350-501 Practice Questions

Question 1

A customer is experiencing intermittent packet loss in an MPLS L3VPN network. The PE routers show no errors on interfaces. Which tool can help isolate the issue by showing the complete path a packet takes through the MPLS network?

Accepted Answer

MPLS traceroute. MPLS traceroute (often implemented via LSP ping with the 'traceroute' option or the 'mpls traceroute' command) is the correct tool because it uses MPLS echo request/reply messages (RFC 8029) to traverse the label-switched path (LSP) hop by hop, revealing the exact label stack and transit LSRs. This allows identification of a failing or misbehaving node in the MPLS core that standard IP traceroute would not see, since IP traceroute relies on TTL expiry in the IP header, which may not occur in an MPLS network where labels are swapped and IP TTL is often copied from the label TTL.

Answer

Traceroute

Answer

Ping

Answer

MPLS ping

Question 2

A network engineer is troubleshooting an OSPF adjacency failure between two directly connected routers, R1 and R2. Both routers are configured with the same OSPF process ID and area. The engineer verifies that the interfaces are up and IP connectivity exists. Which configuration mismatch is most likely causing the adjacency to fail?

Accepted Answer

OSPF network type mismatch between the interfaces. An OSPF network type mismatch, such as one interface configured as point-to-point and the other as broadcast, prevents OSPF adjacency from forming. Options A, B, and C are incorrect: An MTU mismatch (A) usually allows adjacency to form but causes problems during LSA exchange; a passive interface (B) stops the router from sending hellos, so it wouldn't see the neighbor at all, but the question implies both interfaces are operational; an area ID mismatch (C) would prevent adjacency, but the scenario specifies that both routers are configured in the same area. Therefore, the most likely cause of the adjacency failure when interfaces are up and IP connectivity exists is a network type mismatch.

Answer

MTU mismatch between the interfaces

Answer

Passive interface configuration on one router

Answer

Area ID mismatch on the interfaces

Question 3

Which TWO are benefits of using Segment Routing (SR) over traditional LDP-based MPLS?

Accepted Answer

Eliminates the need for LDP and RSVP-TE protocols.. Segment Routing (SR) eliminates the need for the Label Distribution Protocol (LDP) and Resource Reservation Protocol with Traffic Engineering (RSVP-TE) by encoding MPLS labels directly into the Interior Gateway Protocol (IGP), such as OSPF or IS-IS. This simplifies the control plane by removing these protocols entirely, reducing operational complexity and resource overhead.

Answer

Requires only OSPF as the IGP.

Answer

Reduces label imposition at the ingress PE.

Answer

Faster convergence due to BGP PIC.

Question 4

In the DiffServ model, which DSCP value is used for Voice over IP (VoIP) traffic to ensure Expedited Forwarding (EF) per-hop behavior?

Accepted Answer

DSCP 46 (EF). EF PHB is used for low-loss, low-latency traffic such as voice. The recommended DSCP value for EF is 46 (binary 101110).

Answer

DSCP 34 (AF41)

Answer

DSCP 26 (AF31)

Answer

DSCP 0 (Best Effort)

Question 5

Refer to the exhibit. The show command output displays the LDP neighbor state. What does the 'Downstream' label mode indicate about label distribution?

Accepted Answer

Labels are advertised to all neighbors without a request.. In the context of LDP, 'Downstream' label mode means that a router advertises label bindings for all prefixes in its routing table to its LDP neighbors without waiting for an explicit request. This is the default label distribution mode for LDP, where labels are proactively sent to all peers. Option D correctly identifies this behavior.

Answer

Labels are sent only to the peer that advertises the prefix.

Answer

Labels are only distributed when explicitly requested.

Answer

The neighbor must request labels via a Label Request message.

Question 6

A service provider is running OSPF in their core network. After a link failure, routing convergence is taking longer than expected. The engineer suspects the issue is related to the SPF calculation. Which OSPF feature can reduce the SPF calculation time?

Accepted Answer

Incremental SPF (iSPF). Incremental SPF (iSPF) reduces SPF calculation time by recalculating only the affected portion of the Shortest Path Tree (SPT) after a topology change, rather than recomputing the entire tree. This is especially beneficial in large OSPF networks where full SPF runs are CPU-intensive and slow convergence.

Answer

OSPF database overload protection

Answer

Bidirectional Forwarding Detection (BFD)

Answer

LSA pacing timers (pacing-interval)

Question 7

A network engineer is troubleshooting QoS in a service provider environment. Customer traffic is marked with DSCP AF31 (011010) at the CE. On the PE router, the policy maps trust DSCP and sets the CoS to 4. However, core routers remark the DSCP to 0. What is the most likely cause?

Accepted Answer

A policer on the core router is marking down out-of-contract traffic. The core router is likely applying a policer that marks down traffic exceeding the committed information rate (CIR). When traffic is out-of-contract, the policer can re-mark the DSCP to 0 (best effort), which overrides the trusted DSCP AF31 and the CoS 4 set at the PE. This behavior is common in service provider environments where core routers enforce traffic contracts and use policing to penalize excess traffic.

Answer

The core routers use LDP labels and ignore DSCP

Answer

The ingress PE did not set MPLS EXP bits

Answer

The MPLS EXP bits are not copied from DSCP

Question 8

In a hierarchical QoS policy applied to a subscriber aggregation interface, the parent policy shapes to 100 Mbps and the child policy allocates 50% bandwidth to voice and 50% to data. If voice traffic exceeds 50 Mbps, what happens to the excess?

Accepted Answer

Excess voice is dropped or re-marked. The child policy's police command within LLQ drops or re-marks voice traffic exceeding the configured bandwidth to prevent starvation of other queues.

Answer

Excess voice is queued in the data queue

Answer

Excess voice is shaped by the parent policy

Answer

Excess voice is forwarded with best effort

Question 9

Which access technology is commonly used in service provider networks to deliver high-speed internet over existing telephone lines?

Accepted Answer

DSL. DSL (Digital Subscriber Line) uses existing telephone lines to provide high-speed internet access.

Answer

Metro Ethernet

Answer

GPON

Answer

Cable

Question 10

Which RSVP-TE filter style is commonly used for MPLS-TE tunnels to allow multiple senders on the same session?

Accepted Answer

Shared Explicit (SE). Shared Explicit (SE) allows multiple senders, each with its own filter spec, but they share the same reservation. Fixed Filter (FF) is per sender. The question likely refers to SE style.

Answer

Distinct Explicit (DE)

Answer

Fixed Filter (FF)

Answer

Wildcard Filter (WF)

Question 11

Which TWO statements about model-driven telemetry compared to SNMP are correct? (Choose two.)

Accepted Answer

Model-driven telemetry can stream data at sub-second intervals.. Model-driven telemetry (MDT) uses a push model where the network device streams structured data (e.g., YANG-modeled) to a collector, enabling sub-second intervals for real-time monitoring. This contrasts with SNMP's pull model, where the manager polls the device at intervals limited by CPU and network overhead, making sub-second polling impractical.

Answer

Model-driven telemetry uses UDP for transport by default.

Answer

Model-driven telemetry only sends data on change (event-driven).

Answer

Model-driven telemetry uses MIBs to define data structures.

Question 12

An SP uses model-driven telemetry to monitor queue depths on core interfaces. They observe periodic spikes in the queue depth for EF traffic, causing increased latency. Which automation technique could dynamically adjust the QoS policy to mitigate the spikes?

Accepted Answer

Use a Python script that consumes telemetry data and adjusts the EF bandwith percentage via NETCONF when queue depth exceeds a threshold.. It combines model-driven telemetry (to detect queue depth spikes in real time) with a closed-loop automation approach: a Python script consumes the telemetry data, and when the EF queue depth exceeds a threshold, it dynamically adjusts the EF bandwidth percentage via NETCONF. This directly addresses the periodic spikes by modifying the QoS policy on the fly, reducing latency without manual intervention.

Answer

Deploy NETCONF YANG modules for VRF configuration.

Answer

Configure WRED on the EF queue.

Answer

Implement streaming telemetry to collect data every 5 seconds.

Question 13

An SP is troubleshooting an MPLS L2VPN VPLS network where MAC flapping is occurring between two PEs. Which mechanism in VPLS prevents loops and ensures that a broadcast frame from one PE is not reflected back to the originating PE?

Accepted Answer

Split horizon. In a VPLS environment, split horizon is the mechanism that prevents loops by ensuring that a broadcast, multicast, or unknown unicast frame received from a pseudowire (PW) is never forwarded back to any other PW on the same PE. This rule, defined in RFC 4761 and RFC 4762, stops the originating PE from receiving its own reflected frames, thus eliminating the MAC flapping that would occur if the frame were looped through the MPLS core.

Answer

Spanning Tree Protocol (STP)

Answer

Rapid Spanning Tree Protocol (RSTP)

Answer

MAC address aging

Question 14

An SP is deploying MPLS in the core/backbone to support L3VPN services. What is the purpose of MP-BGP in an MPLS VPN architecture?

Accepted Answer

To exchange customer VPN routes and VPN labels between PE routers. MP-BGP (Multiprotocol BGP) carries VPNv4 routes between PE routers, including the VPN label, enabling customer isolation and reachability across the MPLS core.

Answer

To forward data packets across the MPLS core

Answer

To establish IGP adjacencies between PE and CE routers

Answer

To distribute MPLS labels for LSPs

Question 15

An engineer configures an SR-TE policy with a segment list containing adjacency SIDs. The required path includes a strict loose hop through a specific router. Which segment type should be used for the strict hop?

Accepted Answer

Adjacency SID for the desired link.. Adjacency SIDs force the packet to traverse a specific link. For a strict hop, the adjacency SID ensures the packet goes through that exact link. A Node SID would allow any path to the node.

Answer

Anycast SID for the group containing the router.

Answer

Binding SID (BSID) of another policy.

Answer

Node SID (Prefix SID) of the router.

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