14 of 89 questions · Page 2/2 · Automation and Quality of Services · Answers revealed
In the DiffServ model, which field is used in the IP header to mark packets for QoS treatment?
DSCP is the standard marking field in DiffServ.
Why this answer
The DiffServ model uses the 6-bit DSCP field in the IP header to classify packets.
In the Modular QoS CLI (MQC), which component is used to define traffic classes based on match criteria?
Class-map defines traffic classes with match criteria.
Why this answer
In the Modular QoS CLI (MQC), the class-map is the component used to define traffic classes by specifying match criteria such as IP precedence, DSCP values, or access-group references. The class-map groups packets that match one or more conditions, allowing the policy-map to apply QoS actions (e.g., policing, shaping) to that specific class. Without a class-map, the policy-map cannot differentiate between traffic types.
Exam trap
Cisco often tests the distinction between the component that defines traffic (class-map) versus the component that applies actions (policy-map), leading candidates to mistakenly select policy-map when asked about defining traffic classes.
How to eliminate wrong answers
Option A is wrong because a policy-map is used to associate QoS actions (like bandwidth, police, shape) with a class-map, not to define the traffic classes themselves. Option B is wrong because an access-list is a packet filtering tool that can be referenced within a class-map as a match criterion, but it is not the component that defines the traffic class in MQC. Option D is wrong because a service-policy is the command that applies a policy-map to an interface or direction (input/output), not a component for defining traffic classes.
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.)
LLQ ensures low latency; policing prevents voice from starving other classes.
Why this answer
LLQ provides strict priority for voice. CBWFQ with bandwidth guarantees for transactional data ensures it gets capacity. Bulk data can use remaining bandwidth.
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?
leafref with path ensures the value exists in the referenced leaf.
Why this answer
leafref automatically validates that the leaf value matches a leaf in the referenced path.
In the DiffServ model, which DSCP value is used for Expedited Forwarding (EF) to support voice traffic?
EF is DSCP 46 as per RFC 3246.
Why this answer
EF is defined by DSCP 46 (binary 101110) to provide low-loss, low-latency service for voice.
In SR-TE PCE, which protocol does the PCE use to communicate with the PCC (Path Computation Client) to set up LSPs?
PCEP handles path computation and delegation.
Why this answer
In Segment Routing Traffic Engineering (SR-TE) with a Path Computation Element (PCE), the PCE communicates with the Path Computation Client (PCC) using the Path Computation Element Protocol (PCEP), as defined in RFC 5440 and extended for SR by RFC 8664. PCEP is specifically designed for path computation requests and responses, and for initiating and setting up LSPs via the PCE-initiated LSP stateful model (RFC 8281). The PCC sends path computation requests to the PCE, and the PCE responds with computed paths, which the PCC then uses to instantiate SR-TE LSPs.
Exam trap
Cisco often tests the distinction between protocols used for topology discovery (BGP-LS), configuration (NETCONF/RESTCONF), and path computation/setup (PCEP), so the trap here is confusing BGP-LS (which provides topology information to the PCE) with PCEP (which is the actual signaling protocol between PCE and PCC for LSP operations).
How to eliminate wrong answers
Option A is wrong because RESTCONF is a RESTful protocol used for YANG-based data model access (RFC 8040), not for PCE-PCC path computation or LSP setup; it is used for configuration management, not real-time path computation signaling. Option B is wrong because NETCONF (RFC 6241) is a network configuration protocol that uses YANG models for device configuration, but it does not handle dynamic path computation or LSP instantiation between PCE and PCC; it is a management plane protocol, not a control plane protocol for SR-TE. Option C is wrong because BGP-LS (RFC 7752) is used to advertise link-state and topology information (including SR extensions) from routers to a controller or PCE, but it is not used for direct PCE-PCC communication to set up LSPs; it provides topology data, not path computation or LSP setup signaling.
Which DSCP value is recommended for Expedited Forwarding (EF) per-hop behavior, commonly used for voice traffic?
EF is DSCP 46, used for voice.
Why this answer
Expedited Forwarding (EF) per-hop behavior is defined in RFC 3246 and is designed for low-loss, low-latency, and low-jitter services such as voice traffic. The recommended DSCP value for EF is 46 (binary 101110), which corresponds to the PHB that ensures a dedicated forwarding treatment with a minimal queueing delay. This value is widely adopted in Cisco QoS deployments for voice payloads to guarantee strict priority queuing.
Exam trap
Cisco often tests the distinction between DSCP values for voice payload (EF/46) versus voice signaling (CS3/24) or video (AF41/34), and the trap here is that candidates may confuse AF41 (DSCP 34) with EF because both are used for real-time traffic, but only EF provides strict priority queuing.
How to eliminate wrong answers
Option B (DSCP 10) is wrong because DSCP 10 (binary 001010) corresponds to AF11 (Assured Forwarding class 1, low drop probability), which is used for data traffic that requires a guaranteed bandwidth but not the strict low-latency treatment of voice. Option C (DSCP 0) is wrong because DSCP 0 represents Best Effort (BE) forwarding, which provides no QoS guarantees and would cause voice packets to be treated as regular data, leading to unacceptable delay and jitter. Option D (DSCP 34) is wrong because DSCP 34 (binary 100010) maps to AF41 (Assured Forwarding class 4, low drop probability), which is typically used for real-time video or streaming traffic, not for the strict priority queuing required by voice.
A service provider is deploying a QoS policy for a customer with voice, mission-critical data, and best-effort traffic. The voice requires strict priority and low latency. Which queuing mechanism should be used to ensure voice packets are always served before other queues, and what additional measure is necessary to prevent voice traffic from starving other traffic?
LLQ places voice in a strict priority queue; policing prevents voice from exceeding its allocated bandwidth and starving other queues.
Why this answer
LLQ provides a strict priority queue for delay-sensitive traffic like voice. However, to prevent starvation of other traffic, policing must be applied to the priority queue to limit the amount of voice traffic allowed into it.
An SP is deploying SR-PCE for centralized traffic engineering. Which THREE functions does the PCE perform? (Select three.)
Primary function of PCE.
Why this answer
PCE computes paths based on constraints, communicates via PCEP with routers (PCCs), and can instantiate SR-TE policies.
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?
Correct; policing limits voice to 2 Mbps.
Why this answer
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.
In a hierarchical QoS policy applied to a subscriber aggregation interface, the parent policy shapes to 100 Mbps and the child policy allocates 50% bandwidth to voice and 50% to data. If voice traffic exceeds 50 Mbps, what happens to the excess?
LLQ typically includes policing to limit the priority queue.
Why this answer
The child policy's police command within LLQ drops or re-marks voice traffic exceeding the configured bandwidth to prevent starvation of other queues.
Which YANG data modeling construct is used to restrict a leaf value to be a reference to another leaf in the data tree?
leafref ensures the value exists at the referenced leaf.
Why this answer
The 'leafref' type in YANG is used to restrict the value of a leaf to refer to another leaf in the data tree, enforcing referential integrity.
When DSCP markings are mapped to MPLS EXP bits at the ingress PE router, how many EXP values are available for traffic differentiation in the MPLS core?
3 bits yield 2^3 = 8 values.
Why this answer
MPLS EXP (now TC) is a 3-bit field, providing 8 possible values (0-7).
An SP engineer is implementing DSCP classification at the edge. Which two AF classes are typically used for mission-critical data? (Choose two.)
AF4 class is high priority.
Why this answer
AF4x (e.g., AF41/AF42/AF43) and AF3x (e.g., AF31/AF32/AF33) are used for high-priority data. AF1x is low-priority.
Ready to test yourself?
Try a timed practice session using only Automation and Quality of Services questions.