Explain how a packet enters a Multi-Protocol Label Switching (MPLS) network, how it is routed through the network and how it is ultimately delivered. Consider using a simple diagram to help you explain your answer.
Try an answer before revealing the guidance below.
Key Concepts
- Ingress label edge router
- Label push / swap / pop
- MPLS forwarding table
- Label-switched path
- Egress router
Answer Approach
- Describe what the ingress router does to an arriving IP packet.
- Describe what each core router looks at and changes.
- Describe what happens at the egress router.
- Sketch a path of three or four routers with labels on each link.
Full Answer
Answer status: Draft answer (unofficial). Revision notes, not an official marking scheme.
Entry: an IP packet arrives at the ingress (edge) MPLS router, which classifies it (for example by destination) onto a label-switched path and pushes a short MPLS header containing a label between the link-layer and IP headers. Through the network: each MPLS-capable router looks up only the incoming label (and interface) in its MPLS table, swaps it for the outgoing label and sends it out the listed interface — no IP longest-prefix lookup. Paths are set up in advance (e.g. RSVP-TE) and can differ from IP shortest paths for traffic engineering. Delivery: at the egress router the label is popped and the packet is forwarded by normal IP routing to its destination. Diagram: Host → R_in [push 6] → R2 [6→9] → R3 [9→pop] → destination.
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