What is MPLS?
Why Interviewers Ask This
Candidates at the intermediate level are expected to not only know this concept but explain the trade-offs involved. Interviewers use this question to see if you can reason about design decisions, not just recall facts.
Answer
MPLS (Multiprotocol Label Switching) is a routing technique that uses short labels attached to packets to make forwarding decisions, rather than examining the full IP header at each hop. This makes forwarding faster and allows for traffic engineering. How it works: at the ingress MPLS router (LER — Label Edge Router), a label is pushed onto the packet. Intermediate routers (LSRs — Label Switch Routers) forward based solely on the label (swap the label with the next label). At the egress LER, the label is popped and the packet is forwarded normally. MPLS creates LSPs (Label Switched Paths) — predetermined paths through the network. Benefits: traffic engineering (control paths explicitly), QoS (label-based forwarding with different treatment), VPNs (MPLS VPNs separate customer traffic using labels), and faster forwarding. Widely used by ISPs and for enterprise MPLS WAN services. Being replaced by SD-WAN in many deployments.
Pro Tip
Back up your answer with a specific project or situation. Saying 'In my last Networking project, I used this when...' immediately makes your answer more credible and memorable.