MPLS LSP Calculator
Count the LSPs a PE mesh needs, the label space they consume and the RSVP-TE refresh load they generate.
About MPLS LSP Calculator
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.
Worked example
With the default values below, MPLS LSP Calculator returns Unidirectional LSPs of 870. Change any input above and every figure here is recalculated in your browser.
Inputs used
- PE routersaccepts 2–2,000 PEs
- 30 PEs
- Tunnel topologyone of Full mesh between PEs, Hub and spoke
- Full mesh between PEs
- Hub routers (hub-and-spoke only)accepts 1–64 hubs
- 2 hubs
- Platform label spaceminimum 1 labels
- 1,048,576 labels
- RSVP refresh intervalaccepts 1–600 s
- 30 s
Results
- 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
Step by step
- 1Values used
- 2MPLS LSP
- 3Soft-state refresh
- 4Unidirectional LSPs= 870
- 5Ingress Tunnels per Router= 29
- 6Label Space Used= 0.0830
- 7RSVP Refresh Messages= 58.00 msg/s
- 8LSPs if Fully Meshed= 870
- 9Scaling Note= Full mesh is O(n²) — 870 LSPs at 30 PEs, and each new PE adds 60 more
Assumptions and limits
- The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
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.