MPLS LSP Calculator
Count the LSPs a PE mesh needs, the label space they consume and the RSVP-TE refresh load they generate.
Inputs
Unidirectional LSPs
870
Ingress Tunnels per Router
29
Label Space Used
0.0830%
RSVP Refresh Messages
58.00msg/s
LSPs if Fully Meshed
870
Scaling Note
Full mesh is O(n²) — 870 LSPs at 30 PEs, and each new PE adds 60 more
Step by step
Values used
PE routers = 30 PEs; Tunnel topology = Full mesh between PEs; Hub routers (hub-and-spoke only) = 2 hubs; Platform label space = 1,048,576 labels; RSVP refresh interval = 30 s
MPLS LSP
A full PE mesh needs n(n − 1) unidirectional LSPs, since MPLS LSPs are one-way; hub and spoke needs 2 × spokes × hubs.
Soft-state refresh
RSVP refresh load = 2 × LSPs ÷ refresh interval messages per second.
Unidirectional LSPs
= 870
Ingress Tunnels per Router
= 29
Label Space Used
= 0.0830
RSVP Refresh Messages
= 58.00 msg/s
LSPs if Fully Meshed
= 870
Scaling Note
= Full mesh is O(n²) — 870 LSPs at 30 PEs, and each new PE adds 60 more
How it works
Because LSPs are unidirectional the full-mesh count is n(n − 1) rather than half that, and RSVP-TE keeps each one alive with periodic Path and Resv messages. The refresh figure is what turns LSP count into real control-plane load. RSVP-TE refresh is the term that limits TE deployments — a few thousand LSPs is fine, tens of thousands is a control-plane project, and it is the main reason Segment Routing removed per-LSP state from the core entirely.
Formulas
MPLS LSP
A full PE mesh needs n(n − 1) unidirectional LSPs, since MPLS LSPs are one-way; hub and spoke needs 2 × spokes × hubs.
- n
- PE routers in the mesh
- unidirectional
- MPLS LSPs are one-way, so a bidirectional service needs a pair
- refresh
- RSVP-TE Path and Resv soft state must be refreshed, by default every 30 s
Soft-state refresh
RSVP refresh load = 2 × LSPs ÷ refresh interval messages per second.
Frequently Asked Questions
How is MPLS LSP calculated?
A full PE mesh needs n(n − 1) unidirectional LSPs, since MPLS LSPs are one-way; hub and spoke needs 2 × spokes × hubs. Because LSPs are unidirectional the full-mesh count is n(n − 1) rather than half that, and RSVP-TE keeps each one alive with periodic Path and Resv messages. The refresh figure is what turns LSP count into real control-plane load.
Why does MPLS LSP matter?
RSVP-TE refresh is the term that limits TE deployments — a few thousand LSPs is fine, tens of thousands is a control-plane project, and it is the main reason Segment Routing removed per-LSP state from the core entirely.
What values do I need to enter?
This calculator takes 5 inputs: PE routers, Tunnel topology, Hub routers (hub-and-spoke only), Platform label space, RSVP refresh interval. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.
Can I avoid the refresh load without dropping TE?
Refresh reduction (RFC 2961) bundles and acknowledges messages so state does not need constant retransmission, and summary refresh cuts the volume further. Segment Routing removes the problem outright by keeping path state only at the ingress.