MPLS (Multiprotocol Label Switching) moves traffic through a network using short labels rather than looking up a destination address at every hop. It’s the transport layer beneath most carrier VPN services and many traffic-engineered cores.
What it is
Instead of each router making an independent routing decision, MPLS establishes a pre-built path and attaches a label that tells each hop where to send the packet next. Two ideas are worth knowing:
- Label-switched path — the pre-established route a labelled packet follows across the network.
- LDP (Label Distribution Protocol) — how neighbouring routers agree on the labels they’ll use.
MPLS is what lets a provider carry hundreds of separate customer VPNs across one shared core, keeping each one isolated (see network segmentation).
Why it matters
The label plane is largely invisible to address-based monitoring. A VPN site can go completely dark while the underlying IP routing looks perfectly healthy — because the real fault is a broken label-switched path or a dropped label-distribution session between two routers. Teams that only watch the IP layer can spend hours chasing a “site down” ticket whose true cause is one layer below where they’re looking.
What good looks like
Assuring MPLS means making the label plane visible: which paths exist, which interfaces carry them, and whether the label-distribution sessions between routers are healthy. When a session drops, you want to see it immediately and connect it to the customers and services it affects — rather than discovering it only after the tickets pile up.