MSDP is the correct feature when multicast receivers in one autonomous system must be able to learn and receive content from sources in another autonomous system. The scenario describes separate multicast frameworks close to the receivers in each AS and requires source resiliency across AS boundaries. In PIM Sparse Mode, a local RP learns about sources registered in its own domain. MSDP allows RPs in different domains to exchange Source-Active information, enabling receivers in one domain to discover and join toward sources that exist in another domain. Anycast RP is primarily a redundancy and load-sharing technique for RPs, often inside a multicast domain, but it does not by itself provide interdomain source discovery for independent ASs. SSM can be efficient when receivers know the source, but the question emphasizes RP-based framework placement and source availability between ASs. BIDIR-PIM is for scalable many-to-many shared-tree forwarding, not interdomain source discovery. Therefore, the design should include MSDP. Reference topics: MSDP, interdomain multicast, PIM-SM, Source-Active exchange, RP resilience across autonomous systems.

The vSmart controller is the Cisco SD-WAN control-plane element that maintains centralized routing and policy information. WAN Edge routers form secure control connections to vSmart, and vSmart distributes OMP routes, TLOC information, service routes, and policy information across the overlay. It does not forward user traffic in the data plane; forwarding is performed by the WAN Edge routers. It also does not provide the primary management GUI, because that role belongs to vManage. vBond handles orchestration, authentication facilitation, and NAT traversal during control-plane bring-up, not the centralized routing table. The importance of vSmart in the design is that it removes the need for every WAN Edge router to maintain complex direct control-plane relationships with every other edge. It also enforces centralized control policy, which determines which routes and paths are available to which sites. Reference topics: Cisco SD-WAN architecture, vSmart controller, OMP, centralized route table, control policy, WAN Edge. This preserves scalable WAN control-plane behavior.

BFD is the correct mechanism for sub-second failure detection. The question describes unequal failover behavior when an interface on an ISP router toward the campus fails. In many WAN and Internet edge designs, BGP timers or next-hop tracking may not react fast enough when the physical failure is not directly visible to the customer router. Cisco defines Bidirectional Forwarding Detection as a lightweight protocol that provides rapid forwarding-path failure detection independent of the routing protocol timers. When BFD is associated with BGP, OSPF, EIGRP, or static routing, the routing process receives failure notification quickly and can withdraw or replace the route without waiting for longer protocol hold timers. Aggressive timers can improve reaction time but increase session churn and CPU load. Next-hop address tracking improves BGP response when next-hop reachability changes, but it is not the most precise sub-second detection tool. Graceful restart is intended to preserve forwarding during control-plane restart, not detect failures faster. Reference topics: BFD, WAN edge resiliency, BGP failure detection, sub-second convergence.

The correct design is to deploy the application in both data centers, advertise the application prefix from DC1 as a /32, and advertise a broader /24 from DC2. This uses longest-prefix-match behavior to create deterministic primary and backup routing. Cisco routing logic prefers the most specific matching route in the routing table, regardless of administrative distance or metric comparisons among less specific covering routes. As long as the DC1 /32 is present, traffic for that exact application address follows DC1. If DC1, its edge path, or the /32 advertisement fails, the /32 is withdrawn and the remaining /24 covering route from DC2 provides backup reachability. Advertising the same prefix from both locations may result in load sharing or provider-dependent selection, which does not guarantee DC1 primary behavior. IP SLA could be used in some route-tracking designs, but the clean routing solution tested here is prefix specificity. Reference topics: longest-prefix match, BGP advertisement, active/standby data center routing, covering routes, route failover.

Configuring the remote router as an EIGRP stub is the correct action to stabilize the design and reduce Stuck-in-Active risk. A DUAL-3-SIA message indicates that EIGRP did not receive a reply to a query within the active timer. This is common in designs where low-speed or resource-constrained routers are queried for destinations they should never provide transit reachability for. EIGRP stub routing tells upstream neighbors that the router is not a transit device and limits which routes it advertises. As a result, upstream routers do not send unnecessary queries through the stub router, which reduces query scope, CPU load, and convergence delay. Poison reverse and split horizon adjustments are not the right design response to an SIA condition caused by excessive query range. A summary route can help reduce query scope in some places, but the branch or edge router in this scenario should also be explicitly marked as a stub. Reference topics: EIGRP DUAL, Stuck-in-Active, EIGRP stub routing, query scope, low-bandwidth links, convergence stability.

The best answer is to use a single IP address for a static NAT entry that represents the server with a nonoverlapping address. When two companies use overlapping private address space, routing directly between the networks can fail because each side may already have a local route for the same prefix. The requirement is narrow: users in the acquired company need access to one server in Company A, and the design must conserve address space. A static NAT mapping for the server solves the destination overlap problem with one translated address, giving remote users a unique reachable address for that server without renumbering either company. Overload NAT is normally used to translate many inside clients behind one address for outbound access; it is not the most precise solution for publishing one overlapping server. One-to-one NAT for every user wastes addresses and adds operational overhead. Readdressing the acquired company would solve the overlap permanently, but it does not conserve effort or address space. Therefore, a single static NAT entry for the server is the most efficient design.

The IS-IS hierarchy should place access or internal area routers at Level 1, boundary routers at Level 1/2, and backbone routers at Level 2. In this exhibit, users in areas 25 and 55 must send and receive traffic through both backbone areas, while link flaps in areas 35 and 45 must not affect other areas. Cisco IS-IS design uses Level 1 areas to contain local topology changes and Level 2 routing to interconnect areas through the backbone. Level 1/2 routers sit at the area boundary and leak or advertise reachability between the local Level 1 area and the Level 2 backbone. Option C matches that hierarchy by placing A-series routers as Level 1, B-series routers as Level 1/2, and C-series routers as Level 2. Making too many routers Level 1/2 would expand the impact of topology changes and reduce scaling benefits. Making backbone routers Level 1 would break interarea design. Reference topics: IS-IS hierarchy, Level 1, Level 2, Level 1/2 routers, failure-domain containment, enterprise scaling.

Forward error correction and quality of service are the two listed techniques that directly improve application experience in a Cisco SD-WAN design. Forward error correction protects selected application traffic by sending recovery information so packet loss can be corrected without waiting for retransmission. That is especially useful for real-time voice and video traffic, where retransmission delay can degrade quality. QoS improves application treatment by classifying, marking, queuing, shaping, and prioritizing traffic according to business intent and application sensitivity. Cisco SD-WAN designs commonly combine application-aware routing, QoS, and loss-mitigation features to keep critical traffic on the best path and preserve service quality during congestion or impairment. A stateful firewall, AMP, and Cisco Umbrella improve security posture, but they do not primarily improve application experience in the forwarding-quality sense asked by the question. They may protect applications, but they do not directly address loss, delay, jitter, and congestion treatment. Therefore, FEC and QoS are the best answers. Reference topics: Cisco SD-WAN application experience, FEC, QoS, loss mitigation, real-time traffic handling.

The load balancers should be co-located in area 0 because the design is using the same /32 application address from multiple OSPF-enabled devices. This is an anycast-style design: several load balancers advertise the same host route, and the routing protocol sends clients to the topologically best available instance. In OSPF, route preference is strongly affected by route type. Intra-area routes are preferred over interarea routes, and interarea summarization can also hide useful path details. If identical anycast host routes are originated from different nonbackbone areas, hosts may not always select the closest or most resilient load balancer, and failure behavior can become inconsistent. Placing the load-balancer anycast advertisements in area 0 keeps the routing decision consistent and avoids unnecessary interarea preference complications. Configuring X, Y, and Z as different areas would make the problem worse, and placing only one load balancer in area 0 does not provide uniform resilience. Reference topics: OSPF area design, anycast host routes, intra-area preference, backbone area, application load-balancer resilience.

A totally stubby OSPF area blocks Type 3, Type 4, and Type 5 LSAs while allowing a default summary route into the area. In OSPF, Type 3 LSAs are interarea summary LSAs, Type 4 LSAs describe reachability to ASBRs, and Type 5 LSAs carry external routes. A normal area accepts all of these LSA types. A standard stub area blocks Type 5 external LSAs and normally Type 4 ASBR summaries, but it still allows Type 3 interarea summaries. An NSSA blocks normal Type 5 external LSAs but allows Type 7 LSAs for locally redistributed external routes. A totally stubby area is more restrictive: the ABR suppresses interarea summaries and external routing detail, then injects a default route so routers inside the area still have reachability outside the area. This design reduces LSDB size, memory consumption, and SPF processing on low-end routers in remote areas. Therefore, the OSPF area type that blocks Type 3, 4, and 5 LSAs but allows a default summary route is a totally stubby area.