Cisco Updated 300-420 Exam Questions and Answers by dhruv

When eBGP neighbors share the same autonomous system number, normal BGP loop-prevention behavior can block route acceptance because the receiving router sees its own AS in the AS_PATH. Two BGP features address this design condition. The allow-as-in feature permits a BGP router to accept routes even when its own AS appears in the AS_PATH a configured number of times. This is often used in dual-homed or migration designs where the same customer AS appears at multiple sites. The as-override feature is typically used by a provider edge router to replace the customer AS in the AS_PATH with the provider AS before advertising the route to another customer site that uses the same AS. Together, these features allow successful route exchange in same-AS eBGP scenarios while preserving loop-prevention intent through controlled policy. Advertise-best-external influences advertisement of best external paths and does not solve same-AS rejection. Bestpath as-path ignore affects path selection, not route acceptance. Client-to-client reflection is an iBGP route-reflector behavior, not an eBGP same-AS solution.

When BFD echo mode contributes to high CPU utilization on lower-resource devices, the practical design response is to use slower BFD timers for those peers rather than continuing to run an overly aggressive failure-detection profile everywhere. Cisco BFD provides fast failure detection, but timer values must match platform capability, interface scale, and the operational value of subsecond detection. Echo mode can generate frequent packets, and in some environments the processing or punt behavior can create measurable CPU pressure. Slowing the timers reduces packet rate and control-plane stress while keeping BFD available for faster detection than traditional routing protocol dead timers. Moving to asynchronous mode may be appropriate in some designs, but the listed option says “BED” and does not directly address the requirement as cleanly as reducing timer aggressiveness. BFD multihop is used for non-directly connected peers and is not a CPU-relief mechanism. Carrier delay changes interface state notification behavior, but it does not solve BFD packet processing load. Therefore, implement slower timers on peers with limited resources. Reference topics: BFD design, echo mode, failure-detection timers, CPU utilization.

In Cisco SD-Access, an intermediate node is an underlay transit device. Its responsibility is to provide IP reachability between fabric edge nodes, border nodes, and other fabric infrastructure roles. Intermediate nodes forward routed underlay traffic and do not perform endpoint registration, endpoint lookup, or VXLAN encapsulation for user traffic. Endpoint-to-location mapping is handled through the fabric control plane, which is based on LISP map-server and map-resolver functions. Fabric edge nodes identify connected endpoints, apply policy, act as anycast gateways, and encapsulate user traffic into VXLAN. Border nodes connect the fabric to external networks and handle traffic entering or leaving the fabric. The intermediate node is therefore comparable to a routed distribution or core transport device in a traditional hierarchical campus, except it is participating in the SD-Access underlay. It must provide stable Layer 3 reachability and appropriate MTU for encapsulated traffic but does not maintain the host tracking database or add security group information to VXLAN headers. The correct function is transporting IP packets between edge and border nodes.

Policing is the correct mechanism for managing traffic that exceeds the configured threshold. The application is UDP-based, inelastic, and sensitive to delay and jitter, so excess traffic should not be buffered and transmitted later. Cisco defines traffic policing as measuring traffic against a configured rate and applying an action such as transmit, drop, or remark when traffic conforms, exceeds, or violates the policy. In contrast, shaping buffers excess traffic and releases it later to smooth the output rate. That behavior is useful for TCP or provider handoff control, but it can add delay and jitter, which is harmful for inelastic real-time UDP flows. Scheduling has already been addressed by allocating bandwidth and assigning queues; it decides how queues are serviced, not how excess flows are constrained. Remarking changes packet markings but does not by itself enforce a traffic threshold. Therefore, policing is the appropriate QoS conditioning action for traffic above the configured limit. Reference topics: traffic policing, QoS conditioning, shaping versus policing, UDP real-time applications, delay and jitter control.