* A (Design expecting outages and attacks):In WAN design, resiliency requires assuming that failures and attacks will occur. This drives the use of redundancy, diverse paths, and proactive failure mitigation techniques.
Other options explained:
* B/C/D: While important, these focus on operational efficiency rather than core resiliency principles.

* B (Strategic planning): Long-term business-aligned decisions that shape future infrastructure needs.
* D (Tactical planning): Short-to-mid-term activities that adjust design based on evolving operational realities, constraints, and priorities.
Why other options are incorrect:
* A & E: While cost and business optimization are goals, they are not defined planning approaches.
* C: Modular is a design methodology, not a planning approach.
-

* D (L1/L2 for ABRs and L1 for internal routers):This hierarchical approach minimizes LSDB size inside areas (L1 routers only maintain intra-area routes) while L1/L2 routers handle inter-area route propagation, reducing neighbor adjacencies and improving scalability.
Other options explained:
* A: Level 2 everywhere results in larger LSDBs across all routers.
* B: Only L2 routers at ABRs would isolate L1 domains improperly.
* C: Pure L1 design prevents inter-area reachability.

* B (MST - Multiple Spanning Tree): Bundles multiple VLANs into fewer STP instances, minimizing the scope of TCN propagation, which helps reduce MAC table flushing and unicast flooding.
Why other options are incorrect:
* A: PVRSTP still processes TCNs per VLAN, increasing flooding.
* C: Legacy STP behaves similarly but is not optimized for multiple VLANs.
* D: PVSTP+ generates TCNs per VLAN like PVRSTP.
-

In the topology shown, the 10.10.0.0/16 network is redistributed into two different OSPF processes-OSPF 1 and OSPF 2-from router R3. The redistribution uses the external route types:
* Into OSPF 1 as an E1 route with a metric of 100
* Into OSPF 2 as an E2 route with a metric of 100
When redistribution occurs in OSPF, two types of external routes can be injected:
* Type E1: Adds the internal OSPF cost to the external cost (total path cost = external metric + internal OSPF path)
* Type E2: Considers only the external cost and ignores internal path cost (default behavior) In this case:
* The path via R2-R3 (E1) includes both the external cost and the internal cost from R1 to R2 to R3. That is: 10 (R1-R2) + 10 (R2-R3) + 100 (external) = 120.
* The path via R4-R5-R3 (E2) will show a constant metric of 100 regardless of the internal OSPF path (10 + 10 = ignored).
So when the link between R2 and R3 fails, the path R1-R4-R5-R3 (E2 route with constant metric 100) becomes the preferred route.
However, when the link between R2 and R3 is restored, the E1 route will be recalculated as:
* Internal cost (R1-R2-R3) = 10 + 10 = 20
* External metric = 100
* Total = 120
OSPF always prefers E1 routes over E2 routes when both are available, even if the E2 route appears to have a better metric. This is because E1 routes provide more accurate end-to-end cost calculation.
Therefore, upon restoration, the routing table reverts to the E1 route via R1-R2-R3.
This behavior is aligned with OSPF protocol standards and is emphasized in CCDE v3.1 under the "Protocol Design Implications" domain, which focuses on redistribution behaviors, route-type preferences, and convergence consistency in multi-domain IGP designs.