350-501 Practice Test 46 — 15 Questions

Question 1

Refer to the exhibit. An engineer configured Segment Routing on a router. However, the router does not advertise a Prefix-SID for the loopback0 interface. What is the most likely reason?

Accepted Answer

The 'prefix-sid index' command is not associated with the loopback prefix; it should be configured under the OSPF process for that specific prefix.. In Cisco IOS XR, the 'prefix-sid index' command must be configured under the OSPF process for the specific prefix (e.g., 'prefix-sid index 100' under 'router ospf 1' for the loopback0 prefix). Configuring it under the interface does not associate the SID with the prefix in OSPF's link-state database, so the router will not advertise the Prefix-SID. The exhibit shows the command under the loopback interface, which is incorrect for OSPF Segment Routing.

Answer

The loopback is not included in any OSPF network statement.

Answer

The 'segment-routing mpls' command is missing under the loopback interface.

Answer

The 'mpls ip' command is missing on the loopback interface.

Question 2

A service provider is experiencing intermittent multicast issues in their core network. They use PIM-SM with a static RP at 10.1.1.1. The multicast traffic originates from a source connected to PE1 and is received by customers connected to PE2. Recently, after a firewall upgrade between the PE routers and the core, some multicast streams stopped working, while others continue. The network team notices that 'show ip mroute' on PE2 shows the (*, G) entry but not the (S, G) entry for the affected groups. The RP is reachable via OSPF. The firewall logs show no dropped packets for known multicast addresses. Which action should the engineer take to restore full multicast forwarding?

Accepted Answer

Configure a static RP at the customer site (PE2) to bypass the firewall for registration traffic. The firewall upgrade likely blocked PIM register messages from PE1 to the RP, preventing the RP from learning about the source. By configuring a static RP on the customer-facing interface or using a different RP that can receive registration, the (S,G) state can be built. Option A is wrong because PIM-SM is correct for sparse-mode groups. Option C is wrong because adjusting timers would not fix the absence of (S,G). Option D is wrong because Auto-RP would add complexity and might be blocked by the firewall as well.

Answer

Increase the PIM register suppression interval on the source's first-hop router

Answer

Change the multicast mode from PIM-SM to PIM-DM on all interfaces

Answer

Enable Auto-RP on the network to dynamically learn the RP

Question 3

An automation engineer uses RESTCONF to configure a Cisco ASR 9000 router. When sending a PATCH request to update an interface description, the API returns 404 Not Found. What is the most probable issue?

Accepted Answer

The interface does not exist on the device.. A 404 Not Found response to a RESTCONF PATCH request targeting a specific interface indicates that the resource (the interface) does not exist at the given URI. RESTCONF uses the URI path to identify the exact YANG data node; if the interface name in the URI does not match any configured interface on the Cisco ASR 9000, the server returns 404. This is consistent with RFC 8040, where 404 is used when the target resource cannot be found.

Answer

HTTP authentication is required.

Answer

The YANG module is not supported.

Answer

The RESTCONF username/password is incorrect.

Question 4

Which two are benefits of using EVPN-VPWS over traditional VPWS? (Choose two.)

Accepted Answer

Eliminates the need for pseudowire configuration on each PE.. EVPN-VPWS provides multi-homing with fast convergence and simplifies provisioning using BGP.

Answer

Uses MPLS-TE tunnels for path protection.

Answer

Supports only point-to-point topologies.

Answer

Requires LDP for label distribution.

Question 5

Which THREE are requirements for deploying MPLS Traffic Engineering with RSVP-TE?

Accepted Answer

Links must have reservable bandwidth advertised. MPLS TE with RSVP-TE requires that links advertise reservable bandwidth so that the Constrained Shortest Path First (CSPF) algorithm can compute paths that satisfy bandwidth constraints. Without reservable bandwidth advertised via IGP extensions (e.g., OSPF TE or IS-IS TE), RSVP-TE cannot reserve resources along the path.

Answer

OSPFv3 must be the IGP

Answer

LDP must be enabled on all routers

Question 6

A service provider is implementing QoS for voice traffic using the DiffServ model. Which DSCP value should be assigned to voice packets to ensure the highest priority treatment?

Accepted Answer

DSCP 46. DSCP 46 (binary 101110) corresponds to the Per-Hop Behavior (PHB) of Expedited Forwarding (EF), which is defined in RFC 3246 for low-loss, low-latency, and low-jitter treatment. Voice traffic requires the highest priority in a DiffServ domain, and EF (DSCP 46) is the standard marking for voice bearer packets to ensure they receive strict priority queuing.

Answer

DSCP 26

Answer

DSCP 34

Answer

DSCP 0

Question 7

A service provider is deploying MPLS Layer 3 VPNs and wants to ensure that customer traffic is not dropped when a PE-CE link fails. The CE router is using static routing. Which design should be implemented to provide fast convergence?

Accepted Answer

Enable BFD on the PE-CE interface and associate it with the static route.. BFD (Bidirectional Forwarding Detection) provides sub-second failure detection on the PE-CE link, and when associated with a static route, it allows the PE to quickly remove the failed route from the routing table. This triggers fast convergence without waiting for routing protocol timers, which is critical for MPLS Layer 3 VPNs using static routing on the CE side.

Answer

Configure OSPF on the PE-CE link and set hello timers to 1 second.

Answer

Use IP SLA tracking to monitor the CE and adjust the static route.

Answer

Increase the administrative distance of the static route to 255.

Question 8

An SP engineer is configuring hierarchical QoS on a PE router for a subscriber with a 10 Mbps shaped rate. The child policy must guarantee 2 Mbps for voice, 4 Mbps for video, and the rest for data. However, the parent policy shape rate is 10 Mbps. Which configuration ensures proper operation?

Accepted Answer

Parent shape 10 Mbps, child policy with LLQ for voice and CBWFQ for video and data with bandwidth 5 Mbps each. In hierarchical QoS, the child policy must respect the parent shape rate of 10 Mbps while providing low-latency for voice. The correct approach is to use LLQ (priority) for voice to ensure low delay and jitter, and CBWFQ for video and data to allocate bandwidth within the shape. Option A uses LLQ for voice and allocates 5 Mbps each to video and data via CBWFQ, totaling 10 Mbps, which fits within the parent shape. Option D incorrectly uses the bandwidth command for voice, which does not provide the necessary low-latency treatment. Options B and C either miss a data class or use bandwidth percent for voice, not priority.

Answer

Parent shape 10 Mbps, child policy with priority for voice and bandwidth 4 Mbps for video, no data class

Answer

Parent shape 10 Mbps, child policy with bandwidth percent 20 voice, 40 video, and 40 data

Answer

Parent shape 10 Mbps, child policy with bandwidth 2 Mbps for voice, 4 Mbps for video, and default class for data

Question 9

A service provider is deploying MPLS in their core network and wants to ensure that all routers in the MPLS domain can dynamically exchange label bindings. Which protocol should be enabled on all routers to meet this requirement?

Accepted Answer

LDP. LDP (Label Distribution Protocol) is the correct choice because it is the standard protocol used in MPLS networks to dynamically distribute label bindings for all routes in the routing table. It operates by establishing LDP sessions between directly connected LSRs and exchanging label mappings for each prefix, ensuring every router in the MPLS domain has the necessary labels to forward labeled packets.

Answer

OSPF

Answer

iBGP

Answer

RSVP-TE

Question 10

A service provider operates a large MPLS VPN network using OSPF as the IGP and LDP for label distribution. The PE routers (PE1, PE2, PE3) are connected to a core of P routers. Recently, a new link was added between P2 and P3. After the link came up, the engineering team noticed that several VPN routes that were previously reachable via PE2 are now being blackholed when traffic is sent from PE1 to those prefixes. The teams verify that the VPNv4 routes are present in the BGP table on PE1 with valid next-hops, but traffic fails. The traceroute from PE1 to the CE behind PE2 stops at P2. The show mpls forwarding-table on P2 shows the correct label for the VPN prefix, but the outgoing interface is null. Which action should the engineer take to resolve the issue without causing additional disruption?

Accepted Answer

Clear LDP neighbor sessions on P2 to re-initiate label exchange.. The issue is that P2 has a label for the VPN prefix but a null outgoing interface, indicating an LDP label mapping problem. Clearing LDP neighbor sessions on P2 forces re-establishment of LDP sessions and re-exchange of label bindings, which should resolve the missing or incorrect label mapping for the BGP next-hop without disrupting other services.

Answer

Add a static route on P2 for the BGP next-hop pointing to Null0.

Answer

Shut down the new link between P2 and P3.

Answer

Clear BGP sessions on PE1 to force re-advertisement of VPN routes.

Question 11

A service provider is deploying QoS on a Cisco ASR 9000 router to support a triple-play service (voice, video, data) over an Ethernet access network. The network engineer must ensure that voice traffic (EF) is prioritized, video traffic (AF41) is treated with low loss, and data traffic (default) gets best-effort service. Additionally, the engineer must implement a hierarchical QoS policy to shape the aggregate subscriber traffic to 100 Mbps on a GigabitEthernet interface, with the following per-class bandwidth allocations: voice 10 Mbps, video 40 Mbps, and data 50 Mbps. During testing, voice packets are being dropped under congestion, and video traffic is experiencing jitter. The current QoS configuration is as follows:

policy-map CHILD
 class VOICE
  priority level 1
  police rate 10 mbps
 class VIDEO
  bandwidth remaining ratio 40
 class DATA
  bandwidth remaining ratio 50
!
policy-map PARENT
 class class-default
  shape average 100 mbps
  service-policy CHILD
!

Which action should the engineer take to resolve the issues while maintaining the design objectives?

Accepted Answer

Remove the police from the voice class and add 'priority level 1' without police; this ensures voice is priority queued without dropping.. The current configuration uses a police rate of 10 Mbps under the voice priority class, which causes voice packets to be dropped when they exceed that rate, even though priority queuing should guarantee low latency. Removing the police and keeping 'priority level 1' allows voice traffic to be strictly prioritized without a hard rate limit, ensuring no drops for voice while still allowing the parent shaper to enforce the aggregate 100 Mbps. This resolves the voice drops and, by preventing voice from being policed, reduces jitter for video traffic that might otherwise be affected by bursty voice drops.

Answer

Change the video class to 'priority level 2' to give it preferential treatment over data.

Answer

Add a 'police' to the video class with a CIR of 40 Mbps and 'conform-action transmit exceed-action drop' to protect voice.

Answer

Change the voice class to use 'bandwidth remaining percent 10' and remove the police.

Question 12

A service provider is implementing QoS on a PE router for customer traffic. Which tool should be used to classify traffic based on application layer information?

Accepted Answer

MQC with NBAR. NBAR (Network-Based Application Recognition) is a deep packet inspection (DPI) engine within the Modular QoS CLI (MQC) that can identify applications by inspecting payloads up to Layer 7. This allows classification of traffic based on application-layer information such as HTTP, DNS, or proprietary protocols, which is exactly what the question requires.

Answer

Access-list

Answer

Shaping

Answer

Policy-map with police

Question 13

An engineer configures an RSVP-TE tunnel with a bandwidth of 100 Mbps and an explicit path that excludes a specific interface. The tunnel fails to come up. Which term describes the path computation method used by RSVP-TE to find a feasible path given these constraints?

Accepted Answer

CSPF (Constrained Shortest Path First). CSPF (Constrained Shortest Path First) is the algorithm used by RSVP-TE to compute a path that satisfies constraints such as bandwidth, explicit path, and link affinities.

Answer

SPF (Shortest Path First)

Answer

Bellman-Ford

Answer

Dijkstra with ECMP

Question 14

A network engineer is automating BGP configuration using the Cisco IOS-XE YANG model. They want to enable the 'always-compare-med' feature under BGP. Which XPath expression correctly targets this leaf?

Accepted Answer

/native/router/bgp/scope/global/always-compare-med. The Cisco IOS-XE native YANG model (urn:cisco:params:xml:ns:yang:cisco-native) structures BGP configuration under /native/router/bgp/scope/global/always-compare-med. The 'scope' container is required to differentiate between global and VRF-specific BGP settings, and 'always-compare-med' is a leaf within the global scope. This path accurately reflects the hierarchical model used by Cisco for BGP automation.

Answer

/bgp/global/always-compare-med

Answer

/native/router/bgp/always-compare-med

Answer

/router/bgp/global/always-compare-med

Question 15

A service provider's network uses MPLS TE with Fast Reroute (FRR) for link protection. After a fiber cut on a core link, the traffic is rerouted via a backup tunnel, but the backup tunnel's path suffers from high latency. The customer complains about poor voice quality. What can be done to avoid this scenario in future deployments?

Accepted Answer

Configure backup tunnels with explicit paths that avoid high-latency links. Configuring backup tunnels with explicit paths that avoid high-latency links ensures that when MPLS TE Fast Reroute (FRR) activates, the backup path meets the latency requirements for real-time traffic like voice. By specifying explicit routes (e.g., via the 'path-option' command with explicit-path objects), the operator can exclude links known to have high propagation delay or congestion, thus preventing the poor voice quality experienced after the fiber cut.

Answer

Increase the bandwidth reservations on the backup tunnel

Answer

Implement BGP PIC for edge routers

Answer

Use node protection instead of link protection