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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 1.

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

Figure 1: Interconnect LDP Domains over SR-TE in the Core Network
Interconnect 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 2.

Figure 2: LDP over SR-TE between Inter-region Core Networks
LDP over SR-TE between
Inter-region Core Networks

In Figure 2, 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 tunnels are preferred because they have the lowest preference value as compared to RSVP tunnel source protocol.


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 LDP session remains established until the LDP tunneling (ldp-tunneling) configuration is removed, or the SR-TE LSP is removed from the configuration.


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