350-501 Practice Exam 56 — 20 Questions

Question 1

An SP engineer is designing a QoS policy for a business customer with three traffic classes: Voice (critical latency), Transactional Data (low delay), and Bulk Data (no strict requirements). The link is 100 Mbps. Which TWO actions should be part of the policy? (Select two.)

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

Accepted Answer

Apply LLQ to the voice class with a policer to limit its rate to 10 Mbps.. LLQ provides strict priority for voice. CBWFQ with bandwidth guarantees for transactional data ensures it gets capacity. Bulk data can use remaining bandwidth.

Answer

Use FIFO queuing for the bulk data class.

Answer

Use WRED on the voice class for congestion avoidance.

Answer

Set the queue limit to 1000 packets for all classes.

Question 2

An SP is deploying SR-PCE for centralized traffic engineering. Which THREE functions does the PCE perform? (Select three.)

Accepted Answer

Computes path for traffic-engineered LSPs based on constraints.. PCE computes paths based on constraints, communicates via PCEP with routers (PCCs), and can instantiate SR-TE policies.

Answer

Distributes link-state information using BGP-LS.

Answer

Configures MPLS forwarding on routers.

Question 3

In a hierarchical QoS policy, the parent policy shapes to 20 Mbps. The child policy has a class for voice with priority and police rate 2 Mbps, and a class for data with bandwidth 10 Mbps. What is the maximum bandwidth voice traffic can consume under congestion?

Accepted Answer

2 Mbps. Voice is in LLQ with a police rate of 2 Mbps; even though the parent shape is 20 Mbps, policing limits voice to 2 Mbps to prevent starvation.

Answer

8 Mbps

Answer

20 Mbps

Answer

10 Mbps

Question 4

Match each Ethernet OAM protocol to its function.

Accepted Answer

CFM: Fault management and connectivity verification at the Ethernet layer. These are Ethernet OAM and management protocols used in service provider access.

Answer

CFM: Performance monitoring and measurement

Answer

EFM: Fault management across provider networks

Question 5

Which TWO of the following are valid BGP extended community attributes? (Choose two.)

Accepted Answer

Site of Origin (SoO). Site of Origin (SoO) and Route Target (RT) are well-known BGP extended communities. Route Origin (RO) is a standard community, not an extended community. Standard and Large communities are also not extended communities. Thus, only SoO and RT are correct among the options, but the question asks for three; note that Color or Bandwidth are other valid extended communities not listed here.

Answer

Route Origin (RO)

Answer

Standard community

Answer

Large community

Question 6

Which two statements about LDP liberal label retention are true? (Choose two.)

Accepted Answer

It allows faster convergence after a topology change.. Liberal label retention is the default mode in Cisco IOS and does not require explicit configuration; it retains all labels even if not used (D) and speeds convergence because labels are already available after a topology change (B). Option C describes conservative retention, and option E is false because liberal retention is default and does not require manual filtering.

Answer

It requires explicit configuration of the label retention mode in Cisco IOS.

Answer

It conserves memory by only storing labels for next-hop neighbors.

Answer

It requires manual configuration of label filtering.

Question 7

Refer to the exhibit. An engineer checks the policy and notices that the policing is not working as expected—traffic is not being dropped even when exceeding 1 Mbps. What could be the issue?

Accepted Answer

The shape command is interfering with the police command. The shape command can interfere with police by smoothing traffic, which may prevent the policer from seeing bursts that exceed the committed rate. If the shape rate is set higher than the police rate, the policer may not drop traffic because the shaper is already limiting the output to a lower rate. Additionally, the order of operations in a service policy matters: if shape is applied before police, the policer sees shaped traffic, which may not exceed the rate.

Answer

The service-policy is applied inbound, but police should be applied outbound to be effective

Answer

The police command is missing an exceed-action, so traffic is transmitted instead of dropped

Answer

The police rate is too high compared to interface speed

Question 8

In IS-IS, which TLV is used to carry traffic engineering information for MPLS-TE?

Accepted Answer

TLV 22. In IS-IS, TLV 22 (Extended IS Reachability) is used to carry traffic engineering information, such as link attributes (bandwidth, admin group, TE metric), for MPLS-TE. TLV 135 (Extended IP Reachability) carries IP prefix information, not TE link attributes.

Answer

TLV 135

Answer

TLV 229

Answer

TLV 128

Question 9

Match each QoS mechanism to its primary function.

Accepted Answer

Classification: Identifies traffic based on specific criteria.. In QoS, Classification identifies traffic, Marking sets priority, Policing drops/excess, Shaping buffers, and WRED drops probabilistically. Queuing handles packet scheduling, not rate enforcement.

Answer

Queuing: Drops packets that exceed a configured rate.

Question 10

During a network migration from EIGRP to OSPF, you notice that some routes are being redistributed incorrectly, causing routing loops. The OSPF domain uses area 0 and area 1. The EIGRP domain uses AS 100. Which configuration change would best prevent loops during the migration?

Accepted Answer

Use route-maps to tag EIGRP routes and filter them on OSPF routers.. Route-maps allow you to tag redistributed EIGRP routes with a specific tag value (e.g., 'tag 100') and then filter those tagged routes on OSPF routers using a distribute-list in or prefix-list combined with the route-map. This prevents the redistributed routes from being re-injected back into EIGRP, breaking the redistribution loop. Without such tagging and filtering, mutual redistribution between EIGRP and OSPF can cause routing loops due to the two-way redistribution of routes.

Answer

Implement OSPF stub areas to limit external routes.

Answer

Use distribute-list in EIGRP to block OSPF routes.

Answer

Set a high administrative distance on redistributed routes in OSPF.

Question 11

Match each IS-IS term to its definition.

Accepted Answer

IS-IS Area: A set of routers sharing the same area address.. In IS-IS, an area is identified by a common area address, LSPs carry link state data, and Level-1 routers operate intra-area. Common confusions involve swapping area definitions (system ID vs. area address) and mixing Level-1/Level-2 roles.

Answer

IS-IS Area: A set of routers sharing the same system ID.

Answer

Level-1 Router: Routes between areas.

Question 12

A service provider wants to prioritize voice traffic over data traffic on a PE-CE link. Which QoS mechanism should be applied at the PE ingress to classify packets?

Accepted Answer

Class-based marking based on NBAR. Class-based marking using NBAR (Network-Based Application Recognition) is the correct mechanism because it can inspect packets at the application layer to distinguish voice traffic (e.g., RTP, SIP) from data traffic. On a PE-CE link, marking at ingress allows the service provider to set the appropriate DSCP or IP precedence value, which downstream QoS policies can then trust and act upon. This classification step is essential before any policing, shaping, or queuing can be applied.

Answer

Policer

Answer

WRED

Answer

Shaper

Question 13

An SP engineer is designing a QoS policy for a business customer with three traffic classes: Voice (critical latency), Transactional Data (low delay), and Bulk Data (no strict requirements). The link is 100 Mbps. Which TWO actions should be part of the policy? (Select two.)

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

Accepted Answer

policy-map QOS
 class VOICE
  priority 1000
 class PREMIUM
  bandwidth 4000
 class class-default
  fair-queue. LLQ is used for voice (priority), CBWFQ for premium data (bandwidth), and default class for best effort.

Answer

policy-map QOS
 class VOICE
  priority percent 10
 class PREMIUM
  bandwidth percent 40
 class class-default
  shape average 5000

Answer

policy-map QOS
 class VOICE
  bandwidth 1000
 class PREMIUM
  priority 4000
 class class-default
  fair-queue

Answer

policy-map QOS
 class VOICE
  priority 1000
 class PREMIUM
  priority 4000
 class class-default
  fair-queue

Question 14

A small service provider is deploying MPLS in its core network for the first time. They have a simple topology: three P routers in the core and two PE routers connecting customers. They have configured LDP on all interfaces and OSPF as the IGP. They also configured a basic L3VPN for a customer with a single CE attached to each PE. The customer reports that they can ping between CEs for a few minutes, but then the connectivity drops. After a few seconds, it returns. This pattern repeats every few minutes. The engineer checks the LDP neighbors and sees that all LDP sessions are up. The OSPF adjacencies are stable. The engineer also notices that the pings time out exactly when the LDP graceful restart timer is expiring on one of the P routers. What is the most likely cause?

Accepted Answer

The BGP session between the PEs is flapping due to a hold timer mismatch or connectivity issue.. The symptom of periodic connectivity drops that recover after a few seconds, coinciding with the LDP graceful restart timer, indicates that the BGP session between the PEs is flapping. When the BGP session goes down, the VPN routes are withdrawn, causing the CE-to-CE connectivity to drop. Once the BGP session re-establishes, routes are re-advertised and connectivity returns. LDP graceful restart timer expiration is not the cause (LDP sessions are up), but may coincide with BGP hold timer failures. Options B, C, and D are incorrect because LDP graceful restart does not cause the described pattern, OSPF network type does not cause periodic drops, and interface flapping would cause more permanent outages.

Answer

The LDP graceful restart timer is causing periodic re-establishment of LDP sessions.

Answer

The OSPF network type on the core links is set to point-to-point, causing SPF recalculation every 5 minutes.

Answer

One of the core interfaces is flapping due to a hardware issue, triggering IGP and LDP convergence.

Question 15

A service provider is deploying Segment Routing Traffic Engineering (SR-TE) to optimize traffic flows. Which mechanism is used to steer traffic into an SR-TE policy?

Accepted Answer

Color extended community on BGP routes. In Segment Routing Traffic Engineering (SR-TE), traffic is steered into an SR-TE policy using a color extended community attached to BGP routes. The color community acts as a service-level identifier that maps BGP-learned prefixes to a specific SR-TE policy on the headend router, enabling automated traffic engineering without per-flow state or tunnel interfaces.

Answer

RSVP-TE tunnel interface

Answer

BGP prefix-independent convergence (PIC)

Answer

Manual static route pointing to the policy

Answer

LDP label distribution

Question 16

Which MPLS security best practice helps prevent label spoofing attacks where an attacker injects MPLS packets with a forged label stack to bypass ACLs?

Accepted Answer

Disabling TTL propagation on PE-CE interfaces. None of the provided options are the direct best practice to prevent label spoofing. The primary Cisco-recommended method is to disable MPLS on PE-CE interfaces or apply inbound label filtering to reject labeled packets from untrusted sources.

Answer

Enabling MPLS on all interfaces including CE-facing

Answer

Enabling TTL propagation on all PE routers

Answer

Using IP explicit null labels on all LSPs

Question 17

A service provider is deploying multicast service for IPTV using PIM-SM with a single RP. During high traffic periods, the RP becomes overloaded. What is the most scalable solution to distribute the load across multiple RPs?

Accepted Answer

Use Anycast-RP. Anycast-RP allows multiple RPs to share the same IP address, enabling load distribution and redundancy. In PIM-SM, sources register with the nearest RP via unicast routing, and receivers join toward the same Anycast-RP address, which is routed to the closest physical RP. This distributes the registration and join processing load across multiple RPs without requiring protocol changes.

Answer

Use Bidirectional PIM

Answer

Increase the RP's memory and CPU

Answer

Use PIM-DM

Question 18

A large enterprise uses MPLS L3VPN to connect multiple sites. They want to implement inter-AS option B (ASBR-to-ASBR MP-eBGP) for scalability. Which statement about this model is correct?

Accepted Answer

ASBRs exchange VPNv4 routes using MP-eBGP. In Inter-AS Option B, ASBRs use MP-eBGP to exchange VPNv4 routes. They do not maintain VRFs (Option A incorrect), and sessions are per address family, not per VRF (Option C incorrect). Option B is incorrect because a full mesh of MP-iBGP between PEs is not required. Option D correctly describes the VPNv4 route exchange using MP-eBGP.

Answer

Each ASB router maintains VRFs for all attached customers

Answer

Inter-AS option B requires a full mesh of MP-iBGP between all PEs

Answer

MP-eBGP sessions are established per VRF between ASBRs

Question 19

In an MPLS-TE tunnel configuration, which RSVP message carries the Explicit Route Object (ERO) that specifies the explicit path the tunnel must follow?

Accepted Answer

PATH message. In RSVP-TE, the PATH message includes the ERO to define the explicit route the traffic should take. The RESV message is sent back to confirm the reservation.

Answer

RSVP_TEAR message

Answer

RSVP_ERR message

Answer

RESV message

Question 20

A service provider is troubleshooting a BGP route advertisement issue. Routes from a customer are not being advertised to the upstream provider. The PE router is configured with 'neighbor 10.0.0.1 route-map RMAP out'. The route-map RMAP permits the customer prefix. However, the BGP table on the PE shows the prefix as valid but not advertised. What is a likely cause?

Accepted Answer

The next-hop is not reachable from the upstream provider. The BGP table shows the prefix as valid but not advertised, which indicates that BGP has the route but is not sending it to the upstream neighbor. A common cause is that the next-hop for the customer prefix is not reachable from the PE router via the interface used to reach the upstream provider. BGP will not advertise a route if the next-hop is not reachable in the routing table (unless 'neighbor x.x.x.x next-hop-self' is configured), because the upstream router would be unable to forward traffic to that next-hop.

Answer

The BGP session to the upstream provider is flapping

Answer

The prefix is not in the global routing table

Answer

The route-map is applied inbound instead of outbound