3.3 - WAN links and service metrics

Network+ N10-009 objective 3.3 covers WAN service edges and the metrics that describe availability. The demarcation point is where the provider's responsibility ends and the customer's begins on a circuit. A CSU/DSU is the last-mile device that converts the provider's digital signal for a T1 leased line. Load balancers use health checks to test backends: an L4 health check marks a server down after it fails to open a TCP connection, while L7 checks inspect application responses. Reliability metrics matter too: MTTR is the average time to repair after a failure, and MTBF is the mean time between failures - together they gauge how often and how long outages last. You should also know SLAs and uptime. Expect scenario questions that name a boundary, a last-mile device, a health check or a reliability metric and ask what it is.

Memory hook
Provider/customer boundary = demarcation point. Last-mile T1 signal converter = CSU/DSU. Fails TCP connect = L4 health check down. MTTR = avg time to repair; MTBF = mean time between failures.

Practice questions

1. In disaster recovery, RPO (Recovery Point Objective) defines:

  • Max time allowed to restore service
  • Max acceptable data loss, in time (correct answer)
  • The backup media rotation scheme
  • The mean time between failures

RPO = how much data (measured as time since last backup) you can afford to lose. RTO = how long you can be down before recovery. MTBF = reliability metric.

2. A recovery site with hardware ready but data must be restored before use is a:

  • Hot site
  • Cold site
  • Warm site (correct answer)
  • Cloud site

A warm site has hardware and connectivity in place but needs data restored/updated before going live. Hot = fully live/synced, cold = empty space only.

3. Which agreement defines the uptime and performance targets a provider commits to?

  • SLA (correct answer)
  • NDA
  • MOU
  • SOW

An SLA (Service Level Agreement) specifies measurable uptime/performance commitments and penalties. An NDA covers confidentiality, an MOU is a non-binding intent, an SOW scopes work.

4. Which device conditions power and keeps equipment running briefly during an outage?

  • PDU
  • Generator
  • Transfer switch
  • UPS (correct answer)

A UPS provides battery-backed power to ride through short outages and enable clean shutdowns. A PDU only distributes power; a generator covers longer outages.

5. Before a risky change, which change-management item lists how to revert it?

  • Impact analysis
  • Approval record
  • Rollback plan (correct answer)
  • Maintenance window

A rollback (back-out) plan describes how to restore the prior state if the change fails. Impact analysis assesses risk; the window sets timing.

6. Which metric estimates how long a failed component takes to repair on average?

  • RPO
  • RTO
  • MTBF
  • MTTR (correct answer)

MTTR (Mean Time To Repair) is the average time to restore a failed component. MTBF is time between failures; RTO/RPO are recovery targets.

Related objectives

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