Cisco Certified Network Associate (CCNA) 200-301 Practice Test 51
Question 500
A Windows client leases a correct `10.30.0.0/24` address, but its DHCP-provided option 3 points to obsolete gateway `10.30.0.254` rather than the current SVI `10.30.0.1`. On-LAN peers work; off-LAN IP literals do not. Which defect explains this?
- The DHCP pool address prefix is necessarily wrong because the host leased an address inside the expected /24.
- DNS alone is causing failure to reach a numerical remote IPv4 address.
- DHCP is delivering an incorrect default-router (option 3) value even though the lease address itself is in the right subnet.
- A valid DHCPOFFER guarantees that all later gateway options are correct.
Explanation
Correct Answer
C:Option 3 supplies the client's default gateway. Local traffic can be ARPed directly on the /24, but off-subnet forwarding uses that incorrect next hop; correct the server scope option and renew affected leases.
Incorrect Answers
A:The address portion is correct by evidence; the reported fault lies in gateway option 3.
B:DNS is bypassed when testing IP literals, and the wrong default gateway directly explains the routed failure.
D:A lease can complete while carrying an obsolete or wrong router option.
Question 501
Clients in VLAN 60 broadcast DHCP Discover frames. The DHCP server is on another subnet at 10.50.0.20, and the VLAN 60 SVI has `ip helper-address 10.50.0.20`. Which evidence most directly confirms that the relay forwards the clients' requests to the server?
- An NTP association on the router shows an asterisk next to its selected time peer.
- The switch MAC address table contains the client's source Ethernet address on an access port.
- A server-side DHCP capture shows the relayed BOOTREQUEST with the VLAN 60 gateway's nonzero giaddr.
- The router's OSPF neighbor database contains a Full state on its uplink.
Explanation
Correct Answer
C:A DHCP relay listens for client broadcasts on the VLAN interface and forwards them toward the specified server. For a normal relay, the `giaddr` field identifies the relay's gateway address for the client network, enabling the server to choose an appropriate scope and return the offer through the relay. Seeing a forwarded BOOTREQUEST at 10.50.0.20 with the VLAN 60 gateway address in `giaddr` directly tests the helper's forwarding function. Then confirm the reply returns and the client receives valid address, gateway and DNS options. Do not confuse packet arrival at the server with completed lease acquisition; they are sequential checks.
Incorrect Answers
A:NTP synchronization helps correlate timestamps but has no direct relationship to whether DHCP Discover broadcasts cross a routed boundary through a helper. An asterisk in `show ntp associations` can prove a selected time peer, not a DHCP relay transaction. Using this evidence alone would leave the original lease request problem untested and could mask relay or return-routing faults.
B:A learned MAC entry can show that a client's frame arrived on the local VLAN and that Layer 2 learning works. It does not prove the broadcast was relayed as a unicast/relayed DHCP request to a server on another IP subnet. The relay could be missing, filtered or aimed at the wrong server while local switching still learns the client's MAC normally.
D:OSPF Full state indicates a control-plane adjacency with another router, not that DHCP broadcast relay configuration is correct or that DHCP packets reach a specific destination. A working route toward the server may be necessary for the relay's IP packet, but that does not prove the helper address is applied to the correct VLAN interface. Use a DHCP packet trace or server log to test this mechanism directly.
Question 502
A router's WAN interface should obtain IPv4 by DHCP but previously had no lease because `ip address dhcp` was absent. After correction, which operational output directly validates the interface is ready to route?
- The interface is up/up with a valid DHCP-leased address/mask and the expected gateway or installed default route when supplied by service.
- The interface remains administratively down with no IPv4 lease, although DHCP is configured.
- The downstream VLAN's `ip helper-address` is present, while the router WAN has no address.
- A DNS A record resolves on a laptop without inspecting the router WAN IPv4 state.
Explanation
Correct Answer
A:Enabling DHCP-client behavior is not proof until the interface completes a lease exchange. Inspect its assigned IPv4 settings and route state as required; absent carrier or DHCP responses can still block the result.
Incorrect Answers
B:A configured client command cannot succeed over a disabled interface.
C:DHCP relay for clients does not provide the router interface's own required lease.
D:Unrelated name resolution does not demonstrate that this routed interface has obtained usable DHCP parameters.
Question 503
A campus switch receives IP-telephony RTP packets that should receive expedited forwarding treatment across its congested WAN uplink. The QoS design marks those packets with DSCP EF (46) and uses an appropriate constrained priority queue. Which policy pairing is consistent with the goal?
- Classify the RTP traffic, mark or preserve DSCP EF at the trust boundary, and provide bounded low-latency egress treatment.
- Mark all voice packets as DSCP 0 and assume priority queuing happens without a service policy.
- Use an ACL to deny all UDP traffic, because real-time voice should always use TCP retransmissions.
- Increase Ethernet port speed only, without validating packet marking or available WAN queue bandwidth.
Explanation
Correct Answer
A:Voice RTP is latency- and jitter-sensitive; QoS policies commonly identify the relevant flows and preserve or apply the appropriate DSCP marking at a trusted network boundary. DSCP EF is decimal 46 and is typically associated with expedited-forwarding behavior for delay-sensitive traffic when devices are configured to honor it. A bounded priority queue can reduce queuing delay under congestion while preventing the class from starving other traffic. Confirm both ingress trust/marking and egress scheduling with packet captures and policy counters. DSCP values request treatment; they do not themselves guarantee bandwidth or a working voice call end to end.
Incorrect Answers
B:DSCP 0 is best-effort marking rather than a claim for special low-latency treatment. Without a configured service policy and scheduler, changing a mark does not create a priority queue on a congested WAN interface. The requirement calls for verified classification, appropriate marking and bounded egress scheduling.
C:RTP typically uses UDP to avoid retransmission delays in real-time media, while application protocols may use other transport for signaling. Denying all UDP would disrupt the traffic instead of protecting its delay characteristics. QoS schedules eligible packets under congestion; it is not a replacement for RTP transport.
D:A faster local switchport does not change the constrained WAN uplink's egress rate or automatically preserve DSCP and queue treatment. If the bottleneck remains at the WAN edge, voice can still experience jitter and packet loss. Test the actual egress policy and class counters rather than only a local Ethernet speed.
Question 504
Voice RTP packets suffer queuing delay behind bulk backup traffic on a congested WAN egress. Which QoS scheduler choice directly reduces voice waiting time while controlling starvation risk?
- Classify voice into a properly bounded priority/low-latency queue, with suitable policing or capacity safeguards for other classes.
- Put voice and bulk backup traffic in the same FIFO queue and only rename their DSCP labels.
- Raise WAN physical speed in a diagram without checking scheduler or actual congested interface capacity.
- Configure a drop-only policer on voice to force every burst to disappear rather than get timely egress.
Explanation
Correct Answer
A:Low-latency priority service gives delay-sensitive traffic preferred egress treatment under congestion. The priority allocation must be engineered carefully so bulk traffic retains service and voice bursts do not consume all bandwidth.
Incorrect Answers
B:Marking alone does not guarantee different scheduling if the egress queue treats all packets identically.
C:A planned upgrade statement cannot by itself remove queue contention at the operating bottleneck.
D:Aggressive policer drops can worsen voice quality; the requirement is preferential low-delay service, not indiscriminate loss.
Question 505
A branch router sends bursts to a provider with a 20-Mbps committed egress rate. The carrier drops excess burst packets when the instantaneous transmission exceeds the contract. The enterprise can buffer traffic locally and wants to smooth bursts rather than immediately discard them. Which QoS mechanism best fits?
- Apply an ingress policer that drops every packet above 20 Mbps before the local egress queue.
- Disable IP fragmentation on all LAN clients to eliminate any possibility of provider congestion.
- Increase OSPF router priority on the uplink so the provider accepts bursts above the contract.
- Apply traffic shaping to the provider-facing egress at the agreed rate, using local buffering and controlled transmission.
Explanation
Correct Answer
D:Traffic shaping regulates the rate at which packets leave an egress interface and can queue bursts temporarily to meet a configured average or token-based contract. Here the enterprise controls the WAN-facing router, knows the carrier rate, and can buffer packets, so shaping near 20 Mbps is a natural method to reduce excess-rate drops downstream. Monitor shaper queue depth, delay, drops and achieved throughput; too much buffering can add latency, particularly for interactive classes, so pair shaping with appropriate class-based scheduling. This is different from a policer, which typically enforces a rate by dropping or remarking out-of-profile traffic rather than smoothing the burst through a queue.
Incorrect Answers
A:A policer commonly drops or remarks traffic that exceeds its configured allowance. It may be appropriate for strict enforcement, but the stated requirement explicitly favors queueing and smoothing bursts before the carrier link. Policing does not generally buffer an out-of-profile packet for later controlled egress; shaping does.
B:IP fragmentation behavior affects how packets fit a path's MTU and is not a rate-control mechanism for bursts above a contracted 20-Mbps service. Changing fragmentation policy cannot guarantee an acceptable average egress rate or avoid provider policer drops. Configure the egress shaper and monitor actual queue behavior.
C:OSPF router priority applies to designated-router election on relevant OSPF multiaccess interfaces; it does not negotiate a carrier contract or grant a higher physical throughput allowance. Even if a routing path changes, the subscribed rate remains. Shape packets at the provider-facing egress as specified.
Question 506
A WAN router receives packets marked DSCP EF for voice and AF classes for applications. Which interpretation of DSCP is correct when designing per-hop QoS?
- Any DSCP EF packet receives guaranteed zero delay end to end even when routers ignore QoS.
- DSCP requests a traffic class/per-hop treatment; each traversed device must implement suitable classification, queuing or drop policy for it to affect performance.
- DSCP values replace the destination IP address to route high-priority traffic.
- One correct DSCP marking automatically creates a priority queue on every ISP router in the path.
Explanation
Correct Answer
B:DSCP marking carries a codepoint in the IP header. A router can map it to per-hop behavior, but the mark alone does not provide a guaranteed delay or bandwidth contract across unmanaged network hops.
Incorrect Answers
A:A marking is a service request, not a universal bandwidth or latency guarantee.
C:DSCP influences QoS treatment, not Layer 3 destination addressing or route lookup.
D:Each hop needs its own QoS policy/trust and available capacity; remote autonomous networks can re-mark or ignore DSCP.
Question 507
VoIP endpoints send RTP packets marked DSCP EF (46) toward a Cisco access switch. A capture before the switch shows EF, but a capture after the switch's untrusted ingress boundary shows DSCP 0 on those same packets. Delay-sensitive calls lose priority despite sufficient link bandwidth. Which fault is most directly supported?
- The egress priority queue is misconfigured, even though the ingress DSCP marking is preserved.
- The WAN link is saturated solely because its nominal bandwidth is insufficient for the voice codec.
- A policer drops only traffic above a configured byte rate while retaining EF on forwarded packets.
- The switch does not trust or correctly reclassify ingress DSCP, rewriting the voice EF marking to best effort.
Explanation
Correct Answer
D:DSCP EF corresponds to code point 46 and is commonly used to identify latency-sensitive voice traffic. If a Cisco switch receives an EF-marked RTP packet on an untrusted port and the effective QoS policy maps or rewrites it to DSCP 0, downstream classification sees best effort instead of the intended expedited forwarding treatment. Compare captures on either side of ingress and inspect the trust or policy-map configuration to locate the rewrite. This is a fault diagnosis; its paired question verifies restored marking after the boundary.
Incorrect Answers
A:An egress scheduling problem can cause jitter even when packets retain EF markings, but the specific evidence here is a change from DSCP 46 before ingress to DSCP 0 afterward. A queue misconfiguration alone would not establish why the packet's code point was rewritten. The first corrective investigation should focus on the ingress trust boundary, policy map, and classification or marking operation. After restoring the appropriate DSCP value, the team should separately confirm that the egress queue acts on it.
B:Link congestion may contribute to VoIP delay, but the problem gives a packet-marking discontinuity at the switch ingress rather than evidence of utilization saturation. Sufficient bandwidth does not guarantee low latency, yet that fact does not explain why EF becomes zero. Changing link capacity or codec assumptions would leave the observed QoS trust-boundary behavior uncorrected. Trace the packet's DSCP value through the actual interface policy before considering a capacity expansion.
C:Policing can discard packets that exceed a contracted rate and affect voice quality, but the described captures show forwarded packets whose DSCP was changed to zero. This points to marking or trust behavior, not necessarily an over-limit policer drop. A policer might also remark traffic depending on policy, but no such explicit action is stated. Verify the policy's classify, set, trust, and drop counters; the before/after DSCP rewrite is the immediate evidence to explain.
Question 508
Voice RTP jitter increases only when a backup upload fills a WAN interface. Voice and backups use the same FIFO queue and the link still has normal nominal bandwidth. What is the direct cause indicated?
- The voice endpoint must have the wrong DNS A record because jitter appears only during heavy uploads.
- The switch STP root ID changes on every RTP packet and causes queueing only during backup uploads.
- Voice frames wait behind bulk packets in the congested egress queue because no effective priority scheduling separates the classes.
- QoS is already proven effective because both classes have the same FIFO queue.
Explanation
Correct Answer
C:Queue congestion can create variable delay even when the nominal link speed seems adequate at other times. Separate queue treatment for latency-sensitive RTP and capacity management are more relevant than global interface speed alone.
Incorrect Answers
A:A DNS mapping problem would not naturally produce load-dependent output queue delay on an established RTP flow.
B:STP root election is not the scheduler mechanism implied by egress FIFO contention.
D:A shared FIFO provides no preferential service for voice, which is the observed gap.
Question 509
An interface policy drops packets as soon as a configured traffic rate is exceeded. Operators expected it to buffer bursts and release them more slowly. Which QoS mechanism did they confuse?
- A policer enforces a rate by dropping or re-marking excess traffic, whereas a shaper buffers eligible excess and paces transmission.
- Policing always buffers excess packets until link idle, exactly like egress shaping.
- Traffic shaping always drops the first packet of each conforming burst without any queueing.
- A DNS resolver TTL setting decides whether the router buffers or polices WAN packets.
Explanation
Correct Answer
A:Policing compares traffic against a token/rate contract and commonly drops packets beyond it; shaping queues and schedules transmissions to smooth egress. A drop-only policer cannot provide the smoothing behavior expected from a shaper.
Incorrect Answers
B:That confuses the two mechanisms. Policing can discard excess rather than retain it for later transmission.
C:Shaping is intended to smooth traffic rate using buffering and transmission timing, subject to queue limits.
D:DNS caching is independent of QoS traffic-metering and queue disciplines.
Question 510
During congestion, a WAN edge marks voice packets DSCP EF (46), but voice delay and drops remain high. The output policy has no priority queue configured for that traffic, and egress queue-drop counters rise. Which change addresses the observed forwarding problem?
- Configure an appropriate egress low-latency priority treatment for classified voice, with policing or bounds as required.
- Change all voice packets from DSCP EF to CS0 without adjusting egress scheduling.
- Add a DHCP relay helper address on the outgoing WAN interface.
- Enable PortFast on the Ethernet interface carrying the WAN traffic.
Explanation
Correct Answer
A:Classification and marking identify traffic, but they do not by themselves create an egress scheduling policy. Here the packets are correctly marked EF yet suffer congestion loss, and rising queue-drop counters support a data-plane queueing problem. An appropriately sized, platform-supported low-latency priority class in the egress service policy gives eligible voice packets expedited treatment, subject to bandwidth controls that prevent starvation. Verify policy attachment, class counters, offered load, drops and latency after adjustment. Do not promise zero loss under all circumstances; a priority queue still depends on capacity, correct classification and policers. The decisive distinction is between DSCP metadata and the actual per-hop behavior configured on the congested exit link.
Incorrect Answers
B:Remarking EF to default best effort removes the intended differentiated treatment and does not add a queue scheduler. Packets would still compete with bulk transfers in congested output queues and could experience comparable or worse delay. Reclassification could be appropriate if current markings are falsely trusted, but the scenario establishes legitimate voice packets with EF and a missing priority class.
C:DHCP relay forwards client address-assignment requests across IP subnets. It has no role in selecting which data packets depart a congested WAN queue first. The observed EF markings and queue-drop counters concern traffic scheduling, not missing DHCP offers or leases. Configuring a helper on the WAN link would not allocate bandwidth or protect voice latency.
D:PortFast influences Layer 2 spanning-tree transitions for genuine edge access ports. It is unrelated to per-class egress queuing on a WAN bottleneck. Enabling it on an uplink may be inappropriate and could weaken loop-avoidance expectations. The needed verification is on the QoS output policy and queue counters, not the spanning-tree edge timer.
