Cisco 300-420 Exam Dumps

The correct policy is to prepend the AS path on the nonpreferred advertisement for each prefix. For inbound BGP traffic engineering, the enterprise normally influences remote autonomous systems by changing attributes that those systems see when selecting a path. AS-path prepending makes a route appear less attractive by adding repeated instances of the local AS number to the AS path. In this design, network 10.1.1.0/24 should enter through R3-R1, so R2 must advertise that prefix with a prepended AS path to make the R4-R2 path less preferred unless the primary path fails. Conversely, network 10.1.2.0/24 should enter through R4-R2, so R1 must advertise that prefix with prepending to make the R3-R1 path less preferred. This creates inbound load sharing while preserving failover because the prepended route remains available as a backup. Local preference affects outbound path selection inside the local AS, not inbound selection by the provider. Reference topics: BGP traffic engineering, AS-path prepending, inbound path control, prefix-specific policy, multihomed WAN design.

PIM-SSM with IGMPv3 is the correct multicast design. Source-Specific Multicast requires receivers to identify both the multicast group and the permitted source, normally through IGMPv3 membership reports. That source-specific join model prevents unauthorized or spoofed sources from injecting traffic into the group because receivers do not simply accept traffic from any source for a group address. It also improves bandwidth efficiency because the network builds forwarding state only for the requested source and group pair, without requiring shared trees or rendezvous point discovery. PIM-SM with MSDP can interconnect multicast domains, but it is built around Any-Source Multicast behavior and does not provide the same source-specific anti-spoofing property. IGMPv2 cannot signal source-specific joins, which is why IGMPv3 is required for SSM. Since the network may merge with another multicast domain later, SSM also reduces integration complexity by avoiding RP and MSDP dependencies. Therefore, PIM-SSM and IGMPv3 meet both the security and efficiency requirements. Reference topics: PIM-SSM, IGMPv3, source-specific joins, multicast security, bandwidth efficiency.

Cisco Express Forwarding uses the adjacency table to store the Layer 2 rewrite information needed to forward packets efficiently. CEF separates forwarding decisions into the Forwarding Information Base and the adjacency table. The FIB identifies the best next hop based on routing information, while the adjacency table contains the directly connected next-hop details, including the MAC address and encapsulation rewrite string used on the outgoing interface. That is why the correct layer is Layer 2. CEF is not using the adjacency table to populate Layer 3 routes; the routing table and FIB handle Layer 3 reachability. It is also not a Layer 1 function because physical signaling does not determine the next-hop rewrite. Layer 4 is unrelated to CEF adjacency resolution. In a campus or WAN design, CEF matters because it allows hardware or optimized software forwarding without process-switching every packet. Correct ARP, neighbor discovery, and adjacency formation are therefore essential to fast packet forwarding, especially when convergence or high-throughput forwarding is required.

The Cisco SD-Access underlay provides the physical and IP transport foundation that allows fabric nodes to reach each other. In plain terms, it is the routed connectivity built from the switches, routers, links, and loopback reachability that the overlay depends on. Cisco SD-Access then uses the overlay to provide virtualization, segmentation, policy, and endpoint mobility on top of that transport. This is why the underlay is best described as establishing the connectivity between the switches and routers that form the fabric. Option A describes the abstraction performed by the overlay, not the underlay. Option B is closer to a Layer 2 tunneling description and is not the main purpose of the SD-Access underlay. Option D describes encapsulated overlay behavior, especially VXLAN-based data-plane transport, not the physical/routed foundation itself. A good SD-Access design validates underlay reachability, routing adjacencies, MTU, loopback advertisement, and redundancy before onboarding the overlay services. Without a stable underlay, LISP control-plane mappings and VXLAN data-plane forwarding cannot operate predictably. Reference topics: SD-Access underlay, routed campus foundation, fabric node reachability, overlay dependency.

Multiple Spanning Tree is the correct technology for a large Layer 2 domain with hundreds of switches and VLANs when the goals are efficient link use, loop prevention, and low CPU and memory impact. MST maps multiple VLANs into a smaller number of spanning-tree instances, allowing the designer to load-balance groups of VLANs across alternate paths without running a separate STP instance for every VLAN. Cisco Rapid PVST+ improves convergence compared with classic STP, but it maintains a separate instance per VLAN, which becomes costly at scale. PVST+ has similar per-VLAN scaling concerns and slower convergence behavior than rapid variants. Plain RSTP improves convergence but does not provide the same VLAN-to-instance mapping flexibility for hundreds of VLANs. MST is standards-based and particularly useful in multivendor environments where VLAN grouping and predictable root placement are required. The design should define MST regions consistently, map VLANs deliberately to instances, place roots near the Layer 3 gateway, and keep the number of instances small enough to protect supervisor CPU and memory. Reference topics: MST, RSTP, Rapid PVST+, VLAN-to-instance mapping, Layer 2 scalability.