300-420 Exam Dumps : Designing Cisco Enterprise Networks

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.

BGP outbound route filtering is the correct solution because the enterprise wants control over which routes it receives without waiting for a provider change cycle each time the prefix set changes. Cisco BGP prefix-based ORF allows a receiving BGP speaker to send an inbound prefix-list filter to the remote peer, which then applies that list as an outbound filter. This reduces unwanted routing updates before they cross the session and lets the enterprise update the desired prefixes locally when business requirements change. Requesting only fixed prefixes from the ISP would work initially but fails the flexibility requirement because the provider has a two-week change process. Requesting a full table and filtering inbound would provide flexibility but wastes memory and CPU on the customer router because all routes are still received before filtering. A maximum-prefix limit does not select the correct routes. ORF is therefore the scalable operational design for receiving a default route and selected prefixes with minimal provider involvement. Reference topics: BGP ORF, prefix-based filtering, inbound route control, ISP peering design, routing table scale.

Traffic shaping is required in the parent policy when hierarchical QoS uses CBWFQ in the child policy for low-bandwidth remote sites. In Cisco hierarchical QoS, the parent policy commonly shapes outbound traffic to the real available rate of the WAN service, especially when the physical interface speed is higher than the provider committed rate. The child policy then applies class-based queuing, such as CBWFQ and LLQ, inside the shaped rate. This structure creates an artificial congestion point on the router so the child queues can schedule packets predictably instead of allowing the provider to drop traffic after it exceeds the service rate. Policing in the child policy would drop excess packets and is not the normal prerequisite for nested CBWFQ. CBWFQ is supported in child policies within hierarchical designs, and parent shaping is the mechanism that makes the hierarchy effective. This is especially important on low-bandwidth WAN links where voice, video, and business-critical applications require deterministic treatment. Reference topics: hierarchical QoS, CBWFQ, parent shaping, child queuing, WAN QoS design.