Check an SR-MPLS label stack against platform MSD and MTU, and compare state held versus RSVP-TE.
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.
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.
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.
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.
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.
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.
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.