350-501 Practice Exam 57 — 20 Questions

Question 1

A service provider is designing a new MPLS core network using Segment Routing with MPLS data plane. They require traffic engineering capabilities to optimize bandwidth utilization. Which technology should be used to compute optimal paths based on IGP link attributes and bandwidth constraints?

Choose an answer to begin — your selection is scored in the full session.

Accepted Answer

SR-TE (Segment Routing Traffic Engineering). SR-TE (Segment Routing Traffic Engineering) is the correct choice because it uses a centralized or distributed controller to compute optimal paths based on IGP link attributes (such as metric, TE metric, affinity) and bandwidth constraints, encoding the path as a segment list in the packet header. Unlike RSVP-TE, SR-TE does not require per-flow state in the core routers, making it more scalable for bandwidth optimization in an MPLS Segment Routing network.

Answer

RSVP-TE with FRR

Answer

LDP over SR

Answer

OSPF with MPLS-TE extensions

Question 2

Which TWO statements about BGP route reflectors are true?

Accepted Answer

The cluster ID is used to detect routing loops.. BGP route reflectors use the cluster ID to detect and prevent routing loops. When a route reflector receives an update containing its own cluster ID in the cluster-list attribute, it discards the route, breaking the loop. This mechanism is defined in RFC 4456 and is essential for loop-free route propagation in non-full-mesh iBGP topologies.

Answer

Route reflectors modify the AS_PATH attribute.

Answer

Route reflectors must be fully meshed.

Answer

A route reflector passes routes from non-client to non-client.

Question 3

Which DSCP value is used for voice traffic (EF) in the DiffServ model?

Accepted Answer

DSCP 46. EF (Expedited Forwarding) is assigned DSCP 46 for low-latency voice traffic.

Answer

DSCP 8

Answer

DSCP 0

Answer

DSCP 34

Question 4

Which TWO statements correctly describe Segment Routing characteristics? (Select two.)

Accepted Answer

The path is encoded as a label stack at the source router. Segment Routing (SR) encodes the forwarding path as an ordered list of segment identifiers (SIDs) pushed onto a label stack at the source router. This source-routing paradigm allows the ingress node to specify the exact path through the network without requiring intermediate routers to maintain per-flow state.

Answer

All routers must be configured with the same SRGB value

Answer

SR eliminates all per-prefix state from core routers

Answer

Traffic engineering policies are distributed via BGP without any IGP extension

Question 5

Which two statements about MPLS label operations are true? (Choose two.)

Accepted Answer

Push operation adds a label to the packet.. The MPLS push operation adds a new label onto the top of the label stack, encapsulating the packet for forwarding through an LSP. Option E is correct because the pop operation removes the top label, and implicit-null (label value 3) triggers the penultimate hop to pop the label before forwarding the packet to the egress LSR, reducing the egress router's processing load.

Answer

Push operation can only add one label at a time.

Answer

Pop operation can remove multiple labels at once.

Answer

Swap operation removes and replaces two labels.

Question 6

Which TWO are valid benefits of automating QoS policy management in a large SP network? (Choose two.)

Accepted Answer

Reduced human error in configuration.. Automating QoS policy management eliminates manual configuration steps, reducing the risk of syntax errors, misapplied policies, or inconsistent deployments across thousands of devices. Automation tools like Ansible or NSO enforce standardized templates and pre-validated configurations, directly lowering human error rates in large SP networks.

Answer

Eliminates the need for monitoring QoS performance.

Answer

Slower deployment of QoS changes.

Answer

Requires no validation of configurations before apply.

Question 7

Which TWO statements about BGP FlowSpec (RFC 8955) are correct?

Accepted Answer

FlowSpec can be deployed in BGP sessions between a route reflector and a client.. BGP FlowSpec (RFC 8955) can be deployed between a route reflector and its clients. The route reflector propagates FlowSpec NLRI (Network Layer Reachability Information) to its clients, allowing the clients to install traffic filtering rules without requiring a full BGP mesh. This is a common deployment model in service provider networks to distribute flow-spec routes efficiently.

Answer

FlowSpec uses a separate BGP session from the regular IPv4 unicast session.

Answer

FlowSpec is designed to replace ACLs on provider edge routers.

Answer

FlowSpec requires MPLS forwarding to operate.

Question 8

Which THREE of the following are key characteristics of Cisco IOS XR operating system?

Accepted Answer

Each routing protocol runs as a separate process.. Cisco IOS XR uses a microkernel operating system where each routing protocol runs as a separate process (A). It features a two-phase commit configuration model with atomic commit and rollback (B). It also supports non-stop forwarding (NSF) with graceful restart for high availability (C). Option D is incorrect because IOS XR does not have a monolithic kernel; it uses a microkernel. Option E is incorrect because while IOS XR separates control and data planes, it is not a distributed OS architecture; it is a microkernel-based OS designed for a single router.

Answer

Monolithic kernel where all processes run in a single memory space.

Answer

Distributed OS architecture with separate control and data planes.

Question 9

A service provider is deploying a new MPLS core network. The network has four routers: P1, P2, PE1, and PE2. OSPF is used as the IGP. The engineer configures MPLS LDP on all interfaces. After enabling LDP, the engineer notices that the LDP session between P1 and P2 is established, but no labels are exchanged for the loopback0 interfaces of PE1 and PE2. The loopback0 addresses are advertised in OSPF. The engineer verifies that the OSPF routes are present in the routing table of all routers. What is the most likely reason for the missing labels?

Accepted Answer

An access-list is applied under 'mpls ldp advertise-labels' that denies the loopback prefixes.. The most likely reason is that an access-list is applied under 'mpls ldp advertise-labels' that denies the loopback prefixes. LDP by default advertises labels for all prefixes in the routing table, but the 'advertise-labels' command can filter which prefixes receive labels. If the loopback0 prefixes of PE1 and PE2 are denied by such an access-list, no labels will be advertised for them, even though OSPF routes are present.

Answer

LDP is using UDP for label exchange.

Answer

The OSPF cost to the loopbacks is too high.

Answer

The loopback interfaces are not enabled with 'mpls ip'.

Question 10

You are a network engineer at a service provider. Your network uses MPLS L3VPN with OSPF as the IGP and LDP for label distribution. A customer has two sites connected to different PEs (PE1 and PE2) in the same VPN. The customer's CE routers are running eBGP with the PEs. Recently, the customer reports that traffic between the two sites is intermittently dropping. Upon investigation, you find that the BGP session between PE1 and the CE at site A goes down briefly every few minutes. The logs on PE1 show BGP notifications with error code 'Hold Timer Expired'. The CE router at site A is a low-end device with limited CPU. What is the most likely cause and the best course of action?

Accepted Answer

Increase the BGP hold time on PE1 to 180 seconds.. The BGP session drops due to 'Hold Timer Expired' because the low-end CE router's CPU is overloaded, causing it to fail sending BGP keepalives within the default 90-second hold time. Increasing the hold time on PE1 to 180 seconds (option D) gives the CE more time to send keepalives, reducing false timeouts. This directly addresses the root cause—insufficient CPU to maintain timely keepalives—without changing routing protocols or suppressing routes.

Answer

Configure BGP route dampening on PE1 to suppress flapping routes.

Answer

Disable LDP on the link between PE1 and CE1.

Answer

Change the IGP from OSPF to IS-IS to reduce routing updates.

Question 11

A service provider is deploying MPLS-TE with RSVP-TE in their core network. They notice that some LSPs are not being established due to resource contention. Which action would best address this issue without redesigning the entire traffic engineering deployment?

Accepted Answer

Enable preemption on RSVP-TE LSPs with appropriate priority levels.. Enabling preemption on RSVP-TE LSPs with appropriate setup and hold priorities allows higher-priority LSPs to tear down lower-priority LSPs to free up bandwidth, resolving resource contention without redesigning the entire TE deployment. This is the standard mechanism defined in RFC 3209 for managing bandwidth contention in MPLS-TE networks.

Answer

Increase the bandwidth of all core links.

Answer

Configure LSP path-option explicit paths with strict hops.

Answer

Disable RSVP-TE and use LDP for label distribution.

Question 12

An engineer is deploying Segment Routing in an MPLS network. To ensure that routers can forward packets based on SR-MPLS labels without requiring LDP, which requirement must be met?

Accepted Answer

IS-IS or OSPF must have the SR extension enabled.. Segment Routing (SR) can operate without LDP by encoding the forwarding path as a list of MPLS labels derived from the IGP. For routers to allocate and advertise these SR-MPLS labels, the IGP (IS-IS or OSPF) must have the Segment Routing extension enabled. This allows the IGP to distribute prefix-SIDs (labels) that routers use to forward packets, eliminating the need for LDP.

Answer

MPLS LDP must be enabled on all interfaces.

Answer

All routers must run BGP-LU.

Answer

The IGP must have the overload bit set.

Question 13

A network engineer at a service provider is using Cisco NSO to automate the provisioning of VLANs on thousands of access devices. The engineer creates a service using a custom YANG model and deploys it to a set of devices. However, the deployment fails with a 'failed to reach devices' error for some devices, while others succeed. The engineer checks device connectivity and confirms all devices are reachable via SSH and NETCONF. The engineer also verifies that the NSO device list is accurate and includes all target devices. What is the most likely cause of the failure?

Accepted Answer

The devices are not in sync with NSO.. When NSO deploys a service, it first checks whether the target devices are in sync with the NSO CDB (configuration database). If a device is out of sync (e.g., its running configuration differs from what NSO expects), NSO will refuse to push the new service configuration and will report a 'failed to reach devices' error, even though the device is reachable via SSH/NETCONF. This is a safety mechanism to prevent configuration conflicts or overwriting unmanaged changes.

Answer

The service model uses an unsupported feature on those devices.

Answer

The devices have insufficient memory to accept the configuration.

Answer

The NSO package is not loaded on those devices.

Question 14

An MPLS core network uses LDP to distribute labels. An engineer wants to verify that the LDP session between two routers is up and exchanging labels. Which command should be used?

Accepted Answer

show mpls ldp neighbor. The command 'show mpls ldp neighbor' displays the status of LDP sessions, including the neighbor's IP address, session state (e.g., Operational), and label exchange activity. Since LDP is the protocol used to distribute labels in this MPLS core network, this command directly verifies that the session is up and exchanging labels between the two routers.

Answer

show mpls interfaces

Answer

show mpls l2transport binding

Answer

show mpls forwarding-table

Question 15

Which IS-IS TLV is used to carry IPv6 reachability information?

Accepted Answer

TLV 236. TLV 236 (IPv6 Reachability) carries IPv6 prefixes and associated metrics.

Answer

TLV 135

Answer

TLV 232

Answer

TLV 242

Question 16

A service provider is deploying BGP PIC (Prefix Independent Convergence) to improve convergence after a link failure. Which mechanism does BGP PIC leverage to achieve fast failover?

Accepted Answer

It pre-installs a backup path in the routing table and forwarding plane.. BGP PIC uses backup paths that are pre-installed in the RIB/FIB. When the primary path fails, traffic is immediately switched to the backup path without waiting for BGP re-convergence.

Answer

It decreases the BGP update timer to propagate withdrawals faster.

Answer

It reduces the BGP hold timer to detect failures faster.

Answer

It uses BFD to detect failures and triggers BGP to recompute routes immediately.

Question 17

An ISP is deploying BGP confederations to reduce iBGP mesh requirements. Which of the following statements about BGP confederations are true? (Choose two.)

Accepted Answer

The next-hop attribute is unchanged across confederation boundaries. BGP confederations divide an AS into sub-ASes. eBGP-like behavior is used between sub-ASes, but the AS_PATH length calculation treats the confederation as a single AS. The sub-AS number is appended to the AS_PATH with a special type code, but the AS_PATH length for best path selection ignores confederation segments.

Answer

The AS_PATH length calculation includes confederation segments when selecting the best path

Answer

Confederations use a special type of eBGP between sub-ASes, but they retain the IGP metric across the confederation

Answer

The MED attribute is replaced by a confederation-specific metric

Question 18

A service provider is implementing QoS on an MPLS network to support voice, video, and data traffic. Which queuing mechanism provides the lowest latency for real-time traffic?

Accepted Answer

LLQ. LLQ (Low Latency Queuing) is the correct choice because it provides a strict priority queue specifically designed for real-time traffic like voice and video. By placing delay-sensitive packets into a dedicated priority queue that is serviced before all other queues, LLQ ensures minimal and predictable latency, which is essential for maintaining voice quality in an MPLS network.

Answer

FIFO

Answer

WRED

Answer

CBWFQ

Question 19

A client reports intermittent connectivity issues when accessing a server across a provider MPLS network. The issue occurs only during peak hours. Which component should be checked first?

Accepted Answer

Interface errors and discards on the CE router.. Intermittent connectivity during peak hours strongly suggests a bandwidth or queuing issue at the edge of the MPLS network. Interface errors (e.g., CRC, runts) and discards on the CE router indicate congestion or Layer 1/2 problems, which are the most common cause of such time-dependent symptoms. Checking this first aligns with the standard troubleshooting methodology of verifying the physical and data-link layers before moving to higher-layer protocols.

Answer

BGP peering status between CE and PE.

Answer

CPU utilization of the server.

Answer

MPLS labels in the core.

Question 20

An SP engineer is configuring hierarchical QoS on a PE router. The parent policy shapes the traffic to 10 Mbps, and the child policy applies LLQ for voice and CBWFQ for data. What is the primary purpose of using policing within the LLQ child policy?

Accepted Answer

To drop excess voice traffic and prevent starvation of other queues. In hierarchical QoS (HQoS), the parent policy shapes the aggregate traffic to 10 Mbps, while the child policy applies LLQ for voice and CBWFQ for data. The primary purpose of policing within the LLQ child policy is to enforce a strict rate limit on voice traffic, dropping excess packets to prevent voice from consuming bandwidth beyond its allocated share. This prevents voice traffic from starving other queues (e.g., data) within the shaped parent rate, ensuring fairness and protecting other classes from congestion.

Answer

To re-mark voice traffic to a lower DSCP

Answer

To police the parent shape rate

Answer

To buffer excess voice traffic