MPLS Label
Calculate MPLS label stack overhead and effective MTU for labeled traffic.
About MPLS Label
MPLS uses a 20-bit label field, giving a theoretical 2^20 = 1,048,576 label values, but the first 16 (0-15) are reserved for special purposes (e.g. label 3 for Implicit NULL, label 0/2 for IPv4/IPv6 Explicit NULL) per RFC 3032, leaving 1,048,560 usable labels on a device. Dividing usable labels by the number of labels each LSP consumes gives the maximum number of Label-Switched Paths a single label space can support — a practical ceiling that matters for large-scale MPLS/VPN or traffic-engineering deployments with many LSPs.
Formulas
Usable label space
usable_labels = 2^20 − 16 = 1,048,560
Max LSPs supported
max_lsps = usable_labels / labels_per_lsp
Worked example
With the default values below, MPLS Label returns Usable MPLS Labels of 1,048,560. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Labels Consumed per LSPminimum 1
- 1
- Number of LSPs to Provisionminimum 1
- 10,000
Results
- Usable MPLS Labels
- 1,048,560
- Max LSPs Supported (at given labels/LSP)
- 1,048,560
- Labels Needed for Requested LSPs
- 10,000
- Label Space Utilization
- 0.9537%
Step by step
- 1Values used
- 2Usable label space
- 3Max LSPs supported
- 4Usable MPLS Labels= 1,048,560
- 5Max LSPs Supported (at given labels/LSP)= 1,048,560
- 6Labels Needed for Requested LSPs= 10,000
- 7Label Space Utilization= 0.9537
Assumptions and limits
- The result is derived only from the 2 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
Why are the first 16 MPLS labels reserved?
RFC 3032 reserves label values 0-15 for special-purpose signaling — for example, label 0 (IPv4 Explicit NULL), label 2 (IPv6 Explicit NULL), and label 3 (Implicit NULL, used in penultimate-hop popping) — these are never assigned to regular LSPs, leaving 1,048,560 of the full 1,048,576 values for actual label-switched paths.
Is the MPLS label space shared across the whole network or per-device?
Per-device (per label space on that router/switch) — each MPLS-enabled device maintains its own local label allocation, so the 20-bit space is a per-node constraint, not a network-wide budget. Very large networks rarely approach this limit on any single device in practice.
When would an LSP need more than one label?
Label stacking — used for MPLS VPNs (an inner VPN label plus an outer transport label), traffic engineering with explicit paths, or fast reroute mechanisms — consumes multiple labels per logical path, which is why this calculator lets you set labels-per-LSP above 1 for such designs.