350-501 Practice Test 32 — 15 Questions

Question 1

A service provider has a network with PE1, P1, P2, and PE2 connected in a chain: PE1-P1-P2-PE2. The IGP is IS-IS with wide metrics, and MPLS LDP is enabled on all interfaces. The goal is to provide L3VPN services between PE1 and PE2. The engineer has configured MP-iBGP between PE1 and PE2, and the VPNv4 routes are exchanged. However, when a CE router behind PE1 tries to reach the CE behind PE2, the traffic fails. The engineer checks the MPLS forwarding table on PE1 and sees that the label for the BGP next-hop (PE2's loopback) is 'Untagged'. The BGP next-hop is reachable via ICMP. What is the most likely cause of this issue?

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

Accepted Answer

LDP is not enabled on the interface between PE1 and P1.. The MPLS forwarding table on PE1 shows the label for PE2's loopback as 'Untagged', which indicates that LDP has not assigned a label for that prefix on the incoming interface. Since LDP is responsible for distributing labels for IGP routes (like loopbacks) in an MPLS LDP-enabled network, the missing label means LDP is not operational on the link between PE1 and P1. Without a label, PE1 cannot push an MPLS label for the BGP next-hop, causing the VPNv4 traffic to be dropped or forwarded as IP, which fails because the core routers (P1, P2) do not have the VPN route.

Answer

The IS-IS metric between PE1 and P1 is too high, causing suboptimal routing.

Answer

The MPLS TTL propagation is disabled, causing packets to be dropped.

Answer

The MTU on the link between PE1 and P1 is set to 1500, causing fragmentation.

Question 2

Which TWO statements about EVPN Route Type 2 (MAC/IP advertisement) are correct?

Accepted Answer

It is used for MAC learning and ARP suppression. EVPN Route Type 2 is used to advertise MAC and IP addresses of hosts, enabling MAC learning and ARP suppression in EVPN-VXLAN fabrics. By distributing host MAC and IP bindings via BGP, it allows VTEPs to learn remote MAC addresses without data-plane flooding and to suppress ARP requests by responding locally when the IP-to-MAC mapping is known.

Answer

It provides load balancing via aliasing

Answer

It includes the Ethernet Segment Identifier (ESI)

Answer

It is not used for host mobility

Question 3

A service provider is troubleshooting BGP route selection for prefixes received from two different peers. The first peer prepends its AS path twice, making it longer than the second peer's path. However, the router still prefers the route with the longer AS path. Which additional attribute could cause this behavior?

Accepted Answer

The route has a higher local preference. BGP selects the best path based on a sequence of comparison steps. The first step after considering the next-hop reachability is to prefer the route with the highest weight (Cisco proprietary), then the highest local preference. Since the router prefers the longer AS path, a higher local preference (option D) on the first peer's route overrides the shorter AS path of the second peer, as local preference is evaluated before AS path length in the BGP best-path selection algorithm.

Answer

The route has a lower MED

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The route has a higher weight

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The route has a lower origin type

Answer

The route has a lower neighbor router ID

Question 4

An ISP is implementing BGP communities to influence routing behavior for their customers. They want to ensure that a customer's routes are not advertised to a specific transit provider. Which BGP community should be used?

Accepted Answer

NO_EXPORT. The NO_EXPORT community (RFC 1997) tells a router to advertise the route to iBGP peers within the same AS but not to any eBGP peers. In this scenario, the ISP wants to prevent a customer's routes from being advertised to a specific transit provider, which is an eBGP neighbor. Applying the NO_EXPORT community to those routes ensures they stay within the ISP's AS and are not sent to any external transit provider.

Answer

LOCAL_AS

Answer

NO_PEER

Answer

NO_ADVERTISE

Question 5

An engineer is troubleshooting MPLS label switching in a service provider core. They notice that packets are being forwarded correctly between provider edge routers, but when they check the MPLS forwarding table on a P router, they see only implicit-null labels for some destinations. What is the most likely reason for this?

Accepted Answer

The router has a directly connected neighbor that is the egress LER.. When a P router sees only implicit-null labels (label 3) for some destinations in its MPLS forwarding table, it indicates that the router is performing penultimate hop popping (PHP). This occurs when the P router is the penultimate hop before the egress Label Edge Router (LER). The egress LER advertises an implicit-null label to its directly connected neighbor, causing the P router to pop the label and forward the packet using only the IP header. Option B is correct because the P router has a directly connected neighbor that is the egress LER, which triggers PHP.

Answer

The router is using explicit-null label due to security policies.

Answer

The router is misconfigured to use implicit-null for all labels.

Answer

The router is performing penultimate hop popping (PHP) incorrectly.

Question 6

An SP is deploying a Quality of Service (QoS) architecture in its core network to support multiple services: voice, video, and data. The core routers use MPLS and implement QoS based on the MPLS EXP bits. The architecture must ensure that voice packets are never dropped, even during congestion, while allowing video to have higher priority than data. The current design marks voice with EXP 4, video with EXP 3, and data with EXP 0. During a traffic burst, voice traffic is being dropped, which should not happen. The core routers have the following queue configuration: (1) a priority queue (PQ) for EXP 4+5, (2) a bandwidth queue for EXP 2-3, (3) a default queue for EXP 0-1. What is most likely the cause of voice drops?

Accepted Answer

The priority queue is rate-limited by a policer that shapes traffic to a certain bandwidth.. Voice drops occur because the priority queue is often rate-limited by a policer to prevent starvation of other queues. If the policer rate is set too low, voice packets are dropped during bursts. Strict priority without policing does not drop, but policing is common in SP core to protect other queues. The bandwidth queue and default queue are not the issue.

Answer

Voice traffic is being marked with EXP 4 but the priority queue also includes EXP 5, which may cause policing.

Answer

The bandwidth queue for video is configured to borrow from the priority queue.

Answer

The priority queue is not strictly priority; it shares bandwidth with other queues.

Question 7

A network engineer is troubleshooting MPLS L3VPN issues on a PE router. The CE router is receiving routes from the PE, but the CE cannot ping the remote CE. The PE shows the remote prefix in its routing table and in the VRF. What is the most likely cause?

Accepted Answer

The LSP between the PEs is not operational. The CE can receive routes from the PE (indicating that the VRF and route-target import/export are functioning correctly), but the CE cannot ping the remote CE. This points to a failure in the data plane between the PEs. If the LSP (Label Switched Path) between the PEs is not operational, MPLS labels cannot be swapped or forwarded correctly, causing packets to be dropped even though the control plane (routing table and VRF) shows the remote prefix.

Answer

The VRF route-target import/export is misconfigured

Answer

The MTU on the core links is too small

Answer

The RD is different on the two PEs

Question 8

Which protocol is typically used to distribute MPLS labels between directly connected LSRs?

Accepted Answer

LDP. LDP (Label Distribution Protocol) is the standard protocol used to distribute MPLS labels between directly connected LSRs (Label Switch Routers) in a non-traffic-engineered MPLS network. It operates by establishing LDP sessions over TCP (port 646) and exchanging label mappings for each FEC (Forwarding Equivalence Class) based on the underlying IGP routing table, enabling hop-by-hop label switching without requiring explicit path setup.

Answer

RSVP

Answer

BGP

Answer

OSPF

Question 9

What does the output indicate about the TI-LFA protection on R1?

Accepted Answer

Only 1 prefix has a backup path.. The output shows that for the protected prefix 10.1.1.1/32, the backup path is listed as 'N/A', meaning no backup path was computed. However, for the other prefix (likely a different /32), a backup path is present. This indicates that only one prefix has a backup path, making option B correct. TI-LFA computes a backup path only when a viable post-convergence path exists without microloops; if the topology doesn't allow it, the backup is marked as not available.

Answer

All prefixes have backup paths.

Answer

0 prefixes have backup paths.

Answer

2 prefixes have backup paths.

Question 10

Refer to the exhibit. An engineer notices that R1 has an LDP neighbor but 'show mpls forwarding-table' on R1 shows no label bindings for prefixes learned from R2. What is the most likely cause?

Accepted Answer

R1 is configured for 'label distribution on-demand'. R1 shows an LDP neighbor (session is up) but no label bindings for prefixes from R2. This occurs when R1 is configured for 'label distribution on-demand' (RFC 5036), meaning it only requests label bindings for prefixes in its routing table, not all prefixes from the peer. Since R1 has not yet needed those specific prefixes, it has not requested labels, so the forwarding table remains empty.

Answer

The LDP discovery source is incorrect

Answer

The peer LDP identifier is not reachable

Answer

LDP session is not established

Question 11

Which two are characteristics of the Anycast SID in Segment Routing? (Choose two.)

Accepted Answer

It provides redundancy and fast convergence by allowing traffic to be rerouted to another router.. Anycast SIDs are shared among multiple routers in the same anycast group, allowing any of those routers to process traffic destined for that SID. This provides redundancy and fast convergence because if one router fails, traffic is automatically rerouted to another router in the group without requiring control-plane signaling changes.

Answer

It identifies an adjacency between two routers.

Answer

It is typically used as an Adj-SID for load balancing.

Answer

It is unique per node and identifies a specific router.

Question 12

Which TWO of the following are characteristics of Segment Routing (SR-MPLS) compared to traditional LDP-based MPLS? (Select two.)

Accepted Answer

SR-MPLS does not require LDP or RSVP-TE for label distribution.. Segment Routing (SR-MPLS) eliminates the need for label distribution protocols like LDP or RSVP-TE. Instead, SR-MPLS encodes label information directly in the packet header as a segment list, and routers use the IGP (e.g., OSPF or IS-IS) to advertise segment identifiers (SIDs). This simplifies the control plane and removes protocol dependencies.

Answer

SR-MPLS requires a dedicated label range from 16 to 99.

Answer

SR-MPLS does not support traffic engineering.

Answer

SR-MPLS cannot provide fast reroute protection.

Question 13

A network engineer is designing an MPLS core using Segment Routing. They want to minimize label stack depth while still providing fast convergence using TI-LFA. Which prefix-SID assignment strategy should be used?

Accepted Answer

Advertise prefix-SIDs for all IGP prefixes. Advertising prefix-SIDs for all IGP prefixes allows TI-LFA to compute backup paths using any prefix in the network, not just loopbacks. This minimizes label stack depth by enabling TI-LFA to use a single prefix-SID for the repair path, avoiding the need for additional labels or explicit path constructs. Fast convergence is achieved because TI-LFA precomputes a backup next-hop that can be activated immediately upon failure, and having all prefixes as SIDs ensures the backup path can be expressed with minimal label imposition.

Answer

Assign a node-SID to each loopback interface

Answer

Use per-interface label mode from SRGB

Answer

Allocate labels per-VRF on PE routers

Question 14

Which THREE are valid methods to provide fast convergence in an MPLS/Segment Routing network? (Select three.)

Accepted Answer

Topology-Independent LFA (TI-LFA). B is correct because Topology-Independent LFA (TI-LFA) provides fast convergence in Segment Routing networks by computing a backup path that is guaranteed to be loop-free regardless of the network topology. It leverages the Segment Routing data plane to pre-install a repair path using a segment list, enabling sub-50ms failover without relying on IGP convergence.

Answer

BGP Prefix Independent Convergence (PIC)

Answer

IP Fast Reroute (IPFRR)

Question 15

An ISP is deploying EVPN-VXLAN for its data center interconnect. Which statement about control-plane learning is correct?

Accepted Answer

MAC addresses are learned via BGP MP-BGP EVPN address family. In EVPN-VXLAN deployments for data center interconnect, MAC address learning occurs via the control plane using BGP with the MP-BGP EVPN address family (AFI 25 / SAFI 70). This eliminates the need for traditional data-plane flooding and learning, providing better scalability and control. The correct statement is that MAC addresses are learned via BGP MP-BGP EVPN address family.

Answer

MAC addresses are learned via IRB

Answer

MAC addresses are learned via OSPF

Answer

MAC addresses are learned via VXLAN data plane