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ENSLD Question Bank, Part 2 (Domains 3 to 5)

This is the second half of the ENSLD 300-420 v1.1 practice bank. It holds 50 original questions, numbered 56 to 105, covering 3.0 WAN for Enterprise Networks (20 questions), 4.0 Network Services (20 questions) and 5.0 Automation (10 questions). Part 1 (questions 1 to 55) covers domains 1 and 2.

ENSLD is a design exam, so every question asks you to weigh requirements against constraints. Find the deciding constraint first (failure behavior, scale, cost, who controls the routing, what the provider will and will not do), then compare the options. Answer each question before you open the explanation, and log every miss by the blueprint number. Target: 40 of 50 (80%).

3.0 WAN for Enterprise Networks

56. Blueprint 3.1.a

An enterprise has eight sites and wants to run its own OSPF across the WAN, with its routers forming adjacencies directly with each other and no routing relationship with the provider. Which WAN service fits this requirement?

  • A. MPLS Layer 3 VPN, because the provider forwards by IP
  • B. A Layer 2 VPN service such as VPLS or E-LAN, where the provider forwards Ethernet frames and the enterprise routers peer across it
  • C. Dark fiber with no active equipment
  • D. A 5G service with carrier-grade NAT
Answer: B

A Layer 2 VPN is transparent to IP, so the customer's routers form adjacencies with each other and the enterprise owns the routing design. The trade-off is that a multipoint Layer 2 service is one shared broadcast domain, so neighbor counts and flooding grow with the number of sites. In an MPLS Layer 3 VPN (A) the customer peers with the provider's PE router, not with other sites. Dark fiber (C) would also work but is not a service the question implies, and it is costly across eight sites. Carrier-grade NAT (D) prevents the clean any-to-any peering.

57. Blueprint 3.1.b

An enterprise is evaluating an MPLS Layer 3 VPN for 200 sites that need any-to-any connectivity with several QoS classes. Which statement is accurate and should shape the design?

  • A. The enterprise IGP adjacencies pass transparently between customer edge routers
  • B. The customer edge routers must run MPLS and LDP
  • C. Traffic is encrypted by default between sites
  • D. The provider's PE routers hold the customer's routes in a VRF, the CE routers exchange routes with the PE using static routes or a routing protocol such as eBGP, and encryption must be added by the customer if required
Answer: D

In a Layer 3 VPN the provider participates in customer routing, so the PE-CE routing protocol, route redistribution and AS number planning are part of the design. The service gives any-to-any connectivity and QoS classes, but the data is not encrypted, so a security policy may require an overlay such as GET VPN or IPsec. CE routers do not run MPLS (B), and IGP adjacencies end at the PE (A).

58. Blueprint 3.1.c

A metro Ethernet provider offers E-Line, E-LAN and E-Tree services. A company wants its headquarters to reach 12 branch offices at Layer 2, but the branches must not be able to exchange Layer 2 traffic with each other directly. Which service type fits?

  • A. E-Tree, which is a rooted multipoint service where leaf sites communicate only with root sites
  • B. E-Line, one service for each branch
  • C. E-LAN, which connects all sites to each other
  • D. A single EPL service shared by all sites
Answer: A

E-Tree is built for hub-and-spoke at Layer 2, with the leaves isolated from each other. An E-LAN (C) allows every site to reach every other site, which breaks the isolation. Twelve E-Line services (B) would also isolate the branches but cost more and need 12 separate hub handoffs or VLANs. An EPL (D) is a port-based point-to-point service and cannot be shared by 12 sites.

59. Blueprint 3.1.d

Two data centers 80 km apart need very high bandwidth with the lowest possible latency and protocol transparency, since both Ethernet and Fibre Channel replication will share the link. Which WAN option is most suitable?

  • A. MPLS Layer 3 VPN
  • B. 5G fixed wireless
  • C. DWDM wavelengths on leased or owned fiber
  • D. E-LAN with a 1 Gbps port
Answer: C

DWDM multiplexes many wavelengths on a fiber pair, each carrying a protocol-agnostic high-speed channel with minimal added latency. A Layer 3 VPN (A) adds routing and only carries IP. 5G (B) cannot offer the capacity or latency. A 1 Gbps E-LAN (D) is too small and carries only Ethernet.

60. Blueprint 3.1.e

A construction company needs connectivity at temporary sites within a few days of notice. The sites will use 5G routers. The central firewall at headquarters cannot reach the sites' addresses. What is the right design?

  • A. Use static NAT at the carrier to publish management interfaces to headquarters
  • B. Have each 5G router initiate outbound IPsec or SD-WAN tunnels to hubs at headquarters, because the carrier gateway uses carrier-grade NAT and blocks inbound connections
  • C. Have the headquarters firewall initiate the tunnels to the sites
  • D. Use BGP directly to the carrier's mobile core
Answer: B

Mobile carriers typically translate subscribers behind carrier-grade NAT, so only connections that start at the site work reliably. Outbound-initiated tunnels to the hub give a stable overlay, and the same design allows the cellular link to act as a primary or backup transport. Inbound static NAT (A), hub-initiated tunnels (C) and BGP to a mobile core (D) are not options a carrier normally offers.

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