350-501 Practice Exam 61 — 20 Questions

Question 1

In SR-PCE architecture, which protocol is used for communication between the Path Computation Element (PCE) and the Path Computation Client (PCC) to request and report SR-TE paths?

Accepted Answer

PCEP. B is correct because the Path Computation Element Protocol (PCEP), defined in RFC 5440 and extended for Segment Routing in RFC 8664, is the dedicated protocol used between a PCE and a PCC to request, report, and delegate SR-TE paths. PCEP carries explicit path computation requests (PCReq) and replies (PCRep), as well as state reporting (PCRpt) and delegation (PCUpd) messages, making it the standard control-plane protocol for SR-PCE architectures.

Answer

RESTCONF

Answer

NETCONF

Answer

BGP-LS

Question 2

In EVPN with SRv6, which SID behavior is used for integrated routing and bridging (IRB) to forward packets between the IP fabric and the Ethernet segment?

Accepted Answer

End.DT4. End.DT2U is for L2VPN; for IRB, the router uses End.DT4/DT6 for IP forwarding and then bridges to the segment; however, the question asks for the SID behavior that enables IRB. Usually, IRB is achieved by combining a Layer 2 VPN SID and a Layer 3 VPN SID, but for the specific behavior that forwards between IP and MAC, it's often End.DT4 or End.DT6 plus a MAC lookup. But standard SRv6 IRB uses a dedicated SID like End.DT2U? Actually, IRB requires both routing and bridging; the common approach is to use a Layer 3 VPN SID for the routed part and an EVPN Ethernet segment for bridging. In SRv6, the endpoint behavior for IRB is typically End.DT4 or End.DT6 for the routing side, and for bridging, it's End.DT2U. But the question expects a single behavior? Perhaps End.DT2U is for L2VPN only. Let's think: Cisco documentation often describes 'End.DT2U' for L2VPN, and 'End.DT4' for L3VPN. For IRB, the service is provided by a combination. However, a specific SID behavior called 'End.DT2U' can be used for EVPN VPWS. To align with common SIDs, the answer is End.DT2U for L2VPN IRB? Actually, IRB typically uses a Layer 3 gateway SID. I'll choose End.DT4 as it routes IP, and then the router bridges locally. But the question says 'forward packets between the IP fabric and the Ethernet segment' - that is routing. So End.DT4 is correct.

Answer

End

Answer

End.DT2U

Answer

End.X

Question 3

Refer to the exhibit. An engineer is troubleshooting packet loss in the MPLS core. For prefix 10.3.3.0/24, the outgoing label is 'Untagged'. What does this indicate?

Accepted Answer

No MPLS label is being imposed on outgoing packets for this prefix. When the outgoing label for a prefix in the MPLS forwarding table shows 'Untagged', it means that for packets destined to that prefix, no MPLS label is imposed. The router will forward the packet as a standard IP packet (without an MPLS header) out the egress interface. This typically occurs when the next-hop router has signaled an implicit null label (label 3) via LDP, instructing the upstream router to pop the label stack and send the packet as IP.

Answer

The packet will have an implicit null label (label 3) imposed

Answer

The prefix is not reachable and packets will be dropped

Answer

The outgoing label is the same as the local label (label 18)

Question 4

A service provider wants to provide IPv6 connectivity over an existing MPLS IPv4 core using BGP extensions. Which technology should be used?

Accepted Answer

6PE. 6PE (IPv6 Provider Edge) uses BGP to carry IPv6 prefixes with an IPv4 next-hop over an MPLS core, avoiding the need for IPv6 in the core.

Answer

NAT64

Answer

Dual-stack

Answer

6VPE

Question 5

A service provider is migrating its MPLS core from LDP to Segment Routing with MPLS data plane (SR-MPLS). The network currently uses TE tunnels with RSVP-TE for traffic engineering. Which technology can the provider use to continue performing traffic engineering after the migration without requiring RSVP-TE?

Accepted Answer

SR-TE. C is correct because Segment Routing Traffic Engineering (SR-TE) allows the service provider to perform traffic engineering without RSVP-TE by using segment lists (label stacks) to steer traffic along explicit paths. SR-TE leverages the IGP (OSPF or IS-IS) to distribute segment IDs (SIDs) and uses a centralized controller or headend node to compute and enforce TE policies, eliminating the need for RSVP-TE's per-flow signaling and state maintenance.

Answer

LDP

Answer

MPLS-TE (RSVP-TE)

Answer

BGP-LU

Question 6

Which MQC component is used to match traffic based on criteria such as DSCP or IP precedence?

Accepted Answer

Class-map. In the Modular QoS CLI (MQC), the class-map component is used to define traffic classes by matching packets based on criteria such as DSCP (Differentiated Services Code Point) or IP precedence values. The class-map uses match statements (e.g., match ip dscp, match ip precedence) to classify traffic, which is then applied within a policy-map for QoS actions. This makes class-map the correct MQC component for traffic matching.

Answer

Service-policy

Answer

Policy-map

Answer

Access-list

Question 7

Which TWO commands are most effective to verify that an SR-TE policy is active and forwarding traffic? (Choose two.)

Accepted Answer

show segment-routing traffic-eng policy. The 'show segment-routing traffic-eng policy' command displays the operational state of SR-TE policies, including whether the policy is 'up' and actively forwarding traffic. It shows the candidate path, binding SID, and forwarding status, making it the primary verification command for SR-TE policy activity.

Answer

show isis database verbose

Answer

show segment-routing traffic-eng segment-list

Answer

show bgp labels

Question 8

An SP network uses NTP for synchronization. To secure NTP, which authentication mode should be used?

Accepted Answer

NTPv4 autokey. NTPv4 autokey (Option A) is the correct answer because it provides a secure, automated key distribution mechanism using public-key cryptography (RFC 5906). This eliminates the need for manual key exchange, making it the recommended authentication mode for securing NTP in service provider networks.

Answer

NTPv3 symmetric key

Answer

NTPv4 with SHA-1

Answer

NTPv4 with MD5

Question 9

A service provider has implemented model-driven telemetry to monitor the health of its core network. The telemetry collector is a single server running a custom application that receives and processes gRPC streams from 200 routers. The collector is experiencing high CPU usage and is falling behind in processing data, causing some telemetry data to be dropped. The engineer decides to offload processing to multiple collectors. The routers support dial-out mode and can be configured with a list of collector IPs. The engineer wants to distribute the load evenly across collectors without manual configuration per router. Which should the engineer implement?

Accepted Answer

Use a load balancer in front of the collectors and configure all routers to send to the load balancer VIP.. A load balancer distributes incoming gRPC streams from all 200 routers across multiple collectors based on a configured algorithm (e.g., round-robin or least connections), achieving even load distribution without per-router configuration. The routers simply send telemetry to a single virtual IP (VIP), and the load balancer forwards each stream to an available collector, preventing any single collector from being overwhelmed. This matches the requirement to offload processing and avoid manual configuration per router.

Answer

Use a multicast address for telemetry subscription so all collectors receive all data.

Answer

Configure each router with a round-robin DNS name that resolves to multiple collector IPs.

Answer

Divide the routers into groups and assign each group to a different collector IP via the router configuration.

Question 10

Refer to the exhibit. A service provider is receiving BGP prefixes from a customer (AS 64512). The provider wants to tag all routes from that customer that match prefix 10.1.0.0/16 or more specific with community 65000:100, while not modifying other routes. After applying the configuration, which statement is true?

Accepted Answer

Only routes matching 10.1.0.0/16 or more specific will have the community added; other routes remain unchanged.. The configuration uses a route-map applied to the neighbor with a match clause referencing a prefix-list that permits 10.1.0.0/16 le 32. This matches the exact prefix and any more specific prefix (up to /32). The set community 65000:100 action adds the community without using the additive keyword, but because the route-map does not contain a deny clause for non-matching routes, all routes are still accepted; only matching routes have the community added. Thus, only routes matching 10.1.0.0/16 or more specific will have community 65000:100 added, and other routes remain unchanged.

Answer

Routes with a mask longer than /24 will be rejected by the prefix-list.

Answer

All routes from the customer will have their communities replaced with 65000:100.

Answer

Routes not matching the prefix-list will be denied and not installed.

Question 11

In an MPLS VPN environment, which address family is used to exchange VPNv4 routes between PE routers?

Accepted Answer

VPNv4 unicast. In an MPLS VPN environment, VPNv4 routes (which include both the IPv4 prefix and the Route Distinguisher) are exchanged between Provider Edge (PE) routers using the VPNv4 unicast address family under the BGP configuration. This address family allows PE routers to carry customer routes with unique RD values, enabling address overlap across different VPNs.

Answer

Route-target (RT)

Answer

IPv4 unicast

Answer

VPNv6 unicast

Question 12

Segment Routing with TI-LFA (Topology Independent Loop-Free Alternate) provides fast convergence. Which statement accurately describes TI-LFA?

Accepted Answer

TI-LFA computes a backup path that is guaranteed to be loop-free and topology independent.. TI-LFA (Topology Independent Loop-Free Alternate) computes a backup path that is guaranteed to be loop-free regardless of the network topology, using segment lists (label stacks) to steer traffic around a failure. It does not rely on the presence of a natural loop-free alternate in the IGP topology, making it topology independent. This ensures fast convergence (typically sub-50ms) without requiring signaling protocols like RSVP-TE.

Answer

TI-LFA only protects against link failures, not node failures.

Answer

TI-LFA uses a pre-computed backup tunnel signaled via RSVP-TE.

Answer

TI-LFA requires BFD to detect failures.

Question 13

A network engineer notices that voice traffic is being dropped during congestion. The traffic is marked with DSCP EF. After reviewing the QoS policy, it is discovered that the voice traffic is not being placed into a priority queue. Which configuration change would ensure voice traffic receives priority treatment?

Accepted Answer

Add the 'priority' command under the voice class in the policy map. DSCP EF (Expedited Forwarding, per RFC 3246) requires strict priority queuing to guarantee low latency and jitter for voice traffic. The 'priority' command under the voice class in a policy map places the traffic into a strict priority queue (LLQ), ensuring it is serviced before any other queue during congestion. Without this command, the voice traffic is treated as a regular class, subject to bandwidth constraints and potential drops.

Answer

Increase the queue limit to 1000 packets

Answer

Increase the bandwidth percentage for the voice class

Answer

Enable WRED on the voice class

Question 14

A network engineer is troubleshooting an LDP session failure between two directly connected routers. The routers are configured with the 'mpls ldp' command under the interface. The 'show mpls ldp neighbor' command shows no neighbors. Which two additional pieces of information should the engineer verify? (Choose two.)

Accepted Answer

Verify that the interface IP addresses are in the same subnet.. LDP sessions are established only between directly connected LSRs when using the default 'mpls ldp' interface configuration. The LDP Hello messages are sent as UDP packets to the multicast address 224.0.0.2, and if the interface IP addresses are not in the same subnet, the Hello messages will not be received, preventing neighbor discovery.

Answer

Verify that TCP port 646 is open on the interface ACL.

Answer

Verify that 'mpls ip' is enabled globally.

Answer

Verify that OSPF is configured on the interface.

Question 15

A service provider is using Segment Routing with TI-LFA for fast convergence. During a link failure, the router performing the local repair must compute a backup path that avoids the failed link. Which type of Adjacency Segment Identifier (Adj-SID) is required for the backup path to be loop-free?

Accepted Answer

Protected Adj-SID. In Segment Routing with TI-LFA, the router performing local repair must compute a backup path that is guaranteed loop-free. A Protected Adj-SID (Adj-SID with the B-flag set) is required because it indicates that the adjacency segment is protected by a fast-reroute mechanism, allowing the router to use a precomputed backup path that avoids the failed link while ensuring no forwarding loops occur.

Answer

Anycast Adj-SID

Answer

Unprotected Adj-SID

Answer

Backup Adj-SID

Question 16

A service provider is implementing MPLS L3VPN and needs to ensure that BGP route advertisement uses a specific next-hop. Which technique ensures BGP advertises the PE-CE next-hop instead of the PE-PE loopback?

Accepted Answer

next-hop-self. In MPLS L3VPN, the PE-CE next-hop (the PE interface facing the CE) must be advertised to the remote PE so that the remote PE knows to forward traffic directly to the local PE's CE-facing interface. The `next-hop-self` command on the PE forces BGP to set the next-hop to the PE's own IP address (typically the loopback or the interface used for BGP peering), which overrides the default behavior of preserving the original next-hop. This ensures that the remote PE uses the correct next-hop for VPN traffic.

Answer

Route-map with set next-hop

Answer

next-hop-unchanged

Answer

Disable next-hop-check

Question 17

Refer to the exhibit. A telemetry subscription is configured on an IOS-XR router. The collector at 10.1.1.100 is not receiving data. Which configuration error is present?

Accepted Answer

Missing 'protocol' specification in the destination-group. In IOS-XR telemetry, a destination-group must include the 'protocol' keyword (e.g., 'protocol grpc' or 'protocol tcp') to specify the transport protocol for the collector. Without this specification, the router does not know how to send the telemetry data, and the collector at 10.1.1.100 will not receive any data.

Answer

The destination IP address is incorrect

Answer

The sample-interval is too short

Answer

The subscription is not committed

Answer

The sensor-group path is invalid

Question 18

In RSVP-TE, which message is sent by the headend router along the explicit route to reserve resources and establish an LSP?

Accepted Answer

PATH. In RSVP-TE, the headend router sends a PATH message along the explicit route to signal the desired LSP. This message carries the label request, traffic parameters, and the explicit route object (ERO) to each downstream router, which then forwards it hop-by-hop toward the destination. Only after the PATH message reaches the tailend does the tailend respond with a RESV message to actually reserve resources and establish the LSP.

Answer

RESV

Answer

RSVP Hello

Answer

RSVP Tear

Question 19

A network engineer is configuring QoS on a Cisco ASR 9000 router to support multiple traffic classes. The policy must ensure that real-time traffic (EF) is not starved by high-volume bulk data (AF11). Which queuing strategy should be applied to the EF class to provide low latency and strict priority?

Accepted Answer

Priority Queuing (LLQ). Low Latency Queuing (LLQ) is the correct choice because it combines strict priority queuing with CBWFQ, ensuring that EF (real-time) traffic is always served before other queues. This prevents starvation of delay-sensitive traffic by bulk data (AF11) while still allowing fair treatment of other classes. LLQ is the standard Cisco mechanism for supporting real-time traffic like VoIP with guaranteed low latency.

Answer

Weighted Round Robin (WRR)

Answer

Class-Based Weighted Fair Queuing (CBWFQ)

Answer

First-In-First-Out (FIFO)

Question 20

A service provider has deployed segment routing with OSPF as the IGP in its core network. The network consists of 100 routers in a single area. The operations team reports that after a link failure between Router X and Router Y, traffic from Router A to Router B is taking a suboptimal path even though IGP convergence is complete and all routers have updated their LSDB. Router A and Router B are both segment routing capable. The team verifies that no SR-TE policies are configured and that all routers are using the default SPF algorithm. The expected shortest path from A to B should go through the newly restored link, but instead it still traverses an alternate path. Which action should resolve the issue?

Accepted Answer

Execute 'clear ip ospf process' on all routers along the expected path.. The suboptimal path persists despite LSDB convergence due to OSPF not performing a full SPF recalculation after the link restoration. 'clear ip ospf process' forces a complete SPF computation and reflooding of LSAs, which corrects the routing table. Option A is incorrect because adjacency SIDs are dynamically allocated and removing/re-adding them does not address the root cause. Option C is incorrect because SR-TE policies are not required for shortest-path forwarding and may unnecessarily override the IGP. Option D is incorrect because clearing MPLS forwarding labels does not affect the IGP routing decision; the issue is at Layer 3, not label allocation.

Answer

Remove and re-add the adjacency SID configuration on the restored link.

Answer

Configure an SR-TE policy from A to B with an explicit path using the restored link.

Answer

Issue 'clear mpls forwarding labels' on Router A to rebuild the label table.