MPLS Label Stack Overhead Calculator

MPLS Label Stack Overhead

Every MPLS shim header takes up 4 bytes (32 bits). The layout is 20 bits label + 3 bits EXP (Traffic Class) + 1 bit BoS (Bottom of Stack) + 8 bits TTL. Stack a few of those and the overhead just adds up: N × 4 bytes, with N being how deep the stack goes. Most L3VPN or L2VPN setups run two labels, an outer transport label wrapping an inner VPN label.

The end-to-end path is the LSP (Label Switched Path). How do routers learn which labels to use? On paths that follow the IGP, LDP (RFC 5036) hands them out; for traffic-engineered tunnels you reach for RSVP-TE (RFC 3209). The architecture as a whole lives in RFC 3031, while L2VPN pseudowires follow RFC 4447 (PWE3).

Applications

Walk into almost any ISP core and you'll find MPLS running it. In Brazil that means Telefônica/Vivo, Embratel/Claro, Oi and TIM. It carries L3VPN (RFC 4364), L2VPN/VPLS, MPLS-TE for explicit-path routing, and fast reroute (FRR) that fails over in under 50ms. The EXP bits give you QoS, and the whole thing is what Segment Routing (SR-MPLS) builds on.

FAQ

Why is the label only 20 bits? That gives each LSR 1,048,576 distinct label values, which is plenty for forwarding. Values 0–15 are set aside (for example, 0 = IPv4 Explicit Null and 3 = Implicit Null for PHP).

What is the BoS bit? It stands for Bottom of Stack. The innermost label sets it to 1, which tells the router that the next 4 bytes are payload (IP, an Ethernet frame, and so on) rather than yet another MPLS header.

How many labels can be stacked? The standard doesn't set a ceiling. In the field you'll usually see 2–5 labels. Segment Routing sometimes pushes deeper stacks of 10 or more, and that only works on hardware built to parse those extra labels.