350-501 Practice Test 39 — 15 Questions

Question 1

A service provider is migrating their MPLS core network to Segment Routing (SR-MPLS). All existing core routers run IOS-XR and are configured with an SRGB of 16000-23999 and OSPF as the IGP. A new router (R5) is added as a PE. The engineer configures R5 with the same SRGB and enables segment-routing under OSPF. However, when configuring an SR-TE policy from R5 to the remote loopback 10.0.0.1 on R1, the policy remains down. Show commands reveal that R5 is not learning the prefix-SID for 10.0.0.1. On R1, the prefix-SID is configured with index 100. The engineer verifies that OSPF adjacencies are up and that R5 can ping 10.0.0.1. What is the most likely cause of the issue?

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

Accepted Answer

The OSPF process on R5 is not configured with segment-routing mpls.. The most likely cause is that the OSPF process on R5 is not configured with the 'segment-routing mpls' subcommand. Without this configuration, OSPF does not advertise or learn prefix-SIDs, so R5 cannot obtain the prefix-SID for 10.0.0.1. Option B is incorrect: while a color attribute is often used in SR-TE policies, the immediate issue is the lack of prefix-SID learning, not the absence of color. Option C is incorrect: all routers share the same SRGB (16000-23999), so there is no mismatch. Option D is incorrect: index 100 maps to label 16100 (16000+100), which falls within the SRGB range, so this is not the problem.

Answer

The SR-TE policy on R5 is missing the color attribute.

Answer

R1 is not configured with the same SRGB as R5.

Answer

The prefix-SID index 100 is not within the SRGB range of R5 (16000-23999).

Question 2

Refer to the exhibit. A network engineer applies this policy on the PE-CE link. What is the expected behavior for VoIP traffic matching the access list?

Accepted Answer

VoIP traffic is given strict priority queuing with up to 30% bandwidth. The policy-map applies the 'priority' command to the VoIP class, which enables strict priority queuing (LLQ). The 'police cir percent 30' command limits the priority queue's bandwidth to 30% during congestion, ensuring VoIP traffic is always served first but cannot exceed the configured percentage. This matches option A exactly.

Answer

VoIP traffic is dropped if congestion occurs

Answer

VoIP traffic is shaped to 30% of bandwidth

Answer

VoIP traffic is queued in the default class with fair-queue

Question 3

Which multicast RP model is recommended for large-scale networks to provide redundancy and load sharing?

Accepted Answer

Anycast-RP. Anycast-RP is recommended for large-scale networks because it allows multiple RPs to share the same IP address, providing redundancy and load sharing without requiring dynamic RP discovery protocols. This model uses MSDP (Multicast Source Discovery Protocol) or PIM (Protocol Independent Multicast) to synchronize multicast state between RPs, ensuring that sources and receivers can register with the nearest RP for optimal path selection.

Answer

Auto-RP

Answer

BSR

Answer

Static RP

Question 4

A service provider wants to ensure that customer traffic is not impacted during a planned maintenance on a core LSR in an MPLS network. Which MPLS feature should be used?

Accepted Answer

MPLS TE Fast Reroute. MPLS TE Fast Reroute (FRR) is the correct feature because it provides local protection against link or node failures by pre-computing backup paths (bypass tunnels) that are activated within 50 milliseconds of a failure. This ensures that customer traffic is not impacted during planned maintenance on a core LSR, as the backup path is already in place and can be triggered by a manual administrative action (e.g., shutting down the interface) to seamlessly redirect traffic before the maintenance begins.

Answer

MPLS TTL propagation

Answer

MPLS LDP synchronization

Answer

MPLS OAM

Question 5

In Cisco IOS XR architecture, using Secure Domain Routers (SDRs) allows partitioning a physical router into multiple logical routers. Which statement about admin plane and SDRs is correct?

Accepted Answer

The admin plane is a separate SDR that manages hardware resources and system-level functions, while the default SDR handles routing and forwarding.. The admin plane is a special SDR used for managing the system (e.g., hardware inventory, shared resources). Default SDR is the primary forwarding SDR, but admin SDR is separate.

Answer

The admin plane runs only on route processor cards, while default SDR runs on line cards.

Answer

The admin plane and default SDR are the same logical entity and share all configuration.

Answer

SDRs are only supported on Cisco CRS, not on ASR 9000.

Question 6

An SP wants to filter BGP prefixes received from a customer to prevent hijacking. Which two tools can be used together on the provider edge router to implement inbound prefix filtering?

Accepted Answer

Prefix-list and route-map. Prefix-lists and route-maps are commonly used together to match and filter BGP prefixes. Prefix-lists define the prefixes, and route-maps apply them in BGP neighbor statements.

Answer

Distribute-list and ACL

Answer

RPKI and BGP community

Answer

AS-path access-list and community-list

Question 7

A service provider offers L3VPN services to multiple enterprise customers. One customer reports that they cannot reach some remote sites intermittently. The network uses MPLS L3VPN with MP-BGP for VPN route exchange. The PE routers are configured with route-target import and export. The customer's CE router is dual-homed to two different PEs in the same point of presence. The engineer checks the BGP table on both PEs and sees the customer routes with the correct route-target. However, pings from the CE to a remote site fail about 50% of the time, and the flapping pattern suggests load balancing issues. The engineer discovers that the remote site's network prefix is being advertised from both PEs with the same route-target but with different next-hops. The CE has equal-cost paths via both PEs. What is the most likely cause of the intermittent connectivity?

Accepted Answer

The CE is performing per-packet load balancing across the two PEs, causing asymmetric routing.. When a CE router receives two equal-cost paths from two PEs, it may perform per-packet load balancing. This can lead to asymmetric routing where packets from the same flow take different paths, causing out-of-order delivery and connectivity failures about 50% of the time, especially if stateful devices are in the path or if the transport layer is sensitive to ordering. Option A is incorrect because the route-target import on the remote PE is properly configured (as the routes are seen with correct route-target). Option C is incorrect because BGP timer misconfiguration would cause session flapping and complete loss, not a 50% intermittent failure. Option D is incorrect because MTU mismatch would cause consistent packet drops, not intermittent failures.

Answer

The route-target import on the remote PE is missing the customer's route-target.

Answer

The BGP timers are misconfigured, causing the session to flap.

Answer

The MTU on the CE-PE links is mismatched.

Question 8

Which TWO statements about Cisco NSO (Network Services Orchestrator) are true? (Choose two.)

Accepted Answer

NSO provides northbound APIs using NETCONF and RESTCONF.. NSO provides northbound APIs using NETCONF and RESTCONF, enabling integration with higher-level orchestration and management systems. These standard protocols allow external systems to interact with NSO for service lifecycle management, configuration, and operational data retrieval, making B correct.

Answer

NSO automatically generates Python scripts for device configuration.

Answer

NSO eliminates the need for SNMP in network management.

Answer

NSO only supports CLI-based device management.

Question 9

Which MPLS label is used to identify the egress PE in a Layer 3 MPLS VPN?

Accepted Answer

VPN label. In a Layer 3 MPLS VPN, the VPN label (also known as the service label) is assigned by the egress PE router and advertised via MP-BGP (RFC 4364). It uniquely identifies the VPN forwarding instance (VRF) on the egress PE, allowing the correct per-VPN forwarding decision to be made after the transport label has been removed. Without the VPN label, the egress PE would not know which VRF or customer-facing interface should receive the packet.

Answer

Transport label

Answer

LDP label

Answer

IGP label

Question 10

A service provider is implementing IPv6 transition in its core. Which transition mechanism is recommended for carrying IPv6 over an existing MPLS/IPv4 backbone without dual-stacking all routers?

Accepted Answer

6PE (IPv6 Provider Edge). 6PE (IPv6 Provider Edge) is the correct answer because it allows IPv6 traffic to be carried over an existing MPLS/IPv4 backbone without requiring dual-stack on all core routers. In 6PE, only the Provider Edge (PE) routers need to be dual-stack; they encapsulate IPv6 packets in MPLS labels and transport them across the IPv4 MPLS core, which remains IPv4-only. This meets the requirement of not dual-stacking all routers while leveraging the existing MPLS infrastructure.

Answer

ISATAP

Answer

NAT64

Answer

6to4 tunneling

Question 11

Which two are requirements for deploying Segment Routing in a service provider network? (Choose two.)

Accepted Answer

IGP supporting segment routing (OSPF or IS-IS). Segment Routing (SR) relies on an IGP (OSPF or IS-IS) that has been extended to advertise prefix-SIDs and adjacency-SIDs. The IGP extensions (RFC 8665 for IS-IS, RFC 8666 for OSPF) enable routers to compute source-routed paths without a separate signaling protocol. This is a fundamental requirement because SR-MPLS encodes the path as a stack of labels derived directly from the IGP SIDs.

Answer

TE tunnels must be configured

Answer

LDP must be enabled

Answer

All routers must run BGP

Question 12

An SP is designing a core network using Cisco CRS routers running IOS XR. They require process separation to ensure that a failure in the BGP process does not affect OSPF operations. Which IOS XR architectural feature provides this capability?

Accepted Answer

Process separation. IOS XR uses a distributed OS where each protocol runs as a separate process (process separation). This prevents a crash in one protocol process from impacting others, enhancing stability.

Answer

Non-Stop Forwarding (NSF)

Answer

Graceful Restart

Answer

Secure Domain Router (SDR)

Question 13

A BNG (Broadband Network Gateway) is used for subscriber management. Which protocol is typically used between the BNG and the subscriber's modem (CPE) for authentication and IP address assignment in a PPPoE environment?

Accepted Answer

PPPoE with PPP authentication and IPCP. In a PPPoE environment, the BNG acts as the access concentrator. Authentication is performed using PAP or CHAP, and IP address assignment is done via IPCP (IP Control Protocol). The BNG also uses RADIUS for centralized authentication but between BNG and CPE, PPPoE uses PPP for authentication and IPCP for IP address.

Answer

DHCP

Answer

RADIUS directly to CPE

Answer

L2TP

Question 14

A service provider implements CGNAT to conserve IPv4 addresses. Which feature is required to ensure that application-level protocols such as SIP or FTP function correctly?

Accepted Answer

ALG support. Many application protocols embed IP addresses in payload. ALGs (Application Level Gateways) inspect and modify these payloads to ensure proper translation. Without ALGs, these protocols may break.

Answer

Port block allocation

Answer

Logging

Answer

Session limits

Question 15

Based on the exhibit, which statement about label allocation for prefix 10.1.1.0/24 is true?

Accepted Answer

The local router allocated label 161 for the prefix. The exhibit shows that the local router (192.168.1.2) has allocated label 161 for prefix 10.1.1.0/24, as indicated in the output of the 'show mpls forwarding-table' command. This label is assigned by the local router as the ingress LSR to forward traffic toward the egress LSR (192.168.1.1). The correct statement is that the local router allocated label 161 for this prefix.

Answer

Router 192.168.2.2 is the penultimate hop for this prefix.

Answer

The local router will use PHP for this prefix.

Answer

Router 192.168.1.1 is the egress LSR for this prefix.