350-501 Practice Exam 54 — 20 Questions

Question 1

A network engineer is configuring MPLS LDP on a new router. After enabling LDP globally and on the interface, the LDP session does not establish. The IGP (OSPF) is fully operational. What should the engineer verify first?

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

Accepted Answer

The interface has 'mpls ip' enabled.. The most common reason LDP fails to establish after global and interface configuration is that the interface is not enabled for MPLS forwarding. The command 'mpls ip' on the interface is required to activate LDP hello messages and label binding on that link. Without it, LDP remains disabled on the interface even if LDP is enabled globally and OSPF is working.

Answer

The LDP password matches.

Answer

The router-id is a loopback.

Answer

The IGP metric is set.

Question 2

Which THREE statements about implementing QoS in an MPLS VPN environment are correct? (Choose three.)

Accepted Answer

DSCP values can be set at the PE router to classify customer traffic into different classes.. On a PE router, DSCP values can be set or remarked to classify incoming customer traffic into distinct service classes. This classification is performed at the ingress edge of the MPLS VPN network, allowing the provider to apply appropriate per-hop behaviors (PHBs) before the traffic is label-switched.

Answer

QoS policies cannot be applied to MPLS interfaces due to label encapsulation.

Answer

802.1p CoS marking is preserved across the MPLS backbone by default.

Question 3

Refer to the exhibit. Which statement is true regarding the forwarding entry for 10.2.2.0/24?

Accepted Answer

This entry uses explicit label request (not PHP).. The forwarding entry for 10.2.2.0/24 shows an outgoing label of 20, which means the egress LSR is not performing penultimate hop popping (PHP). In MPLS, when the outgoing label is not the implicit-null label (3) or explicit-null label (0), the penultimate hop must push that label, and the egress LSR will perform a full label lookup. This is an explicit label request, not PHP.

Answer

The outgoing interface uses penultimate hop popping.

Answer

The outgoing label is Untagged.

Answer

The local label is 20.

Question 4

An engineer is using RESTCONF to configure an interface on a Cisco IOS-XE device. The request returns a 400 Bad Request error. What is the most likely cause?

Accepted Answer

The JSON payload contains incorrect data types or missing mandatory leafs. A 400 Bad Request in RESTCONF indicates a client-side error, typically malformed syntax. The most common cause is an invalid JSON payload, such as incorrect data types (e.g., string instead of integer) or missing mandatory leafs defined in the YANG module. RESTCONF validates the payload against the YANG schema before applying the configuration, and any schema violation triggers a 400 error.

Answer

The device does not support RESTCONF

Answer

The user does not have sufficient privileges

Answer

The URI is incorrect

Answer

The YANG module is not loaded

Question 5

A service provider is troubleshooting an MPLS L3VPN scenario where a customer in VRF Blue cannot reach a server in VRF Blue at a remote site. The PEs are running MPLS with LDP. The VRF on both PEs shows the remote prefix. The PE at the local site shows the label from the remote PE for the prefix in the BGP table. However, when pinging from the CE, the packets are dropped. A packet capture on the core shows MPLS packets with the correct VPN label, but the transport label is missing. Further investigation shows that the LDP session between the two PEs is up, but the LDP label binding for the remote PE's loopback is not present. What is the most likely cause and correct action?

Accepted Answer

Check MPLS LDP interface configuration and enable LDP on all core interfaces. The LDP session is up, but the label binding for the remote loopback is missing. This could be because LDP is not enabled on the underlying interfaces between the PEs, or because the label space is configured incorrectly. Typically, if LDP is enabled on all core interfaces, label bindings for loopbacks are automatically exchanged. The missing binding suggests that either LDP is not configured on some interfaces, or an access-list is blocking LDP. The most direct action is to verify that LDP is enabled on all core interfaces using the command 'show mpls ldp interface'.

Answer

Increase the TTL on the MPLS packets

Answer

Change the transport address in LDP to the loopback IP

Answer

Configure the remote PE's loopback to be advertised via BGP

Question 6

In the context of MPLS QoS, at which point are EXP bits typically set based on the DSCP value of the IP packet?

Accepted Answer

At the ingress PE router before the packet enters the MPLS core.. In MPLS QoS, the EXP bits are set at the ingress PE router because this is where the IP packet first encounters the MPLS domain. The ingress PE maps the incoming IP packet's DSCP value to the MPLS EXP bits using a configured trust boundary or marking policy (e.g., 'policy-map' with 'set mpls experimental imposition'). This ensures that the QoS treatment is applied consistently across the MPLS core, as P routers typically forward based on the EXP bits without inspecting the original IP header.

Answer

At the P routers in the core based on routing protocols.

Answer

At the egress PE router when packets are forwarded to the CE.

Answer

At the CE router before sending to the PE.

Question 7

A multicast service provider is designing an IPv4 PIM Sparse Mode (PIM-SM) network with multiple RPs. The network must support Anycast RP using MSDP. Which THREE statements are correct regarding this design?

Accepted Answer

Anycast RP allows multiple RPs to share the same IP address, providing redundancy and load balancing. Options A, C, and E are correct. Option A: Anycast RP uses the same IP address on multiple RPs, providing redundancy and load balancing. Option C: MSDP peering is required between the anycast RPs to exchange active source information. Option E: IGMPv3 is needed on last-hop routers to support Source-Specific Multicast (SSM) if used together with Anycast RP. Option B is incorrect: Auto-RP is not required; Anycast RP can work with static RP assignments. Option D is incorrect: MSDP is used for Anycast RP, not specifically for SSM; SSM does not require MSDP.

Answer

Auto-RP must be used to distribute the RP information to all routers

Answer

MSDP is only required if the network uses Source-Specific Multicast (SSM)

Question 8

An SP network engineer is designing a new segment routing traffic engineering deployment within a single IGP area. The network consists of 50 core routers running IS-IS and MPLS. The engineer needs to steer traffic from Router A to Router D over a path that avoids high latency links. Which technology should be used to define and instantiate the explicit path?

Accepted Answer

Configure an SR-TE policy with an explicit path using segment lists.. SR-TE policies with explicit paths using segment lists allow steering traffic over a specific sequence of nodes or links, avoiding high latency links. Option A is wrong because LDP distributes labels for IGP shortest path forwarding, not explicit path control. Option B is wrong because while PCE can assist with path computation, the question asks for the technology to define and instantiate the explicit path, which is the SR-TE policy itself. Option C is wrong because BGP-LU distributes labels for BGP prefixes, not for traffic engineering paths.

Answer

Use LDP to distribute labels and rely on IGP shortest path.

Answer

Set up a Path Computation Element (PCE) and delegate path computation.

Answer

Implement BGP LU to create an explicit path via local policies.

Question 9

Which two statements about MPLS Layer 3 VPN route target (RT) communities are correct?

Accepted Answer

RTs must be manually configured on every PE router for each VRF.. Route targets (RTs) are BGP extended community attributes used to control the import and export of VPN routes between VRFs on PE routers. Option D is correct. While auto-RT can automatically generate RTs, many deployments require manual configuration on each PE for each VRF, especially when custom RTs are needed; therefore, option A is also correct. Option B is incorrect because loop prevention in MPLS VPNs uses mechanisms like AS-override and allowas-in, not RTs. Option C is incorrect because the next-hop IP address for VPN prefixes is determined by the BGP next-hop attribute, not RTs. Option E is incorrect because RTs are carried within BGP VPNv4 updates as extended communities, not in the IP header.

Answer

RTs are used to prevent routing loops within the MPLS VPN network.

Answer

RTs are used to determine the next-hop IP address for VPN prefixes.

Answer

RTs are carried in the IP header to enable per-VPN forwarding.

Question 10

A service provider needs to offer L3VPN services to multiple customers, each with overlapping IP addresses. The provider plans to use MPLS VPNs with VRFs. Which statement is TRUE regarding the configuration of VRFs on the PE routers?

Accepted Answer

Route targets (RTs) are used to control the import and export of VPNv4 routes between VRFs. Route targets (RTs) are used on PE routers to control which VPNv4 routes are imported into a VRF and which routes from a VRF are exported to MP-BGP. This allows proper segregation and connectivity for MPLS L3VPNs. Option A is incorrect because VRF-to-CE routing can use any protocol, not just OSPF. Option C is incorrect because MPLS labels are automatically assigned per VRF by the router, not manually per prefix. Option D is incorrect because the route distinguisher (RD) only needs to be unique per VRF on a single PE router; it can be the same across different PE routers if the RTs are configured correctly.

Answer

The VRF must run OSPF to exchange routes with the CE router

Answer

MPLS labels are assigned per prefix inside a VRF by manual configuration

Answer

Each VRF must have a unique route distinguisher across the entire provider network

Question 11

An engineer is configuring MPLS LDP on a Cisco router. Which TWO statements about LDP operation are correct? (Choose two.)

Accepted Answer

LDP discovers neighbors using UDP hello messages on port 646, and then establishes a TCP session on port 646.. LDP uses downstream unsolicited mode by default in Cisco IOS; liberal label retention means it keeps all labels even if not directly reachable. Option B is correct because LDP uses UDP hello on port 646 for neighbor discovery and then TCP on port 646 for session establishment. Option D is correct because liberal label retention stores all received labels, even from non-next-hop neighbors, improving convergence. Option C is incorrect because LDP uses downstream unsolicited, not downstream-on-demand. Options A and E are incorrect.

Answer

The LFIB (Label Forwarding Information Base) in LDP is used to store only the outgoing label mapping for each FEC.

Answer

LDP uses a downstream-on-demand label distribution method, where labels are sent only when requested by an upstream router.

Answer

LDP label allocation happens only after the IGP converges, and labels are allocated for all IGP routes by default.

Question 12

An SP is planning to offer L3VPN services. Which TWO components are essential for BGP/MPLS IP VPN operation?

Accepted Answer

VRF (Virtual Routing and Forwarding) on provider edge routers.. VRF (D) provides per-customer routing isolation, and MP-BGP with VPN-IPv4 address family (E) is used to exchange VPN routes between PEs. MPLS labels are used for data plane forwarding but are not considered control plane components for VPN operation.

Answer

Full mesh of IPsec tunnels between all PEs.

Answer

OSPF as the IGP within the MPLS core.

Answer

A single global routing table shared among all customers.

Question 13

An SP engineer is configuring a policy-map to implement hierarchical QoS. Which two statements are correct when using MQC for hierarchical QoS? (Choose two.)

Accepted Answer

The child policy-map must include the bandwidth and priority commands to define queuing behavior.. In hierarchical QoS using MQC, the child policy-map must include bandwidth/priority commands to define queuing behavior (B). The parent policy-map contains the shape command and references a child policy-map (C). The service-policy command is applied directly to the interface, referencing the parent policy (D). Therefore, options B, C, and D are correct. Note: The question stem asks for two, but all three are accurate statements.

Answer

Hierarchical QoS supports only two levels of nesting.

Answer

Both shape and priority commands must be configured in the same policy-map.

Question 14

A service provider is transitioning from LDP-based MPLS to Segment Routing (SR-MPLS). Which TWO statements correctly describe differences or interoperability considerations? (Choose two.)

Accepted Answer

The Segment Routing Global Block (SRGB) is a reserved label range that is used to allocate Prefix-SIDs, ensuring end-to-end label consistency.. In SR-MPLS, the Segment Routing Global Block (SRGB) is a reserved label range used to allocate Prefix-SIDs, ensuring end-to-end label consistency across the SR domain. An Adjacency-SID is a local label that represents a specific link and is used for traffic steering over that link. LDP and SR-MPLS can coexist with proper mapping, making option B incorrect. Node-SIDs are allocated from the SRGB and are unique domain-wide, not just within an area. In SR-MPLS, the ingress router pushes a label stack, and intermediate nodes perform penultimate hop popping (PHP) or continue to forward, but they do not swap labels as in LDP. Therefore, options C and D are correct.

Answer

In SR-MPLS, the Node-SID is allocated from the dynamic label range and is unique only within the IGP area.

Answer

LDP and SR-MPLS cannot coexist on the same router; the router must be configured exclusively for one or the other.

Answer

In SR-MPLS, the label stack is built by the ingress router and nodes swap the top label without pushing additional labels.

Question 15

A network engineer is designing a Segment Routing over IPv6 (SRv6) deployment. Which TWO statements are true about the SRv6 SID format and behaviors? (Choose two.)

Accepted Answer

End.DT6 behavior is used for IPv6 L3VPN services.. SRv6 SID is 128 bits and consists of locator, function, and optional argument. End.DT6 is used for IPv6 L3VPN services. End.DT4 is for IPv4 L3VPN, not for L2VPN. End.X is not the default behavior; the default is End. The SRH is an extension header that does not replace the IPv6 header. Therefore, the correct answers are B and D.

Answer

End.X behavior is a default behavior for any SID that is not explicitly defined.

Answer

End.DT4 behavior is used for EVPN L2VPN services over SRv6.

Answer

The SRH (Segment Routing Header) is an extension header that replaces the IPv6 header when SR is used.

Question 16

A network engineer is implementing QoS on a Cisco ASR 1000 for a customer with multiple service classes. The customer requires that mission-critical data not be starved when voice traffic bursts. Which two actions should the engineer take? (Choose two.)

Accepted Answer

Configure a policer on the voice class to limit its bandwidth. Policing the voice class limits its bandwidth to a configured rate (e.g., using the `police` command under the class-map), preventing voice bursts from starving mission-critical data. This ensures that voice traffic does not exceed its allocated share, leaving enough bandwidth for other classes. Option D is correct because allocating a minimum bandwidth guarantee to the data class (e.g., using the `bandwidth` command under the class-map) ensures that even when voice bursts occur, the data class receives a reserved amount of bandwidth, preventing starvation. Together, these actions enforce admission control and bandwidth reservation, aligning with the customer's requirement.

Answer

Enable WRED on the voice class

Answer

Use tail drop with a high threshold for the data class

Answer

Configure a shape on the voice class to 75% of interface bandwidth

Question 17

In SR-MPLS traffic engineering, which three components are essential for configuring an SR Policy on a headend router? (Choose three.)

Accepted Answer

Headend router. In SR-MPLS traffic engineering, an SR Policy on a headend router is defined by three essential components: the headend router itself (the router that originates the policy), the endpoint (destination prefix), and a color value that distinguishes multiple policies to the same endpoint. Bandwidth reservation (A) is optional and not part of the basic policy identifier. Explicit path with ERO (B) is a way to specify the path but is not a required component of the policy definition; segment lists are used instead. Therefore, the correct choices are C (headend), D (endpoint), and E (color).

Answer

Bandwidth reservation value

Answer

Explicit path with ERO

Question 18

In SR-MPLS, which mechanism replaces LDP for label distribution?

Accepted Answer

IGP (IS-IS or OSPF) with Segment Routing extensions. Segment Routing replaces LDP by using IGP extensions (IS-IS or OSPF) to advertise Prefix-SIDs (Node-SIDs) and Adjacency-SIDs. In IS-IS, TLV 242 (SR Capabilities) advertises the SRGB, while Node-SIDs are advertised via Prefix-SID sub-TLVs (e.g., TLV 135 for IPv4), eliminating the need for a separate label distribution protocol.

Answer

RSVP-TE

Answer

MPLS-TE

Answer

BGP-LU

Question 19

Which TWO of the following are characteristics of MPLS LDP?

Accepted Answer

Label distribution based on IP routing table. B is correct because MPLS LDP distributes labels for routes found in the IP routing table, not for BGP routes. LDP peers exchange label bindings for each prefix in the IGP routing table, enabling label-switched paths (LSPs) for those destinations. This is fundamental to LDP's operation as defined in RFC 5036.

Answer

Distributes labels for BGP routes

Answer

Requires an IGP like OSPF for session reachability

Answer

TCP for both discovery and session

Question 20

Drag and drop the steps to configure a GRE tunnel on a Cisco router into the correct order.

Accepted Answer

Step 1: Create tunnel interface (interface tunnel0), Step 2: Configure tunnel source (tunnel source source-ip), Step 3: Configure tunnel destination (tunnel destination dest-ip), Step 4: Assign IP address (ip address ip mask). GRE tunnel configuration involves creating the tunnel interface, setting source and destination, and assigning an IP address.

Answer

Step 1: Create tunnel interface, Step 2: Assign IP address, Step 3: Configure tunnel source, Step 4: Configure tunnel destination

Answer

Step 1: Configure tunnel destination, Step 2: Configure tunnel source, Step 3: Create tunnel interface, Step 4: Assign IP address

Answer

Step 1: Assign IP address, Step 2: Create tunnel interface, Step 3: Configure tunnel source, Step 4: Configure tunnel destination