350-501 Practice Test 46 — 15 Questions

Question 1

A service provider wants to synchronize clocks across its core routers so that timing accuracy supports one-way delay measurements and strict SLA reporting. The engineer must choose a synchronization technology that delivers sub-microsecond accuracy and is designed for packet networks. Which technology should be selected?

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

Accepted Answer

Precision Time Protocol (PTP) operating as IEEE 1588v2 with hardware timestamping.. PTP as defined by IEEE 1588v2, combined with hardware timestamping, is designed to deliver sub-microsecond synchronization over packet networks. That accuracy is what one-way delay measurement and strict SLA reporting require, whereas NTP and SNTP only reach millisecond precision and GPS alone does not distribute time to the rest of the core.

Answer

NTP with symmetric key authentication configured on all core routers.

Answer

SNTP configured in unicast client mode on every router.

Answer

GPS receivers installed only on the route reflectors, with no time distribution to other routers.

Question 2

An engineer is troubleshooting an SR-MPLS TE tunnel in a Cisco IOS XR network. The headend router has an SR Policy with an explicit candidate path that contains a segment list with the SIDs of the desired path. However, the tunnel is not forwarding traffic. The engineer checks the segment list and sees that the first SID is a node SID of an intermediate router, and the last SID is the node SID of the destination. What is the most likely cause of the issue?

Accepted Answer

The headend router does not have a route to the first SID in the segment list, so the SR Policy is not valid.. An SR Policy requires that the headend router can resolve the first SID in the segment list. If the first SID is a node SID of an intermediate router, the headend must have a route to that node's loopback. Without this route, the SR Policy is invalid and traffic will not be forwarded. The other options are not requirements for SR-MPLS TE.

Answer

The last SID in the segment list must be an adjacency SID to the destination, not a node SID.

Answer

The segment list is missing a SID for the penultimate router, causing a label stack mismatch.

Answer

The segment list must include the local node SID as the first SID.

Question 3

A service provider runs an MPLS L3VPN over an SR-MPLS core. The operator wants to ensure that customer traffic follows a specific path that avoids a congested link, without using RSVP-TE. Which mechanism should be used to steer the traffic?

Accepted Answer

Segment Routing Traffic Engineering (SR-TE) with an explicit segment list. SR-TE uses a segment list to define an explicit path. The headend router pushes the list onto the MPLS label stack, and each segment directs the packet along the desired path. This allows the operator to avoid congested links without RSVP-TE, satisfying the design constraint.

Answer

MPLS LDP with IGP shortest path

Answer

BGP Link State (BGP-LS) with a centralized controller

Answer

RSVP-TE with explicit route objects

Question 4

A service provider is using Cisco IOS XR and wants to implement a routing policy that sets the BGP community attribute on routes learned from a specific neighbor. The policy should also prepend the AS path three times for those routes. Which configuration correctly applies this policy?

Accepted Answer

Create a route-policy that sets the community and prepends the AS path, then apply it as an inbound policy on the neighbor.. In IOS XR, route-policies are used to manipulate BGP attributes. To modify routes learned from a neighbor, apply the policy inbound. A single route-policy can set the community and prepend the AS path. Applying it outbound would affect routes sent to the neighbor, not received from it.

Answer

Create a route-policy that sets the community and prepends the AS path, then apply it as an outbound policy on the neighbor.

Answer

Use a route-policy to set the community, and use a separate route-policy to prepend the AS path, then apply both inbound.

Answer

Use a route-policy to set the community, and use the 'as-path prepend' command directly under the neighbor configuration.

Question 5

A service provider uses BGP on IOS XR routers to exchange routes with customers. An engineer notices that a customer is advertising a prefix with an unexpected AS_PATH that includes the provider's own AS number, which the provider's inbound policy does not filter. The provider wants to prevent its own AS from appearing in routes learned from customers without disrupting legitimate customer routes. Which BGP feature should be applied on the customer-facing sessions?

Accepted Answer

Apply an inbound route-policy that matches the provider's own AS in the AS_PATH using an as-path-set and drops those routes.. To stop customers from advertising routes that contain the provider's own AS number, the provider applies an inbound route-policy that matches its AS in the AS_PATH and drops those updates. This is targeted filtering at the customer edge. Enforce-first-as only checks the leading AS, route refresh does not validate paths, and bestpath as-path ignore affects local selection rather than rejecting the routes.

Answer

Enable BGP route refresh capability on the customer-facing neighbors so that invalid AS_PATHs are automatically corrected.

Answer

Configure 'bgp enforce-first-as' on the customer-facing neighbors to reject updates where the first AS in the AS_PATH is not the sending peer's AS.

Answer

Configure the 'bgp bestpath as-path ignore' command on the customer-facing neighbors to disregard the AS_PATH during best-path selection.

Question 6

A service provider is using Cisco IOS XR routers in its core. The network operations team requires that configuration changes be validated before committing to the running configuration, and that they be able to roll back to a previous configuration state if a change causes issues. Which IOS XR feature should be used to meet these requirements?

Accepted Answer

Two-stage configuration with commit and rollback. The two-stage configuration model in IOS XR allows administrators to make changes in a target configuration, validate them, and then commit. If issues arise, rollback can revert to a previous committed configuration. This provides the required safety and recovery. Other options are either subsets of this model or do not provide the full validation and rollback functionality.

Answer

Configuration session with exclusive lock

Answer

Configuration replace with load commit

Answer

Candidate configuration with automatic commit

Question 7

A service provider is deploying a Segment Routing (SR) solution using Cisco IOS XR. The SP wants to minimize the number of labels in the MPLS stack while still providing traffic engineering capabilities. Which SR component allows a headend router to steer traffic through a specific path without relying on RSVP-TE?

Accepted Answer

Segment Routing Traffic Engineering (SR-TE) with a segment list. SR-TE with a segment list allows a headend router to steer traffic through a specific path by imposing a stack of segments (MPLS labels or SRv6 SIDs). Each segment represents a node or adjacency, and the headend controls the path explicitly. This eliminates the need for RSVP-TE signaling, reducing control plane overhead and simplifying operations. It provides traffic engineering capabilities with greater scalability and flexibility.

Answer

IGP Flexible Algorithm (Flex-Algo)

Answer

BGP Link-State (BGP-LS) with PCE

Answer

RSVP-TE with explicit route objects (EROs)

Question 8

A service provider runs Cisco IOS XR on its core routers and wants to ensure high availability by separating control plane and data plane processes. The SP is concerned about a routing protocol process failure affecting packet forwarding. Which IOS XR architectural feature best addresses this concern?

Accepted Answer

Microkernel-based modular architecture with independent process spaces. The microkernel-based modular architecture of IOS XR ensures that each routing protocol runs as an independent process with its own memory space. If one process fails, it does not affect other processes or the forwarding plane, which continues to forward packets based on the last known good forwarding state. This design provides high availability and fault isolation, directly addressing the SP's concern.

Answer

Distributed Route Processor (DRP) architecture with separate control plane and forwarding plane

Answer

Nonstop Forwarding (NSF) with Stateful Switchover (SSO) between dual route processors

Answer

Multi-Protocol Label Switching (MPLS) with Traffic Engineering (TE) tunnels

Question 9

A service provider is deploying a large-scale MPLS network with Cisco ASR 9000 routers running IOS XR. The network operations team wants to ensure that a software defect in a single routing protocol process does not cause the entire router to reload. Which IOS XR architecture feature addresses this requirement?

Accepted Answer

Process Isolation with Memory Protection. Cisco IOS XR uses a microkernel architecture with process isolation, where each process runs in its own memory space. This prevents a software defect in one process, such as a routing protocol, from affecting other processes or causing a router reload. Other features like NSR and modular packages provide different benefits but do not provide this level of fault containment.

Answer

Modular Software Packages

Answer

Preemptive Multitasking

Answer

Non-Stop Routing (NSR)

Question 10

A service provider is deploying a new core network using Cisco IOS XR. The network operations team wants to ensure that a configuration change on a router can be automatically rolled back if the router loses connectivity during the commit. Which IOS XR feature should be used?

Accepted Answer

Commit confirmed with a rollback timer.. The commit confirmed feature in IOS XR is designed to prevent lockout during remote configuration changes. When an administrator commits a change with a confirmation timer, the router applies the configuration but automatically rolls back to the previous state if the commit is not confirmed within the specified time. This ensures that if connectivity is lost, the router returns to a known-good configuration, which is exactly what the operations team needs.

Answer

Configuration rollback via the load rollback command.

Answer

Atomic configuration replace with a timer.

Answer

Two-stage configuration commit with a rollback delay.

Question 11

A service provider is deploying Segment Routing with TI-LFA (Topology Independent Loop-Free Alternate) for fast convergence in its core network. The network uses IS-IS as the IGP. An engineer must ensure that TI-LFA is operational. Which statement accurately describes a requirement for TI-LFA?

Accepted Answer

TI-LFA requires that all routers in the network support SR and have a consistent SRGB.. TI-LFA is a Segment Routing-based fast reroute mechanism that provides loop-free alternates for any topology. It requires that all routers in the network support Segment Routing and have a consistent SRGB so that SIDs are interpreted correctly. RSVP-TE is not used. Wide metrics are recommended but not required. Prefix SIDs must be unique, not identical. Thus, the correct requirement is consistent SRGB and SR support.

Answer

TI-LFA requires RSVP-TE to be enabled on all interfaces to establish backup tunnels.

Answer

TI-LFA requires that the IGP use narrow metrics to compute backup paths.

Answer

TI-LFA requires that all routers be configured with the same prefix SID for their loopback interfaces.

Question 12

An engineer configures an MPLS L3VPN on a Cisco IOS XR PE router. The customer VRF uses a route distinguisher of 65000:100 and a route target of 65000:100 for both import and export. A second PE router in the same AS advertises the same customer prefix with a different route distinguisher but the same route target. Which statement describes how the receiving PE handles these VPNv4 routes?

Accepted Answer

Both routes are imported into the VRF, and the BGP best-path selection determines which one is installed in the VRF routing table.. Route targets decide which VRFs import a VPNv4 route, while route distinguishers only ensure uniqueness of the VPNv4 NLRI. Since both PEs export and import the same route target, both routes enter the VRF, and the normal BGP decision process selects the best path for the VRF routing table.

Answer

Only the route whose route distinguisher matches the local VRF configuration is imported; the other is discarded.

Answer

Neither route is imported because the route targets on the two PEs must match exactly, including the RD portion.

Answer

Both routes are imported, and the PE load-balances traffic across them because they have different route distinguishers.

Question 13

A service provider is deploying Segment Routing over MPLS (SR-MPLS) in its core network. The design team must ensure that the network can quickly recover from link failures using TI-LFA (Topology Independent Loop-Free Alternate). Which two statements about TI-LFA in an SR-MPLS environment are correct? (Choose two.)

Accepted Answer

TI-LFA provides 100% coverage for single link and node failures in any topology, including ring and square topologies.. TI-LFA provides 100% coverage for single link and node failures in any topology by using the post-convergence path as the backup. It leverages Segment Routing to build a repair list of segments that avoids the failed component and prevents microloops. It does not require LDP or RSVP-TE and is supported across multiple IOS XR platforms.

Answer

TI-LFA requires the use of RSVP-TE to establish backup tunnels.

Answer

TI-LFA requires that all routers in the network support LDP for label distribution.

Answer

TI-LFA is only supported on Cisco ASR 9000 series routers and not on other IOS XR platforms.

Question 14

A service provider is deploying MPLS L3VPN services using Cisco IOS XR. The PE routers run MP-BGP and need to forward customer traffic. Which label operation does the ingress PE perform on packets destined for a remote VPN prefix?

Accepted Answer

Push the VPN label assigned by the egress PE, then push the IGP label for the BGP next-hop.. In an MPLS L3VPN, the ingress PE receives an IP packet from the customer, looks up the destination in the VRF, and finds a VPNv4 route with a BGP next-hop. It then imposes a label stack consisting of the VPN label advertised by the egress PE and the transport label to reach that next-hop. This two-label stack allows the core to forward based on the outer label while the egress PE uses the inner label to identify the correct VRF.

Answer

Pop the VPN label and forward the packet based on the IP header to the next-hop router.

Answer

Push a single label that combines the VPN and transport information, using a per-VRF label space.

Answer

Swap the incoming label with a new label allocated for the VPN prefix by the ingress PE.

Question 15

A service provider is implementing QoS in its MPLS network using Cisco IOS XR. The provider wants to classify and mark traffic at the edge and then use the MPLS Experimental (EXP) bits to carry QoS information across the core. Which two statements about MPLS EXP bits are correct? (Choose two.)

Accepted Answer

The EXP bits can be used to implement Differentiated Services (DiffServ) in the MPLS core.. The MPLS EXP bits are a 3-bit field in the shim header used for QoS marking. They enable DiffServ in the MPLS core by allowing core routers to apply per-hop behaviors based on the EXP value. They are not used for label stacking, and by default they are not automatically copied from IP headers. Their limited length requires mapping from DSCP.

Answer

By default, IOS XR copies the IP Precedence or DSCP value into the EXP bits when imposing an MPLS label.

Answer

EXP bits are only used for label stacking and not for QoS.

Answer

EXP bits are 6 bits long and can carry the entire DSCP value.