGP peering endpoint.

  4. Configure routing options to identify the router in the domain.

  5. Enable OSPF protocols on the interface.

Results

From configuration mode, confirm your configuration by entering the show chassis, show interfaces, show routing-options, and show protocols commands. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

Verification

To confirm that the configuration is working properly, perform the following tasks:

Verifying LDP Tunneling over SR-TE

Purpose

Verify that the LDP over SR-TE tunnel is enabled and the LDP tunnel to the remote edge router is taking the right path.

Action

From operational mode, run the show spring-traffic-engineering lsp detail command.

On R1

On R2

Meaning
  • On R1, the LDP tunnel is established with the remote edge router 192.168.66.66 in the SR-TE core network. You can also see the SID label values 80104, 80204, 80304 in the output.

  • On R2, the LDP tunnel is established with the remote edge router 192.168.33.33 in the SR-TE core network. You can also see the SID label values 80504, 80300, 80200 in the output.

Verify LDP Forwarding to the Remote PE Device

Purpose

Verify that the route to the remote PE router uses LDP forwarding and is tunneled over SR-TE.

Action

From operational mode, run the show route destination-prefix table inet.3 command.

On R1

Verify that the route to the remote PE (PE2) router is through LDP over SR-TE tunnel.

On R2

Verify that the route to the remote PE (PE1) router is through LDP over SR-TE tunnel.

On PE1

Verify that the route to the remote PE (PE2) router is through a targeted LDP session to the remote PE.

On PE2

Verify that the route to the remote PE (PE1) router is through a targeted LDP session to the remote PE.

Meaning
  • On R1, you can see the LDP label as 17 and the SR-TE label stacks as 80304, 80204, 85003, 85004.

  • On R2, you can see the LDP label as 17 and the SR-TE label stacks as 80200, 80300, 85004, 85003.

  • On PE1, you can see the LDP label as 18.

  • On PE2, you can see the LDP label as 19.

Verify LDP Sessions Between Edge and PE Routers

Purpose

Verify that LDP sessions are established between the PE and Edge routers, between the edge routers (through SR-TE tunneling), and between the CE devices over the SR-TE core.

Action

From operational mode, run the show ldp session command.

On R1

Verify that LDP sessions are established with the connected PE (PE1) router and the remote edge router (R2).

On R2

Verify that LDP sessions are established with the connected PE (PE2) router and the remote edge router (R1).

On PE1

Verify that the LDP session is established with the connected edge router in the SR-TE core.

On PE2

Verify that the LDP session is established with the connected edge router in the SR-TE core.

Meaning
  • On R1, LDP sessions are established with the connected PE1 (192.168.22.22) router and with the remote edge router R2 (192.168.66.66), which is a tunneled LDP session that extends the LDP domain over the SR-TE core.

  • On R2, LDP sessions are established with the connected PE2 (192.168.77.77) router and with the remote edge router R2 (192.168.33.33), which is a tunneled LDP session that extends the LDP domain over the SR-TE core.

  • On PE1, the LDP session is established with the connected edge router R1 (192.168.33.33), which is in the local LDP domain.

  • On PE2, the LDP session is established with the connected edge router R2 (192.168.66.66), which is in the local LDP domain.

Verifying the Advertised Label

Purpose

Verify the labels advertised for the forwarding equivalence class (FEC).

Action

From operational mode, run the show ldp database command.

On R1

Verify the labels advertised towards the directly connected PE (PE1) and the labels received from remote edge router (R2).

On R2

Verify the labels advertised towards the directly connected PE (PE2) and the labels received from remote edge router (R1).

On PE1

Verify the label for the remote PE (PE2) device's loopback address is advertised by edge device R1 to the local PE (PE1) device.

On PE2

Verify the label for the remote PE (PE1) device's loopback address is advertised by edge device R2 to the local PE (PE2) device.

Meaning
  • On R1, you can see label 18 is advertised towards the directly connected PE (PE1) and the label 17 is received from remote edge router (R2).

  • On R2, you can see label 19 is advertised towards the directly connected PE (PE2) and the label 17 is received from remote edge router (R1).

  • On PE1, you can see label 18 is received from the local edge router (R1)..

  • On PE2, you can see label 19 is received from the local edge router (R2).

Label Operations

Figure 5 depicts an LDP LSP being tunneled through an RSVP LSP. (For definitions of label operations, see MPLS Label Overview.) The shaded inner oval represents the RSVP domain, whereas the outer oval depicts the LDP domain. RSVP establishes an LSP through routers B, C, D, and E, with the sequence of labels L3, L4. LDP establishes an LSP through Routers A, B, E, F, and G, with the sequence of labels L1, L2, L5. LDP views the RSVP LSP between Routers B and E as a single hop.

When the packet arrives at Router A, it enters the LSP established by LDP, and a label (L1) is pushed onto the packet. When the packet arrives at Router B, the label (L1) is swapped with another label (L2). Because the packet is entering the traffic-engineered LSP established by RSVP, a second label (L3) is pushed onto the packet.

This outer label (L3) is swapped with a new label (L4) at the intermediate router (C) within the RSVP LSP tunnel, and when the penultimate router (D) is reached, the top label is popped. Router E swaps the label (L2) with a new label (L5), and the penultimate router for the LDP-established LSP (F) pops the last label.

Figure 5: Swap and Push When LDP LSPs Are Tunneled Through RSVP LSPsSwap and Push When LDP LSPs Are Tunneled Through RSVP LSPs

Figure 6 depicts a double push label operation (L1L2). A double push label operation is used when the ingress router (A) for both the LDP LSP and the RSVP LSP tunneled through it is the same device. Note that Router D is the penultimate hop for the LDP-established LSP, so L2 is popped from the packet by Router D.

Figure 6: Double Push When LDP LSPs Are Tunneled Through RSVP LSPsDouble Push When LDP LSPs Are Tunneled Through RSVP LSPs

LDP Session Protection

LDP session protection is based on the LDP targeted hello functionality defined in RFC 5036, LDP Specification, and is supported by the Junos OS as well as the LDP implementations of most other vendors. It involves sending unicast User Datagram Protocol (UDP) hello packets to a remote neighbor address and receiving similar packets from the neighbor router.

If you configure LDP session protection on a router, the LDP sessions are maintained as follows:

  1. An LDP session is established between a router and a remote neighboring router.

  2. If all of the direct links between the routers go down, the LDP session remains up so long as there is IP connectivity between the routers based on another connection over the network.

  3. When the direct link between the routers is reestablished, the LDP session is not restarted. The routers simply exchange LDP hellos with each other over the direct link. They can then begin forwarding LDP-signaled MPLS packets using the original LDP session.

By default, LDP targeted hellos are set to the remote neighbor so long as the LDP session is up, even if there are no more link neighbors to that router. You can also specify the duration you would like to maintain the remote neighbor connection in the absence of link neighbors. When the last link neighbor for a session goes down, the Junos OS starts an LDP session protection timer. If this timer expires before any of the link neighbors come back up, the remote neighbor connection is taken down and the LDP session is terminated. If you configure a different value for the timer while it is currently running, the Junos OS updates the timer to the specified value without disrupting the current state of the LDP session.

LDP Native IPv6 Support Overview

IPv6 connectivity often relies on tunneling IPv6 over an IPv4 MPLS core with IPv4-signaled MPLS label-switched paths (LSPs). This requires the IPv4-signaled LSPs to be configured statically or established dynamically by IPv6 provider edge routers. Because of the growing demand of IPv6, it has become imperative to deploy an IPv6 MPLS core with an IPv6-signaled LSP to provide IPv6 connectivity. In Junos OS, LDP is supported in an IPv6 network only, and in an IPv6/IPv4 dual-stack network as described in RFC 7552. Apart from providing a single session for both IPv4 and IPv6 networks, Junos OS LDP supports separate IPv4 sessions for IPv4 only, and IPv6 sessions for IPv6 only.

You can configure the address family as inet for IPv4 or inet6 for IPv6, or both. If the family address is not configured, then the default address of family inet is enabled. When both IPv4 and IPv6 are configured, you can use the transport-preference statement to configure the prefered transport to be either IPv4 or IPv6. Based on the preference, LDP attempts to establish a TCP connection using IPv4 or IPv6. By default, IPv6 is selected. The dual-transport statement allows Junos OS LDP to establish the TCP connection over IPv4 with IPv4 neighbors, and over IPv6 with IPv6 neighbors as a single-stack LSR. The inet-lsr-id and inet6-lsr-id IDs are the two LSR IDs that have to be configured to establish an LDP session over IPv4 and IPv6 TCP transport. These two IDs should be non-zero and must be configured with different values.

Longest Match Support for LDP Overview

LDP is often used to establish MPLS label-switched paths (LSPs) throughout a complete network domain using an IGP such as OSPF or IS-IS. In such a network, all links in the domain have IGP adjacencies as well as LDP adjacencies. LDP establishes the LSPs on the shortest path to a destination as determined by IGP. In Junos OS, the LDP implementation does an exact match lookup on the IP address of the forwarding equivalence class (FEC) in the routing information base (RIB) or IGP routes for label mapping. This exact mapping requires MPLS end-to-end LDP endpoint IP addresses to be configured in all the label edge routers (LERs). This defeats the purpose of IP hierarchical design or default routing in access devices. Configuring longest-match allows LDP to set up LSP based on the routes aggregated or summarized across OSPF areas or IS-IS levels in the inter-domain.

Release History Table
Release
Description
20.3R1
Starting in Junos OS Release 20.3R1, support for MPLS to provide LDP signaling protocol configuration with the control plane functionality.
15.1
Beginning with Junos OS Release 15.1, multi-instance support is extended to LDP over RSVP tunneling for a virtual router routing instance.