✍ 12: Wide Area Networks — Exercises¶
Tip
Practice — try each question first, then expand the answer to check your reasoning.
Work through each question, then click ▶ Show answer to check yourself. Review the notes if you get stuck.
🌍 Q1. At which OSI layers do LANs and WANs differ, and which do they share?¶
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They **differ at Layers 1–2** (media, access methods, framing) and **share Layers 3+** (IP, TCP/UDP).🧭 Q2. Match the metric/behavior to the routing protocol: hop count (max 15), cost/bandwidth link-state, between autonomous systems.¶
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- Hop count (max 15) → **RIP** - Cost/bandwidth, link-state → **OSPF** - Between autonomous systems → **BGP**💡 Q3. A router learns a route to the same network via OSPF and via a static route. Which is installed, and why?¶
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The **static route** — it has a lower **administrative distance** (static = 1 vs. OSPF = 110), so it's more trusted.🌍 Q4. Compare hub-and-spoke and full-mesh WAN topologies (redundancy vs. cost).¶
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- **Hub-and-spoke** — cheap and simple, but the hub is a single point of failure and spoke-to-spoke traffic takes two hops. - **Full mesh** — every site directly linked → maximum redundancy and optimal paths, but expensive/complex (many links).🔹 Q5. A branch office needs cheap, encrypted connectivity to HQ over the Internet. What fits?¶
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A **site-to-site VPN** (typically IPsec) over the public Internet — far cheaper than a dedicated circuit, with an encrypted tunnel. **SD-WAN** can add multi-link steering/failover.🔹 Q6. Rank by typical bandwidth/latency for last-mile access: dial-up, DSL, fiber (FTTH), satellite (geostationary).¶
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Best → worst: **Fiber (FTTH) > DSL > Satellite (geostationary — high latency) > Dial-up.**❓ Q7. What is the "last mile," and what is a CSU/DSU used for?¶
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- **Last mile** — the connection from the provider's network to the customer premises. - **CSU/DSU** — terminates a digital leased line (T1/T3), converting between the router and the WAN circuit's signaling.🔹 Q8. How does MPLS forward traffic, and what advantage does that give?¶
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MPLS forwards by **labels** (label swapping) rather than full IP lookups at each hop, enabling **fast forwarding, QoS, and traffic engineering** across the provider network.❓ Q9. What is a default route, and when is it used?¶
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`0.0.0.0/0` — the **gateway of last resort**. It matches any destination not otherwise in the routing table (commonly the route toward the ISP/Internet from a stub site).🔹 Q10. Contrast single-homed and multihomed Internet connectivity.¶
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- **Single-homed** — one ISP, one link; no redundancy. - **Multihomed** — multiple ISPs/links (often via BGP) for failover and load sharing.💡 Q11. Which PPP authentication method is secure, and why?¶
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**CHAP** — it uses a challenge-response **hash**, so the password is never sent in clear text (and it re-authenticates periodically). **PAP** sends credentials in plaintext and is insecure.❓ Q12. What is convergence, and why does faster convergence matter?¶
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Convergence is when **all routers agree** on the topology after a change. Faster convergence (OSPF/EIGRP over RIP) means **less disruption** and quicker rerouting during failures.⬅️ Prev: Module 11 · 📚 All Exercises
🧭 Bonus Scenario Practice¶
🧩 Case A: Route Selection¶
A router knows these routes:
10.0.0.0/8through OSPF10.10.0.0/16through RIP10.10.5.0/24as a static route
Which route is used for destination 10.10.5.44, and why?
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The static `10.10.5.0/24` route is used because it is the longest prefix match. Route specificity is checked before administrative distance.🧩 Case B: WAN Choice¶
A rural branch office needs connectivity for email and cloud apps, but fiber and cable are unavailable. Latency-sensitive voice calls perform poorly over the current satellite link. What should the design discussion focus on?