350-501 Practice Exam 62 — 20 Questions

Question 1

A network engineer needs to design a multicast solution for IPTV distribution across a service provider network. Which protocol is best suited for building a shared distribution tree where the RP is dynamically elected?

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

Accepted Answer

PIM-SM with BSR. PIM-SM with BSR (Bootstrap Router) is the correct choice because it provides a dynamic, automated mechanism for electing an RP (Rendezvous Point) within a PIM-SM domain. BSR uses a distributed election process where candidate BSRs exchange bootstrap messages to elect a single BSR, which then collects candidate-RP advertisements and distributes the RP-set to all routers, enabling shared distribution trees without manual RP configuration.

Answer

PIM-DM

Answer

PIM-SM with Auto-RP

Answer

MSDP

Question 2

In EVPN multihoming with all-active mode, what is the purpose of the aliasing capability?

Accepted Answer

It enables one PE to advertise MAC addresses for another PE in the same ES.. In EVPN all-active multihoming, the aliasing capability allows a PE that has learned a MAC address via local attachment to the Ethernet Segment (ES) to advertise that MAC address on behalf of other PEs in the same ES. This enables remote PEs to load-balance traffic destined to that MAC across all multihomed PEs, even if only one PE actually learned the MAC locally. Without aliasing, traffic would be forced to the specific PE that learned the MAC, defeating the purpose of all-active redundancy.

Answer

It permits the use of a single ESI label across all PEs in the ES.

Answer

It reduces the number of BGP updates by aggregating MAC routes.

Answer

It allows load balancing of traffic across all PEs.

Question 3

Which two of the following are characteristics of MPLS Traffic Engineering that differentiate it from pure MPLS forwarding?

Accepted Answer

Allows bandwidth reservation. MPLS Traffic Engineering (MPLS-TE) explicitly supports bandwidth reservation, which allows an operator to guarantee a certain amount of bandwidth for a traffic-engineered LSP. This is a key differentiator from pure MPLS forwarding, where LSPs are created without any bandwidth awareness and simply forward packets based on the label-swapping mechanism.

Answer

Requires LDP for LSP creation

Answer

Provides optimal load balancing based on topology

Answer

Uses RSVP for label distribution

Question 4

A service provider is migrating from LDP to Segment Routing in an IS-IS network. After enabling IS-IS with segment-routing on all routers, they observe that some prefixes still receive labels from LDP instead of from SR. Which configuration is most likely missing on these routers?

Accepted Answer

Missing 'segment-routing mpls' command under IS-IS. When migrating from LDP to Segment Routing in an IS-IS network, the 'segment-routing mpls' command must be explicitly enabled under the IS-IS routing process. Without this command, IS-IS will not allocate MPLS labels for prefixes using the Segment Routing (SR) extension, causing the router to fall back to LDP for label distribution. This is the most common missing configuration when SR labels are not being assigned.

Answer

OSPF segment-routing is still configured

Answer

TI-LFA is not enabled

Answer

SRGB range conflicts with MPLS label range

Question 5

A service provider is deploying MPLS L3VPN over an OSPF backbone. The PE routers are configured with OSPF as the IGP. The CE router of customer A is connected to two PEs for redundancy. Which configuration is required on the PE routers to ensure that the CE router can load-balance traffic across both PEs without loops?

Accepted Answer

Configure OSPF with the capability vrf-lite and enable the down-bit on the PE-CE link.. Configuring OSPF with the capability vrf-lite and enabling the down-bit on the PE-CE link prevents routing loops in a multi-homed CE scenario. The down-bit is set by the PE when redistributing routes into OSPF, ensuring that the CE does not re-advertise those routes back to another PE, which would cause a loop. The vrf-lite capability allows the CE to understand the down-bit without requiring full MPLS/VPN functionality, enabling load-balancing across both PEs safely.

Answer

Use OSPF sham-links between the two PEs.

Answer

Use the BGP cost community to adjust the path selection on the CE.

Answer

Disable the DN-bit on the PE-CE OSPF interface.

Question 6

A service provider is designing a QoS policy for a multi-service MPLS VPN network that carries voice, video, and data traffic. The network uses DiffServ and MPLS EXP markings. The design must ensure that voice traffic is given priority over video and data, while video traffic should have better treatment than data but not at the expense of voice. The provider plans to use a hierarchical QoS (HQoS) policy at the PE-CE interfaces. Which configuration approach best meets these requirements?

Accepted Answer

Apply a parent policy with a shape for the total bandwidth and a child policy with LLQ for voice and CBWFQ for video and data.. Hierarchical QoS (HQoS) allows the service provider to enforce a total bandwidth shape at the parent level while using a child policy to apply LLQ for voice (ensuring strict priority) and CBWFQ for video and data (ensuring video gets better treatment than data without starving voice). This meets the requirement that video should not degrade voice, as the parent shape prevents any single class from monopolizing the link, and the child policy’s LLQ guarantees voice priority over all other traffic.

Answer

Mark voice traffic with EXP 5, video with EXP 4, and data with EXP 0, and rely on the core to prioritize based on EXP.

Answer

Use a single-level policy with LLQ for voice and video together, and CBWFQ for data.

Answer

Apply class-based shaping to each traffic class separately on the interface.

Question 7

A service provider wants to use Segment Routing TE with Flex-Algo to engineer traffic around links with low bandwidth. Which configuration approach should be taken?

Accepted Answer

Define a Flex-Algo with a metric type of 'min-unidirectional-link-bandwidth'.. Flex-Algo allows the definition of a custom metric type, such as 'min-unidirectional-link-bandwidth', which enables Segment Routing TE to compute paths that avoid low-bandwidth links. This approach directly addresses the requirement to engineer traffic around links with low bandwidth by using bandwidth as the path selection metric.

Answer

Configure a policy-map to apply QoS marking on TE tunnels.

Answer

Use the 'min-delay' metric in Flex-Algo to prefer low-delay links.

Answer

Use standard IS-IS metric and adjust it on low-bandwidth links to a high value.

Question 8

An SP uses NSO with a YANG service model to provision an L3VPN. The service model references a leafref to a VPN ID that must exist in a separate list. Which YANG statement ensures the leafref value is valid against the referenced list?

Accepted Answer

type leafref { path '/vpn-list/vpn-id'; }. leafref automatically validates that the leaf value matches a leaf in the referenced path.

Answer

must 'current() = /vpn-list/vpn-id';

Answer

mandatory true;

Answer

type string { pattern '[0-9]+'; }

Question 9

Which IS-IS router type maintains a full Layer 2 routing table of all areas and can act as a gateway between areas?

Accepted Answer

Level 2. In IS-IS, Level 2 routers route between areas and maintain a complete L2 database. Level 1 routers route within an area, and Level 1/2 routers act as both, but the question asks for the type that maintains a full L2 routing table of all areas. Level 2 routers specifically perform inter-area routing.

Answer

Level 0

Answer

Level 1/2

Answer

Level 1

Question 10

An engineer notices that an MPLS LSP in the core is not establishing. Which troubleshooting step should be performed first?

Accepted Answer

Verify LDP neighbor adjacency. The first step in troubleshooting an MPLS LSP that is not establishing is to verify LDP neighbor adjacency because LDP is the protocol responsible for distributing labels across the MPLS domain. Without a working LDP session between adjacent LSRs, label bindings cannot be exchanged, and the LSP cannot be built. This is the foundational layer for MPLS forwarding, so it must be checked before higher-layer issues like routing or reachability.

Answer

Ping the far-end loopback interface

Answer

Check BGP IPv4 unicast neighbors

Answer

Check OSPF neighbor adjacency on all links

Question 11

When automating configuration changes across a large network using a tool like Cisco NSO, what is the best practice to minimize the risk of negative impact?

Accepted Answer

Use a staging environment with identical configuration to test before production. Using a staging environment with identical configuration to test before production is the best practice because it allows validation of automation scripts and configuration changes in a controlled setting that mirrors the production network. This approach identifies potential errors, compatibility issues, or unintended side effects without risking service disruption, aligning with Cisco NSO's recommended deployment workflow that emphasizes pre-production testing to ensure change reliability.

Answer

Rely on rollback automation

Answer

Limit automation to read-only commands

Answer

Automate only during maintenance windows

Answer

Apply changes directly to production devices

Question 12

Refer to the exhibit. A network automation engineer uses NETCONF to retrieve the QoS policy 'POLICE-CUSTOMER'. Based on the response, what is the effect of this policy?

Accepted Answer

It polices all traffic to a maximum of 256 kbps and drops excess.. The NETCONF response shows a 'police' configuration under the QoS policy 'POLICE-CUSTOMER' with a committed information rate (CIR) of 256000 bps (256 kbps) and a conform-action of 'transmit' with an exceed-action of 'drop'. This is a standard policing action that meters traffic to the specified rate and drops any packets that exceed it, as defined in RFC 2697 (Single Rate Three Color Marker). Option A correctly identifies this behavior.

Answer

It sets DSCP CS4 on traffic exceeding 256 kbps.

Answer

It guarantees a priority queue for traffic up to 256 kbps.

Answer

It shapes traffic to an average rate of 256 kbps.

Question 13

Drag and drop the steps for the BGP route selection process (best path selection) in order.

Accepted Answer

Weight, then Local Preference, then Locally Originated, then AS_PATH Length, then MED. BGP best path selection follows a specific order: weight, local preference, locally originated, AS_PATH length, MED.

Answer

Local Preference, then Weight, then Locally Originated, then AS_PATH Length, then MED

Answer

Weight, then Local Preference, then AS_PATH Length, then MED, then Locally Originated

Answer

Weight, then AS_PATH Length, then Local Preference, then Locally Originated, then MED

Question 14

In Segment Routing over MPLS (SR-MPLS), how are Prefix-SIDs advertised in IS-IS?

Accepted Answer

Via IS-IS TLV 135 and TLV 149. In SR-MPLS, Prefix-SIDs are advertised in IS-IS using TLV 135 (Extended IP Reachability) to carry the prefix and its metric, and TLV 149 (IS-IS Router Capability) to carry the SID/Label Binding sub-TLV, which maps the prefix to a specific label (Prefix-SID). This allows IS-IS to distribute both the prefix reachability and its associated segment identifier in a single protocol update, enabling source routing without LDP or RSVP-TE.

Answer

Via LDP label mapping messages

Answer

Via RSVP-TE PATH messages

Answer

Via BGP VPN labels

Question 15

An engineer is configuring VPLS on a Cisco ASR 9000. After verifying the pseudowire status, they notice that the 'pw status' shows 'down' for one of the PWs. Which two conditions could cause this? (Choose two.)

Accepted Answer

Mismatched encapsulation type on the pseudowire (e.g., Ethernet vs. VLAN). A mismatched encapsulation type (e.g., Ethernet vs. VLAN) on the pseudowire causes the PW to fail to come up. In VPLS, the encapsulation must match between the local and remote PE for the pseudowire to be operational; otherwise, the PW status will show 'down' due to a negotiation failure.

Answer

VPLS ID mismatch on the same bridge domain

Answer

SNMP MIB not loaded

Answer

LDP session is missing

Question 16

Which QoS mechanism is most effective for preventing congestion on a service provider edge link when traffic exceeds the link capacity?

Accepted Answer

Traffic shaping. Traffic shaping is the most effective QoS mechanism for preventing congestion on a service provider edge link when traffic exceeds link capacity because it buffers excess packets and smooths the transmission rate to match the configured committed information rate (CIR). Unlike policing, which drops or re-marks packets, shaping avoids tail drops and packet loss by queuing bursts, making it ideal for edge links where preserving traffic integrity is critical. Cisco IOS uses the Generic Traffic Shaping (GTS) or Class-Based Shaping (CB-Shaping) to enforce a rate limit while maintaining TCP fairness and reducing retransmissions.

Answer

Priority queuing

Answer

Traffic policing

Answer

WRED

Question 17

Based on the exhibit, which prefix is missing a label binding and likely causing traffic to be dropped?

Accepted Answer

10.4.4.0/24. The exhibit shows an MPLS LDP label binding table. Prefix 10.4.4.0/24 has no local label assigned (shown as 'no label' or missing), meaning LDP has not established a label binding for this FEC. Without a label, traffic destined for 10.4.4.0/24 cannot be label-switched and will be dropped or forwarded via IP lookup (if enabled), but in an MPLS core, this typically results in packet loss.

Answer

10.2.2.0/24

Answer

10.1.1.0/24

Answer

10.3.3.0/24

Answer

10.5.5.0/24

Question 18

A service provider is troubleshooting BGP route advertisement for a VPNv4 prefix. The PE router receives the prefix from the route reflector but does not install it in the VRF routing table. The BGP table shows the prefix as valid but not best. What is the most likely cause?

Accepted Answer

The next-hop is not reachable via the IGP with an MPLS label.. For a VPNv4 prefix to be installed in the VRF routing table, BGP must select it as the best path. A key requirement for best-path selection is that the next-hop address must be reachable via the IGP with an associated MPLS label (via LDP or other label distribution protocol). If the next-hop is not reachable with a label, the route remains valid but not best, and thus is not installed in the VRF.

Answer

The VRF does not have the correct route-target import.

Answer

The BGP table is full and cannot accept more prefixes.

Answer

The MPLS label is missing in the BGP update.

Question 19

A network operator is migrating from traditional MPLS LDP to Segment Routing (SR) with IS-IS. The network consists of four routers (R1-R4) in a square topology. The engineer has enabled SR on all routers and configured prefix-SIDs for loopbacks. However, when checking the MPLS forwarding table on R1, the engineer sees that some prefixes have label values that are not the prefix-SIDs. For example, the prefix for R4's loopback shows label 16004 instead of the expected 16004 (which is correct). But for another prefix, the label is 16003 instead of 16003 (correct). The engineer does not see any labels for some external routes. What is the most likely reason that some labels are missing?

Accepted Answer

External routes are not covered by prefix-SIDs; they require LDP or another label distribution mechanism.. Segment Routing (SR) with IS-IS only assigns prefix-SIDs to prefixes that are part of the IS-IS domain and explicitly configured with a SID. External routes, such as those redistributed from another protocol (e.g., BGP or OSPF), are not covered by prefix-SIDs and require a separate label distribution mechanism like LDP or a manually configured explicit-null label to be forwarded with MPLS encapsulation. The engineer's observation that some labels are missing for external routes directly points to this limitation.

Answer

The IS-IS wide metrics are not enabled on all interfaces.

Answer

Route redistribution from another protocol is not configured.

Answer

The prefix-SIDs are inconsistent across routers.

Question 20

In a carrier's network, MPLS-TE tunnels are used to steer traffic away from congested links. The operator configures a tunnel with a bandwidth of 200 Mbps and a dynamic path. The CSPF computation shows a path with sufficient bandwidth, but the tunnel fails to come up. The RSVP neighbor is established. What is the most likely cause?

Accepted Answer

The path message is rejected due to resource reservation failure. The tunnel fails to come up because the CSPF-computed path cannot reserve the requested 200 Mbps bandwidth. Even though the path has sufficient bandwidth on paper, the RSVP resource reservation process may fail due to a lack of available bandwidth at the exact time of reservation, or due to a mismatch in bandwidth pool configuration (e.g., global vs. subpool). Since the RSVP neighbor is established, the issue is specifically with the reservation step, not with reachability or LDP.

Answer

The tunnel destination is not reachable via IGP

Answer

The tunnel interface is down

Answer

MPLS LDP is not configured