Cisco 300-420 - Designing Cisco Enterprise Networks (ENSLD) v1.1

A logical topology that virtually connects devices over an arbitrary physical network is an overlay. In Cisco SD-Access, the underlay provides basic IP reachability between fabric nodes, while the overlay creates the logical network used by endpoints, virtual networks, and policy segmentation. The overlay is not constrained to the exact physical cabling topology; instead, it uses encapsulation and control-plane mapping to carry user traffic across the routed fabric. Cisco SD-Access uses LISP in the control plane for endpoint-to-location mapping and VXLAN in the data plane for encapsulation. This allows the fabric to present consistent logical connectivity even when the physical network is built from routed links and different infrastructure layers. The data plane is the forwarding mechanism, the control plane resolves location information, and the underlay is the physical and routed transport. The logical topology built on top of those components is the overlay. Reference topics: Cisco SD-Access overlay, underlay, VXLAN, LISP, fabric logical topology.

Explicit QoS trust should be configured at the network boundary points where markings first enter the managed DiffServ domain and are known to be reliable. In a DiffServ design, traffic classification and marking are typically performed at the access edge or at a controlled ingress boundary, and downstream devices then honor DSCP or CoS markings based on the trust boundary. Cisco QoS design stresses that the trust boundary must be intentionally placed; endpoints or unmanaged devices should not be trusted by default because they can mark traffic incorrectly and steal priority treatment. The exhibit answer identifies B and E as the correct trust points, meaning those are the controlled ingress locations where markings should be accepted. Options that trust uncontrolled endpoint-facing points or internal transit locations would either create a security problem or add no practical value. Once traffic is trusted and marked at the correct boundary, queuing, policing, and shaping policies can use the DSCP values consistently across the campus or WAN. Reference topics: DiffServ, QoS trust boundary, DSCP marking, classification, enterprise QoS design.

Cisco Identity Services Engine is the SD-Access component that integrates with Cisco DNA Center to provide identity-based policy, Security Group Tags, and Security Group ACL policy enforcement. Cisco SD-Access uses Cisco TrustSec technology concepts for group-based policy, but ISE is the actual policy and identity services platform that integrates with Cisco DNA Center. DNA Center automates fabric deployment and policy orchestration, while ISE provides endpoint identity, group assignment, scalable group information, and policy authorization. SGACLs define permitted or denied communication between security groups, and SGTs classify users or endpoints into those groups. APIC-EM is not the SD-Access policy platform. Cisco Network Data Platform supports analytics and assurance functions, not identity policy enforcement. Cisco TrustSec is the underlying policy technology, but the question asks for the component that integrates with Cisco DNA Center. Therefore, the verified answer is Cisco Identity Services Engine. In design terms, ISE should be planned as a critical policy-plane dependency with redundancy, pxGrid integration, RADIUS services, and consistent scalable group policy design.

The ORF filter should be applied so the route reflector or upstream peer suppresses the unwanted Branch A prefix before sending updates toward the resource-constrained branch. Cisco BGP outbound route filtering lets a receiving router advertise a prefix-list filter to its peer, causing that peer to apply the filter outbound. This is useful when a router has limited CPU or memory because unwanted prefixes are never sent across the session. In the described design, HQ must prevent Branch A prefixes from reaching R4, while Branch B has limited resources. The answer using the 10.10.10.0/24 prefix list on R2 aligns with filtering the Branch A prefix at the point where HQ can control route reflection or advertisement toward the constrained branch path. Applying the filter on the wrong router or using the wrong prefix would still allow unnecessary updates. ORF is preferred over simple inbound filtering when the goal is to reduce received route volume. Reference topics: BGP ORF, prefix-list signaling, route-reflector filtering, branch route scale, outbound update suppression.

The correct IS-IS hierarchy places area 49.000 as Level 2 and areas 49.001, 49.002, and 49.003 as Level 1. In IS-IS, Level 2 forms the interarea backbone, while Level 1 areas contain local topology details. This design supports growth in the backbone because Level 2 routers exchange interarea reachability without carrying every local topology change from every access area. Areas 49.002 and 49.003 can perform summarization and route leaking at their Level 1/Level 2 boundaries, allowing routers such as A1 and A2 to avoid suboptimal routing by learning selected more-specific routes from the backbone when necessary. If the nonbackbone areas were made Level 2, the design would flatten the hierarchy and expose more of the network to topology changes. If the backbone were Level 1, interarea routing would be broken or badly constrained. The selected hierarchy preserves scaling, allows route control, and avoids unnecessary propagation of local link flaps. Reference topics: IS-IS hierarchy, Level 1 areas, Level 2 backbone, route leaking, summarization, scaling.

The SD-Access overlay should reduce subnet sprawl and avoid overlapping IP subnets unless there is a specific operational requirement. Cisco SD-Access creates overlay virtual networks over an IP underlay and uses anycast gateway, VXLAN encapsulation, LISP control-plane mapping, and policy constructs to simplify endpoint mobility and segmentation. Reducing the number of subnets simplifies DHCP scope management, default-gateway placement, and operational troubleshooting. Avoiding overlapping address space across overlay networks is also a practical design recommendation because overlapping subnets complicate shared services, fusion routing, security logging, and policy troubleshooting. LAN automation and a dedicated IGP process are underlay deployment considerations, not overlay design choices. Layer 3 to the access design is also associated with routed underlay and campus fabric transport, not the overlay segmentation model. A clean overlay design should define virtual networks, scalable groups, IP pools, shared-services reachability, and external connectivity before deployment. Reference topics: Cisco SD-Access overlay design, anycast gateway, virtual networks, DHCP simplification, overlapping subnet avoidance.

The drag-and-drop mapping separates WAN connectivity technologies by the service characteristics they deliver. Cisco enterprise WAN designs distinguish private WAN options, such as MPLS Layer 3 VPN, Ethernet WAN, VPLS, and VPWS, from Internet-based options such as broadband, LTE, and VPN overlays. Layer 3 MPLS VPN gives the provider responsibility for routing between customer sites and is commonly used when private any-to-any routing, QoS treatment, and segmentation are required. Layer 2 services such as VPWS and VPLS extend customer Layer 2 adjacency across a provider network; VPWS is point-to-point, while VPLS is multipoint. Internet and cellular links are typically lower-cost and fast to deploy, but they require overlay security, such as IPsec, DMVPN, or SD-WAN encryption, when used for enterprise private traffic. The selected mapping therefore aligns each description with the WAN category that actually provides that behavior. In design work, the architect must compare latency, resiliency, SLA, QoS preservation, encryption requirements, routing control, and operational ownership before selecting the WAN type. Reference topics: enterprise WAN connectivity, MPLS VPN, VPLS, VPWS, Internet VPN, cellular backup.

gRPC provides efficient integration by using protocol buffers to serialize and deserialize structured data over the network. In network programmability and telemetry designs, gRPC is commonly used as the transport framework for streaming model-driven telemetry or for remote procedure calls between collectors, controllers, and network devices. Protocol buffers define compact structured messages that are faster and less ambiguous than parsing text output. This makes gRPC suitable for high-volume telemetry and programmatic device interaction. LDAP is a directory and authentication-related protocol, not the integration mechanism provided by gRPC. XMPP is associated with messaging and presence systems, not gRPC data serialization. Graph API is not a gRPC feature. The operational value of gRPC is that it provides a modern RPC framework with efficient message encoding, service definitions, and support for secure transport through TLS when implemented by the platform. Reference topics: gRPC, protocol buffers, model-driven telemetry, network programmability, structured data serialization. It improves controller interoperability.

Dual stack is the correct migration strategy when the application chooses IPv6 or IPv4 based on DNS resolution. In a dual-stack deployment, hosts and network devices run IPv4 and IPv6 at the same time. Applications can query DNS and then select the protocol based on the returned records, such as using an AAAA record for IPv6 or an A record for IPv4. This gives the organization a controlled migration path because IPv6-capable applications and services can move to IPv6 while legacy IPv4 services remain reachable during the transition. Address-family translation for public web presence is useful when IPv6-only and IPv4-only endpoints must communicate through a translation boundary, but it does not provide native application choice between the protocols. Host-initiated tunnels and site-to-site IPv6-over-IPv4 tunnels encapsulate one protocol over another and add operational overhead. The question’s key phrase is that the application chooses between IPv6 and IPv4 based on DNS; that is the normal dual-stack behavior. Reference topics: IPv6 migration, dual stack, DNS A and AAAA records, transition strategy, application protocol selection.

MSDP is the correct solution because the question explicitly references RFC 3618, which defines the Multicast Source Discovery Protocol. The design has separate PIM Sparse Mode multicast domains, with RP functions already configured on edge routers in each autonomous system. Cisco explains that MSDP allows rendezvous points in different PIM-SM domains to discover active multicast sources outside their local domain. When a source becomes active, the local RP can advertise source-active information to MSDP peers, allowing receivers in another domain to learn and join the stream. PIM-SSM is not correct because SSM removes the RP dependency and requires receivers to know the source directly. MP-BGP can carry multicast routing information, but it does not perform RP-to-RP source discovery. BIDIR-PIM is a multicast mode for many-to-many shared-tree forwarding, not an interdomain RP discovery mechanism under RFC 3618. Reference topics: MSDP, RFC 3618, PIM Sparse Mode, interdomain multicast, Source-Active messages, rendezvous points.