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

Bandwidth percentage with policing is the correct QoS approach. The requirement has two separate parts: each subnet must receive a minimum share during congestion, and no subnet should introduce delay into download traffic. Bandwidth percentage is used to allocate a minimum bandwidth guarantee among classes during congestion while still allowing a class to use unused bandwidth when other classes are idle. Policing enforces a rate by dropping or remarking excess traffic rather than buffering it. That behavior is important because the question explicitly says download traffic must never experience delay. Shaping would be wrong because shaping buffers excess packets and transmits them later, which intentionally adds delay. Rate- limiting is a broad term, but when the design choice offers policing explicitly, policing is the precise mechanism that matches the no-delay requirement. A parent or class policy can reserve 25 percent per subnet on a 40 Mbps service, giving each subnet 10 Mbps during congestion while unused bandwidth remains available. Reference topics: CBWFQ bandwidth percent, traffic policing, shaping versus policing, Internet edge QoS, congestion management.

Loop guard should be used on SW2 to reduce the chance of Layer 2 forwarding loops caused by a unidirectional fiber failure. In spanning-tree operation, a root or alternate port expects to receive BPDUs from the designated bridge. If BPDUs stop arriving because the fiber becomes unidirectional, the port may incorrectly assume that the path is no longer receiving a superior BPDU and transition toward forwarding. Cisco loop guard prevents this by moving the port into a loop-inconsistent state when BPDUs are unexpectedly lost on a non-designated port. That behavior protects the topology from a port role change that could create a Layer 2 loop. BPDU filter is dangerous in this context because it can suppress BPDUs and hide the control-plane signal STP needs. BPDU guard is mainly for PortFast edge ports that should never receive BPDUs. Root guard prevents superior BPDUs from changing root placement, but it does not address unidirectional loss of expected BPDUs. Therefore, loop guard on SW2 is the proper design control. Reference topics: STP loop guard, unidirectional fiber failure, BPDU loss, loop-inconsistent state, Layer 2 resiliency.

IntServ with RSVP is the correct model when an application must request a specific service level and receive guaranteed bandwidth or delay treatment. Cisco describes the Integrated Services model as a flow-based QoS architecture in which applications signal their resource requirements to the network. RSVP is the signaling protocol used to reserve resources along the path. This differs from DiffServ, which marks packets and applies per-hop behavior without creating per-flow reservations. The question states that the application can request a particular type of service to support its delay requirements and that bandwidth must be guaranteed. Those are classic IntServ and RSVP characteristics. DiffServ with DSCP is scalable and widely used in enterprise networks, but it cannot guarantee resources end to end unless every hop is engineered and provisioned accordingly. IntServ with DSCP is not the standard model because RSVP is the reservation mechanism. Reference topics: Integrated Services, RSVP, resource reservation, guaranteed service, QoS architecture, delay-sensitive applications.