Your corporate network incorporates an MPLS network purchased from an ISP to link your US and Australian offices. Explain how a packet in your offices in Sydney is routed through the MPLS network to arrive in San Francisco.
Try an answer before revealing the guidance below.
Key Concepts
- Ingress label edge router
- Label push / swap / pop
- Label-switched path
- Label switching routers
- Egress router
Answer Approach
- Follow the packet from the Sydney LAN to the ISP’s first MPLS router.
- Describe what that router adds to the packet and how the label is chosen.
- Describe what each core router looks at and does.
- Describe what happens at the San Francisco end.
Full Answer
Answer status: Draft answer (unofficial). Revision notes, not an official marking scheme.
The packet leaves the Sydney office as an ordinary IP packet and reaches the ISP’s ingress label edge router. That router classifies it (by destination and possibly source or class) onto a pre-established label-switched path and pushes an MPLS label between the layer-2 and IP headers. Each label switching router along the path looks only at the incoming label, swaps it for the outgoing label in its MPLS table and forwards it on the listed interface, without examining the IP header. At the egress router near San Francisco the label is popped and the packet is forwarded as normal IP to the San Francisco office. Paths are set up in advance with a signalling protocol such as RSVP-TE or LDP and can be chosen for traffic engineering.
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