SR-MPLS Calculator
Check an SR-MPLS label stack against platform MSD and MTU, and compare state held versus RSVP-TE.
About SR-MPLS Calculator
Segment Routing puts the path in the packet, so only the ingress holds path state and the core forwards on labels it already advertised. The cost moves to the packet header, which is why MSD and MTU become the binding constraints instead of control-plane scale. MSD is a hardware limit that fails silently at the wrong layer: a controller computes a valid path, the ingress cannot impose the stack, and traffic falls back to the shortest path without any obvious error.
Formulas
SR-MPLS
Each SID in the stack is a 4-byte MPLS label, so a strict explicit path of k hops adds 4k bytes and must fit the platform's maximum SID depth.
- MSD
- Maximum SID Depth — how many labels the forwarding ASIC can impose, commonly 5 to 10
- Binding-SID
- A single SID that expands into a pre-programmed stack, used to compress deep paths
- path state
- Per-path forwarding state — O(n) SIDs for SR versus O(n²) LSPs for a full RSVP-TE mesh
Control-plane state
State: SR needs one SID per node; a full RSVP-TE mesh needs n(n − 1) LSPs with state at every hop.
Worked example
With the default values below, SR-MPLS Calculator returns MTU Required with the Stack of 1,516. Change any input above and every figure here is recalculated in your browser.
Inputs used
- SIDs in the label stackaccepts 1–20 SIDs
- 4 SIDs
- Platform maximum SID depthaccepts 1–20 SIDs
- 5 SIDs
- Customer IP MTUaccepts 576–9,000 bytes
- 1,500 bytes
- Nodes in the domainaccepts 2–10,000 nodes
- 100 nodes
Results
- MTU Required with the Stack
- 1,516 bytes
- Label Stack Overhead
- 16 bytes
- Path State Entries with SR
- 100
- Path State Entries with RSVP-TE
- 9,900
- State Reduction vs RSVP-TE
- 98.99%
- Fits the Platform MSD
- Yes — a 4-SID stack fits an MSD of 5
Step by step
- 1Values used
- 2SR-MPLS
- 3Control-plane state
- 4MTU Required with the Stack= 1,516 bytes
- 5Label Stack Overhead= 16 bytes
- 6Path State Entries with SR= 100
- 7Path State Entries with RSVP-TE= 9,900
- 8State Reduction vs RSVP-TE= 98.99
- 9Fits the Platform MSD= Yes — a 4-SID stack fits an MSD of 5
Assumptions and limits
- The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax or compounding frequency into account.
- Figures are rounded for display to 2 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 SR-MPLS calculated?
Each SID in the stack is a 4-byte MPLS label, so a strict explicit path of k hops adds 4k bytes and must fit the platform's maximum SID depth. Segment Routing puts the path in the packet, so only the ingress holds path state and the core forwards on labels it already advertised. The cost moves to the packet header, which is why MSD and MTU become the binding constraints instead of control-plane scale.
Why does SR-MPLS matter?
MSD is a hardware limit that fails silently at the wrong layer: a controller computes a valid path, the ingress cannot impose the stack, and traffic falls back to the shortest path without any obvious error.
What values do I need to enter?
This calculator takes 4 inputs: SIDs in the label stack, Platform maximum SID depth, Customer IP MTU, Nodes in the domain. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.
How do I get around a low MSD?
Use fewer, coarser segments. A loose path with node-SIDs at key waypoints needs far less depth than a strict hop-by-hop path, and a Binding-SID collapses a whole sub-path into one label the ingress imposes.