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Layer 3 Practice Questions

15 original questions covering ENCOR 3.2.a to 3.2.d. Answer every one before opening any explanation. Target: 12 of 15 (80%).

1. Blueprint 3.2.a

R1 runs EIGRP with default K values and variance 2. It learns 10.50.0.0/24 from four neighbors.

Neighbor Total metric through this neighbor Reported distance
R2 3072 2816
R3 3328 2560
R4 3584 3072
R5 5120 3328

Which two neighbors does R1 use to forward traffic to 10.50.0.0/24? (Choose two.)

  • A. R2
  • B. R3
  • C. R4
  • D. R5
Answer: A, B

R2 has the lowest metric, so it is the successor and the FD is 3072. A path is a feasible successor only if its reported distance is strictly less than the FD. R3 (2560) qualifies, R4 (3072, equal) and R5 (3328) do not. Variance only applies to feasible successors, so R3's 3328 (below 2 x 3072 = 6144) is installed and R4 and R5 are not used, even though their metrics are also below 6144.

2. Blueprint 3.2.a

R1 learns 172.16.5.0/24 from three OSPF sources at the same time: as an intra-area route with cost 150, as an inter-area route with cost 20, and as an external type 2 route with metric 1. Which route is installed?

  • A. The inter-area route, because it has the lowest cost of the internal routes
  • B. The intra-area route
  • C. The external type 2 route, because its metric is lowest
  • D. All three, as unequal-cost paths
Answer: B

OSPF compares route types before cost: intra-area, then inter-area, then E1, then E2. Cost is only compared between routes of the same type, so the intra-area route wins despite its higher cost. OSPF never load balances over unequal-cost paths (D).

3. Blueprint 3.2.a

A large hub-and-spoke EIGRP network has routes going stuck in active when a hub link flaps. Which two changes reduce the scope of EIGRP queries? (Choose two.)

  • A. Configure eigrp stub on the spoke routers
  • B. Summarize the spoke routes on the distribution routers toward the core
  • C. Increase variance on the hub
  • D. Change the hello interval to 1 second
  • E. Raise the EIGRP administrative distance
Answer: A, B

Stub routers tell their neighbors not to send them queries, and a router that receives a query for a prefix it only knows as part of a summary replies immediately without querying further. Both stop queries spreading across the AS. Variance (C) only affects load balancing, faster hellos (D) only speed up failure detection, and AD (E) only matters between routing sources.

4. Blueprint 3.2.a

Refer to the exhibit. All four routers are in OSPF area 0 with default settings.

R1# show ip ospf interface brief
Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C
Te0/0/0      1     0               10.1.2.1/30        1     P2P   1/1
Gi0/0/1      1     0               10.1.3.1/30        1     P2P   1/1

The engineer wants OSPF to prefer the 10 Gbps link. Which action achieves this with costs that stay consistent across the domain?

  • A. Configure bandwidth 10000000 on Te0/0/0 on R1
  • B. Configure auto-cost reference-bandwidth 100000 on every OSPF router
  • C. Configure maximum-paths 1 on R1
  • D. Configure auto-cost reference-bandwidth 100000 on R1 only
Answer: B

With the default 100 Mbps reference, every link of 100 Mbps or faster costs 1. Raising the reference to 100 Gbps (the value is in Mbps) gives 1 Gbps a cost of 100 and 10 Gbps a cost of 10, but it must be done on every router or the routers calculate on different scales. D fixes R1 only. A sets the interface bandwidth to the value it already has. C does not change the costs.

5. Blueprint 3.2.b

Four routers boot at the same time on one Ethernet segment.

Router ip ospf priority Router ID
RA 1 10.0.0.4
RB 50 10.0.0.1
RC 50 10.0.0.2
RD 0 10.0.0.9

Which routers become DR and BDR?

  • A. DR RB, BDR RC
  • B. DR RD, BDR RA
  • C. DR RC, BDR RB
  • D. DR RA, BDR RC
Answer: C

Priority is compared first, so RB and RC (50) beat RA (1). RD has priority 0 and can never be elected, even with the highest RID. Between RB and RC the higher router ID wins, so RC is DR and RB is BDR.

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