350-501 Practice Test 5 — 25 Questions

Question 1

In the MQC (Modular QoS CLI) framework, which component is used to match traffic based on DSCP values?

Accepted Answer

Class-map. Class-maps define match criteria (e.g., match ip dscp 46) to classify traffic in MQC.

Answer

Service-policy

Answer

Access-list

Answer

Policy-map

Question 2

An SP network uses NSO for service orchestration. Which component in NSO provides the abstraction layer to communicate with different device types using protocols like NETCONF or CLI?

Accepted Answer

NED. NEDs (Network Element Drivers) are plugins that translate NSO's service models to device-specific configurations.

Answer

YANG

Answer

RESTCONF

Answer

CDB

Question 3

In an SP network using NSO for service orchestration, which component abstracts device-specific CLI or SNMP interactions into a common model for NETCONF/RESTCONF southbound communication?

Accepted Answer

NED (Network Element Driver). The Network Element Driver (NED) provides the abstraction layer between NSO and network devices, translating YANG models to device-specific configurations (e.g., CLI, SNMP).

Answer

YANG model

Answer

CDB (Configuration Database)

Answer

RESTCONF northbound API

Question 4

Which YANG data modeling construct is used to restrict a leaf value to be a reference to another leaf in the data tree?

Accepted Answer

leafref. The 'leafref' type in YANG is used to restrict the value of a leaf to refer to another leaf in the data tree, enforcing referential integrity.

Answer

when

Answer

must

Answer

choice

Question 5

When DSCP markings are mapped to MPLS EXP bits at the ingress PE router, how many EXP values are available for traffic differentiation in the MPLS core?

Accepted Answer

8. MPLS EXP (now TC) is a 3-bit field, providing 8 possible values (0-7).

Answer

16

Answer

64

Answer

32

Question 6

A customer is experiencing packet loss during congestion on a link. The service provider wants to implement a QoS policy that drops less important traffic before more important traffic. Which queuing mechanism is best suited?

Accepted Answer

Class-Based Weighted Fair Queuing (CBWFQ) with WRED. CBWFQ with WRED is the best choice because CBWFQ provides per-class queuing with guaranteed bandwidth, while WRED proactively drops less important traffic (based on IP precedence or DSCP) before the queue becomes full, preventing tail drop and ensuring that higher-priority traffic is preserved during congestion. This combination allows the service provider to selectively discard lower-priority packets under congestion, meeting the requirement to drop less important traffic before more important traffic.

Answer

FIFO queuing

Answer

Low Latency Queuing (LLQ)

Answer

Traffic shaping

Question 7

Which two statements correctly describe the use of DSCP values in the DiffServ model? (Choose two.)

Accepted Answer

DSCP is a 6-bit field allowing 64 distinct values.. DSCP is a 6-bit field with 64 possible values. Per-hop behaviors (PHBs) like EF and AF are defined by DSCP values.

Answer

DSCP values are mapped to MPLS EXP bits at the egress PE.

Answer

DSCP values are used only for voice traffic.

Answer

DSCP value 0 indicates low latency traffic.

Question 8

A service provider is designing a multicast solution for a Layer 3 VPN. They want to use MVPN with BGP signaling (draft-rosen). The PE routers are configured with VRF and multicast routing enabled. Which BGP address family must be enabled between PE routers to carry multicast routing information?

Accepted Answer

MCAST-VPN address family. In a draft-rosen MVPN (Multicast VPN) implementation, BGP is used to signal multicast routing information between PE routers. The MCAST-VPN address family (AFI 25, SAFI 5) is specifically defined to carry multicast VPN routes, including Intra-AS I-PMSI A-D routes and S-PMSI A-D routes, enabling the exchange of multicast state and tunnel information across the MPLS/VPN backbone.

Answer

MVPN does not use BGP; it uses PIM.

Answer

VPNv4 address family

Answer

IPv4 multicast address family

Question 9

An engineer wants to secure NTP on IOS XR routers. Which configuration is required to prevent unauthorized time synchronization?

Accepted Answer

NTP authentication with a key. NTP authentication using a key ensures that only trusted NTP servers can update the router's clock, preventing spoofing attacks.

Answer

NTP broadcast mode

Answer

NTP access group with ACL

Answer

NTP version 4 only

Question 10

In an MPLS network using LDP, which statement correctly describes the label distribution behavior for a given FEC?

Accepted Answer

Labels are advertised unsolicited to all neighbors, and all labels are retained even if not used (liberal label retention).. LDP uses downstream unsolicited mode by default, where each LSR advertises labels for all FECs to all neighbors without being asked, and liberal label retention stores all received labels even if not currently used.

Answer

Labels are advertised only in response to a request from an upstream neighbor (downstream on demand).

Answer

Labels are advertised unsolicited but only the best label for each FEC is retained (conservative label retention).

Answer

Labels are advertised only when the LSR has an outgoing label for the FEC (ordered label distribution).

Question 11

An engineer is configuring management plane hardening on an IOS XR router. The requirement is to authenticate users against a central server and provide granular command authorization. Which protocol and feature should be used?

Accepted Answer

TACACS+ with AAA and task groups. TACACS+ provides separate authentication, authorization, and accounting, and is commonly used with AAA for centralized management. Role-based access is achieved via task groups in IOS XR.

Answer

SSH with local username/password

Answer

SNMPv3 with ACLs

Answer

RADIUS with local authentication

Question 12

An SP is implementing BGP FlowSpec to mitigate DDoS. The FlowSpec rule should match traffic with destination port 80 and DSCP value 0. Which FlowSpec component is used to specify the destination port?

Accepted Answer

Type 3: Destination Port. FlowSpec defines traffic matching criteria using components like destination port (type 3), source port (type 2), etc. Destination port is type 3.

Answer

Type 2: Source Port

Answer

Type 4: ICMP Type

Answer

Type 1: Destination Prefix

Question 13

In an MPLS L3VPN network, which security measure should be taken on PE-CE links to prevent MPLS label spoofing?

Accepted Answer

Disable MPLS on the PE-CE link. Disabling MPLS on the PE-CE link ensures that the customer cannot send labeled packets into the MPLS core, preventing label spoofing. Additionally, disabling TTL propagation can hide the core topology but does not prevent spoofing.

Answer

Enable MPLS OAM

Answer

Use BGP prefix filtering

Answer

Enable TTL propagation

Question 14

In Segment Routing over IPv6 (SRv6), what is the function of the 'Locator' part of an SRv6 SID?

Accepted Answer

It identifies the node and is used for routing the packet to that node.. The Locator is the portion of the SRv6 SID that identifies the node (or the segment endpoint) and is used for routing to that node.

Answer

It indicates the length of the SID.

Answer

It encodes an argument for the SRv6 function.

Answer

It specifies the function to be performed on the packet.

Question 15

Which LDP label retention mode stores all received label bindings even if they are not currently used for forwarding?

Accepted Answer

Liberal label retention. Liberal label retention stores all label bindings, allowing quick convergence when a path changes, while conservative retention stores only bindings from the next-hop.

Answer

Conservative label retention

Answer

Ordered label distribution

Answer

Downstream unsolicited

Question 16

An engineer is troubleshooting an MPLS network where some routers are not exchanging labels via LDP. Which two steps are most appropriate for diagnosing the issue? (Choose two.)

Accepted Answer

Verify LDP neighbor adjacencies using 'show mpls ldp neighbor'.. Verifying LDP sessions with 'show mpls ldp neighbor' and checking interfaces with 'show mpls ldp interface' are standard first steps. Checking MTU or OSPF neighbors are less direct.

Answer

Check the MTU size on all interfaces.

Answer

Reboot all routers to reset LDP sessions.

Answer

Verify that OSPF neighbor relationships are established.

Question 17

In SRv6, which endpoint behavior is used for decapsulating the outer IPv6 header and performing an IPv4 lookup (e.g., for L3VPN)?

Accepted Answer

End.DT4. End.DT4 is the SRv6 endpoint behavior that decapsulates the outer IPv6 header and then performs a lookup in the IPv4 VPN table. End.DT6 is for IPv6, and End.DT2U is for L2VPN.

Answer

End.DT6

Answer

End.DT2U

Answer

End.DX4

Question 18

A service provider is deploying a BNG for subscriber management. Which protocol is typically used to authenticate subscribers and assign IP addresses in a PPPoE-based broadband network?

Accepted Answer

RADIUS. RADIUS is the standard protocol for authentication, authorization, and accounting (AAA) in BNG environments. DHCP assigns IP addresses, but authentication is via RADIUS. TACACS+ is for device administration; IPoE uses DHCP but not typically for authentication.

Answer

DHCP

Answer

IPoE

Answer

TACACS+

Question 19

In SR-MPLS, when a router receives a packet with an MPLS label corresponding to an Adjacency SID, what operation does it perform?

Accepted Answer

It pops the label and forwards the packet out of the specific interface.. An Adjacency SID represents a specific link; when a router receives a packet with that label, it pops the label and forwards the packet out of the associated interface, because the Adjacency SID is a 'penultimate hop popping' type of SID.

Answer

It performs a recursive lookup in the FIB to determine the next hop.

Answer

It pushes an additional label and forwards.

Answer

It swaps the label with the next-hop label and forwards.

Question 20

Which two actions are performed by an MPLS label edge router (LER) when it receives an unlabeled IP packet destined for a prefix with an LSP? (Choose two.)

Accepted Answer

It forwards the packet based on the MPLS label.. The LER (or ingress LSR) imposes a label stack (push operation) on the packet, then forwards it based on the label. It does not swap labels (that's done by LSRs), nor does it pop the label (egress LER does).

Answer

It performs a recursive lookup in the LFIB.

Answer

It performs a label swap operation.

Answer

It pops the top label and forwards based on the inner header.