300-420 Exam Dumps : Designing Cisco Enterprise Networks

Root guard is the correct feature when the design must allow switches to be attached for testing but prevent those switches from influencing the spanning-tree root placement. Cisco root guard protects the intended STP root by placing a port into a root-inconsistent state if superior BPDUs are received on a port where the root should never appear. This directly addresses the risk created by rogue or lab switches connected by end users: if a user switch advertises a better bridge priority, the production access topology is not allowed to reconverge around that unauthorized device. BPDU guard is usually used on PortFast edge ports to err-disable a port that receives any BPDU, which is excellent for strict host-only ports but less aligned with the wording that users may connect their own switches for testing. Loop guard protects root or alternate ports from losing BPDUs and accidentally forwarding. BPDU skew detection is a diagnostic feature, not the correct design control. Therefore, root guard is the appropriate protection against rogue switches affecting STP topology. Reference topics: STP root placement, root guard, superior BPDUs, campus Layer 2 protection.

A /22 subnet is the smallest IPv4 block in the answer set that closely fits a branch expected not to exceed roughly one thousand users while avoiding unnecessary address waste. A /22 contains 1024 total addresses and 1022 usable host addresses after the network and broadcast addresses are reserved. That aligns with the requirement to provide the maximum number of host addresses without selecting a larger block than necessary. A /21 provides 2048 total addresses and 2046 usable hosts, which is materially larger than the stated requirement and wastes address space. The specific 192.168.8.0/22 block also aligns on a valid /22 boundary because /22 networks increment by four in the third octet. A 192.168.16.0/22 is also mathematically valid, but the exhibit determines the available address range; the selected option uses the appropriate block from the displayed plan. Reference topics: IPv4 subnet sizing, CIDR, usable host calculation, branch address planning, VLSM.

A totally stubby OSPF area is the best choice for a small remote site with low-end routers and no requirement to carry transit traffic. A standard stub area blocks Type 5 external LSAs, but it still receives interarea Type 3 summary LSAs. A totally stubby area goes further by blocking external routes and most interarea summary routes, while allowing a default route to be injected by the ABR. That greatly reduces LSDB size, SPF processing, memory use, and routing-table entries on the low-end routers. The remote site still has full connectivity to the rest of the OSPF domain through the default route. NSSA and totally NSSA designs are useful when the area must inject external routes from an ASBR inside the area, but the question does not state that the remote site needs redistribution. A regular stub area is acceptable but does not minimize resources as much as a totally stubby area. Because there is no demand for traffic to pass through this area, the most efficient Cisco design is totally stubby. Reference topics: OSPF stub areas, totally stubby area, default routing, LSDB reduction, branch OSPF design.