Replacing an aging LDP-based core with Segment Routing across four points of presence in two countries, with zero service interruption for L3VPN customers.
An end-to-end redesign of a regional carrier's MPLS backbone. The objective was to replace an aging LDP-based core with a modern Segment Routing architecture and migrate all enterprise L3VPN customers to the new transport plane with zero service interruption. The project spanned four points of presence across two countries and required comprehensive redesign of the IGP, label distribution, and BGP policies.
The carrier was operating a legacy MPLS backbone built on LDP for label distribution and OSPF as the underlay IGP. The architecture had accumulated significant technical debt and was displaying critical scaling limitations.
LDP flooding behaviour caused severe instability during topology changes, resulting in unacceptably high convergence times.
All traffic followed the IGP shortest path with no mechanism to steer data around congested links.
The OSPF domain had grown into a single flat area, so LSA flooding placed excessive CPU load on backbone routers during failures.
L3VPN customers shared a single BGP route reflector with no redundancy, creating a single point of failure for the entire VPN control plane.
Planned expansion to integrate two new PoPs would have pushed the architecture past its structural design limits.
The backbone was redesigned from the IGP level upward, introducing Segment Routing as the primary transport plane and restructuring the BGP hierarchy for scalability and redundancy.
Migration from flat OSPF to IS-IS with a structured multi-area design. Backbone PoPs sit in L1/L2 areas providing full core topology visibility while isolating LSP flooding within each area. IS-IS was selected for its performance during topology churn and native SR-MPLS integration, with BFD enabled on all adjacencies for sub-second failure detection.
LDP was decommissioned entirely and replaced with Segment Routing over MPLS. Node SIDs were assigned to all backbone routers and advertised via IS-IS TLV extensions, eliminating LDP adjacency state. Adjacency SIDs were provisioned on core-facing interfaces for explicit path control. Embedding label distribution into the IGP removes per-hop LDP state and dramatically reduces control plane complexity.
SR-TE policies dynamically steer high-priority customer VPN traffic away from congested links. Explicit segment lists define primary and backup paths between all major PoP pairs, with a Path Computation Element integrated to compute dynamic paths from real-time topology and bandwidth data.
Dual route reflectors deployed across separate PoPs provide full redundancy. Each vRR peers with all PE routers, and route reflectors peer with each other for inter-cluster exchange. L3VPN customers continue using MP-BGP VPNv4, so no customer-facing modifications were required. BGP communities enforce per-customer policies and route filtering at PoP boundaries.
IS-IS ran parallel to OSPF until preferred via administrative distance, then OSPF was gracefully decommissioned. SR-MPLS was enabled within IS-IS while LDP continued forwarding production traffic. LDP was then removed router by router from edge inward, with SR label stacks verified per hop using MPLS OAM before each teardown. Customer PE interfaces remained untouched throughout, making the final cutover transparent to end users.
Thirty minutes with a senior engineer, covering your current environment and the gaps worth addressing first. No obligation, no sales handoff.
