SR-TE LSPs Through a Multi-Domain Network

By Ricardo Dominguez — Juniper TechPost (posted 2025-04-30)

Establishing SR-TE (CSPF) LSPs through inter-OSPF areas is a challenge, because these LSPs rely on the Traffic Engineering Database (TED), and the TED is per OSPF area or IS-IS level. This post explores how to create four types of SR-TE LSPs across an inter-OSPF-areas network, with particular emphasis on their reliance on the TED.

Introduction

SR-TE LSPs can be leveraged to migrate traffic from LDP to SR. From a JUNOS perspective, static LSPs are mainly configured through CLI or Netconf, while dynamic LSPs are PCE Initiated (e.g., Juniper Paragon), BGP-SRTE, and ODN (dynamic tunnels).

This post considers four types of LSPs with transport class and BGP color community, each with different constraints on OSPF areas:

• IGP Metric
• TE Metric
• Delay Metric
• Admin Groups

When setting up LSPs, dynamic paths through OSPF require TED from multiple areas, as TED is area-specific. To bring up an LSP, a BGP-LS producer (typically ABRs), a BGP-LS consumer (such as ingress PEs or PCE), and a Route Reflector are necessary.

Test Topology

The test topology consists of three OSPF areas: Area 0, Area 1, and Area 2. Routers P1–P4 are connected through OSPF Area 0. PE11 and PE12 are connected to P1 and P2 in OSPF area 1, while PE13 and PE14 are connected to P3 and P4 in OSPF area 2.

The entire network is based on IPv4/OSPF with Segment Routing as the MPLS signaling protocol. BGP is used among PEs and the Route Reflector for inet, inet-vpn, inet6, inet6-vpn, and evpn address families. The core network is BGP-free and IPv6-free, with a shared L3VPN across all PEs for both IPv4 and IPv6 CE traffic. 6VPE and 6PE are utilized to enable IPv6 communication among CEs over an IPv4/MPLS network. EVPN E-Line (EPL) is used between PE11 and PE13 for IPv4 and IPv6 traffic; EVPN E-Line (EVPL) for VLAN 600 and EVPN E-LAN for VLAN 700 are used between PE12 and PE14.

Hardware used in the test bed: P1/P2: ACX7100-32C, P3/P4: ACX7100-48L, PE11: MX304, PE12/PE13: MX204, PE14: ACX7024, CEs: ACX5448 / ACX7024.

Conclusion

SR-TE LSPs can be established across multi-OSPF-area networks by relying on BGP-LS to propagate TED information between areas. Additionally, Juniper Paragon PCE-initiated LSPs can compute SR-TE LSPs across a multi-domain network from a central point.

Useful Links

• MPLS Applications User Guide: https://www.juniper.net/documentation/us/en/software/junos/mpls/mpls.pdf
• Day One: Configuring Segment Routing with JUNOS
• Day One: Migrating to Segment Routing
• Service Mapping to Colored MPLS Paths (Juniper TechPost)
• Inter-domain On-demand SR-TE LSPs with BGP-LS (Juniper TechPost)

Glossary

6VPE: IPv6 VPN Provider Edge · 6PE: IPv6 Provider Edge · ABR: Area Border Router · BGP-LS: Border Gateway Protocol Link State · CE: Customer Edge · CSPF: Constraint Shortest Path First · TED: Traffic Engineering Database · E-Line: Ethernet Line · EPL: Ethernet Private Line

原文链接：https://community.juniper.net/blogs/ricardo-dominguez/2025/04/30/sr-te-lsps-through-a-multi-domain-network