CCNA Spcor Automation Qos Questions

The NED (Network Element Driver) in Cisco NSO is the component that abstracts the native device configuration and translates YANG data models into device-specific CLI commands or protocols (e.g., SNMP, NETCONF). This allows NSO to manage heterogeneous devices using a unified YANG-based service model, making option A correct.

Hierarchical QoS uses a parent policy for shaping and a child policy for classification and queuing. The parent policy applies shape, and the child policy is attached to a class within the parent.

DSCP EF (46) is commonly mapped to MPLS EXP 5 to preserve priority across the MPLS core.

EF is defined as DSCP 46 (binary 101110) for low-loss, low-latency traffic like voice.

'bandwidth remaining percent' allocates a percentage of the remaining bandwidth after the priority queue is serviced. This ensures other classes share leftover bandwidth.

In the DiffServ Tunneling Model, the Short Pipe model specifically instructs the egress PE to ignore the MPLS EXP bits for queuing and instead use the original IP DSCP value. This preserves the end-to-end QoS marking across the MPLS core, making option D correct. The Pipe model, by contrast, uses the EXP bits at the egress PE, which is why option B is incorrect.

LLQ provides a strict priority queue for real-time traffic (voice) and uses CBWFQ for other data classes with bandwidth guarantees.

NETCONF uses <edit-config> to modify the config; <get-config> retrieves; <lock> prevents concurrent edits.

RESTCONF is a southbound interface that NSO uses to communicate with network devices. It is a RESTful protocol that uses HTTP methods (GET, POST, PUT, DELETE) to access YANG-defined data models, making it suitable for modern programmable devices. NSO leverages RESTCONF to configure and manage devices that support this protocol, providing a standardized, stateless interface for automation.

NETCONF uses RPCs like get-config and edit-config; it runs over SSH (RFC 6242) and uses XML encoding.

AF41 is Assured Forwarding class 4, low drop probability, with DSCP decimal value 34 (binary 100010).

EF (Expedited Forwarding) is assigned DSCP 46 for low-latency voice traffic.

Leafref references an existing leaf in the YANG data tree, ensuring consistency.

Best Effort (DF, Default Forwarding) is DSCP 0.

AF4 has three drop probabilities: low (AF41 = DSCP 34), medium (AF42 = DSCP 36), high (AF43 = DSCP 38).

Traffic shaping buffers excess traffic that exceeds the configured rate, smoothing bursts and delaying packets to conform to the desired rate. Traffic policing, on the other hand, immediately drops or re-marks packets that exceed the rate limit, without buffering. This fundamental difference in handling excess traffic—buffering versus dropping/re-marking—is the key distinction between the two QoS mechanisms.

On Cisco routers, you can use 'set mpls experimental imposition' to set EXP on the imposed label, or use a QoS policy with 'set mpls experimental' in a policy-map.

NSO uses a Configuration Database (CDB) for storing device configurations, YANG models for service definitions, and NEDs (Network Element Drivers) for device abstraction.

get-config with <source>running</source> retrieves the running configuration. The <get> RPC retrieves both configuration and state data.

AF41 uses DSCP 34 (010001) and AF31 uses DSCP 26 (011010).

In MQC, a class-map defines the match criteria to classify traffic.

Policing within the LLQ class limits the amount of traffic that can use the priority queue, preventing starvation.

In the DiffServ model, QoS is implemented by classifying and marking packets at the network edge using the DSCP field in the IP header. This marking determines how each packet is treated within the core network, enabling scalable, class-based service differentiation without per-flow state.

The bandwidth command can specify a percentage or absolute bandwidth, but the sum must not exceed the available bandwidth (interface bandwidth minus LLQ).

'must' is an XPath expression that enforces a constraint on the data; if the condition is false, validation fails.

GET retrieves data; POST can create but typically returns data; PATCH modifies; DELETE deletes.

Policing drops or re-marks excess traffic; shaping buffers excess and smooths output.

WRED (Weighted Random Early Detection) drops packets probabilistically based on queue depth and IP precedence/DSCP, avoiding synchronization and providing congestion avoidance.

DSCP 26 (binary 011010) corresponds to AF31, which is part of the Assured Forwarding class with low drop probability and medium priority.

Policing drops or re-marks traffic exceeding the configured rate, enforcing a hard limit. Shaping buffers excess traffic, which does not enforce a hard drop.

Shaping buffers excess traffic to smooth bursts, while policing drops or re-marks excess packets. Shaping introduces delay due to buffering; policing can cause TCP retransmissions.

Option B is correct because the DiffServ model uses the 6-bit Differentiated Services Code Point (DSCP) field in the IP header to mark packets, providing 2^6 = 64 possible values (0-63). This allows for up to 64 distinct per-hop behaviors (PHBs), enabling scalable QoS classification beyond the limited 8 classes of the older IP Precedence model.

LLQ provides a strict priority queue for EF traffic and includes a policer to limit the priority traffic rate, preventing starvation of other queues.

RESTCONF uses POST to create a new data resource. PUT is used to replace or create, but POST is the standard for creation.

MQC uses class-map, policy-map, and service-policy; class-map can match multiple criteria; policy-map defines actions.

The DiffServ model defines EF (Expedited Forwarding), AF (Assured Forwarding), and Default (BE) PHBs. CS is Class Selector, but not a separate PHB; it is part of AF? Actually CS is a backward-compatible PHB. Standard ones are EF, AF, and Default.

In MPLS QoS, the EXP bits are set at the ingress PE router because this is where the IP packet first encounters the MPLS domain. The ingress PE maps the incoming IP packet's DSCP value to the MPLS EXP bits using a configured trust boundary or marking policy (e.g., 'policy-map' with 'set mpls experimental imposition'). This ensures that the QoS treatment is applied consistently across the MPLS core, as P routers typically forward based on the EXP bits without inspecting the original IP header.

SR-PCE acts as a central controller that computes paths for SR-TE policies and communicates them via PCEP.

Shaping buffers excess traffic to smooth bursts, while policing drops or re-marks excess traffic to enforce a hard rate limit.

The 'bandwidth percent' command allocates a percentage of the interface bandwidth to the class.

YANG models are the foundation for service definitions in NSO because they define the data structures and constraints for network services using a standardized modeling language (RFC 6020/7950). NSO uses YANG to model both the service layer (customer-facing services) and the device layer (native device configurations), enabling service abstraction and multi-vendor orchestration.

Shaping buffers excess packets to smooth traffic and reduce bursts, while policing drops or re-marks excess packets to enforce a strict rate limit.

RESTCONF uses POST to create a new resource, and PUT can also create if the resource does not exist.

DSCP is a 6-bit field with 64 possible values. Per-hop behaviors (PHBs) like EF and AF are defined by DSCP values.

A 'must' expression in YANG provides a XPath condition that must evaluate to true for the data to be valid.

MQC consists of class-map (classification), policy-map (define actions), and service-policy (apply). class-map uses match statements; policy-map can include bandwidth, priority, shape, etc.

PCEP (Path Computation Element Protocol) is used for communication between a PCE (controller) and PCC (router) to compute and instantiate SR-TE policies.

NSO (Network Services Orchestrator) uses NETCONF as its primary protocol for configuration management because NETCONF provides a standardized, transaction-based mechanism for editing device configurations using YANG data models. This allows NSO to perform atomic, rollback-capable configuration changes across multiple network devices, which is essential for service orchestration.

WRED (Weighted Random Early Detection) and tail drop are congestion avoidance mechanisms that manage queue fullness before congestion occurs.

PCEP (Path Computation Element Protocol) is used between PCCs (e.g., routers) and a PCE (e.g., SR-PCE) for path computation requests and responses.

A common mapping is to copy the three most significant bits of DSCP to EXP, which maps DSCP PHBs into corresponding EXP values.

PCEP is the protocol used between PCE and PCC for path computation and setup.

NSO uses NETCONF as the southbound protocol to communicate with devices. NETCONF provides transactional configuration operations like edit-config and get-config.

NSO's CDB (Configuration Database) holds the configuration and state of all managed devices, acting as the authoritative data store.

Option B is correct because LLQ provides strict priority queuing for voice (EF), ensuring it is never dropped during congestion, while CBWFQ allows the remaining bandwidth to be shared among video (AF41) and data (AF21) in a 2:1 ratio. LLQ is designed to guarantee low latency for real-time traffic, and CBWFQ enables explicit bandwidth allocation for non-real-time classes. This combination meets the requirement of zero-drop for voice and proportional sharing for video and data.

LLQ is used for voice (priority), CBWFQ for premium data (bandwidth), and default class for best effort.

Class-maps define match criteria (e.g., match ip dscp 46) to classify traffic in MQC.

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.

LLQ provides a strict priority queue for voice (low latency) and CBWFQ for other classes with bandwidth guarantees. Video conferencing can be assigned to a class with a bandwidth percent, and bulk data to the default class with fair queuing.

LLQ is strict priority, so voice gets priority up to its police rate; if voice exceeds, it may be dropped. Video and data get their allocated bandwidths, but if voice exceeds its allowed priority, it may be policed, preventing starvation.

RESTCONF uses PUT to replace a resource entirely; PATCH is for partial updates.

CDB stores the intended configuration and operational state for managed devices, enabling NSO to compare and sync with actual device configurations.

Option C is correct because NSO's NED (Network Equipment Driver) is the component that abstracts the southbound communication between NSO and network devices. NEDs can use NETCONF, SNMP, or CLI as the southbound protocol to translate NSO's device-independent configuration into device-specific commands, enabling multi-vendor management.

The correct answer is C because the NETCONF `get-config` operation is specifically designed to retrieve a configuration datastore, such as the running configuration. Unlike the `get` operation, which retrieves both configuration and state data, `get-config` targets only the configuration data, making it the precise operation for fetching the entire running configuration from a network device.

In hierarchical QoS, the parent policy applies shaping; the child policy applies queuing actions like bandwidth and priority. The child policy is attached to a class in the parent.

Option B is correct because the YANG service model is a core component of NSO architecture. It defines the service logic and data model that translates high-level service requests into device-specific configurations. NSO uses YANG to model both services and devices, enabling automation of service provisioning.

In hierarchical QoS (HQoS), the parent policy shapes the aggregate traffic to 10 Mbps, while the child policy applies LLQ for voice and CBWFQ for data. The primary purpose of policing within the LLQ child policy is to enforce a strict rate limit on voice traffic, dropping excess packets to prevent voice from consuming bandwidth beyond its allocated share. This prevents voice traffic from starving other queues (e.g., data) within the shaped parent rate, ensuring fairness and protecting other classes from congestion.

Hierarchical QoS uses a parent policy for shaping and a child policy for classification and priority queuing. The child is applied to the parent's class-default.

Hierarchical QoS (HQoS) is required when you need to shape traffic to a contracted rate (e.g., 10 Mbps) while also applying per-class actions like priority and bandwidth guarantees. Option A correctly uses a child policy to define class-based QoS (priority for voice, bandwidth for critical data) and a parent policy that applies the shape average command to the entire child policy aggregate, enforcing the 10 Mbps contract. This two-level MQC structure ensures that shaping occurs at the parent level before the child policy’s per-class queuing and scheduling are applied.

NEDs (Network Element Drivers) are plugins that translate NSO's service models to device-specific configurations.

The Network Element Driver (NED) provides the abstraction layer between NSO and network devices, translating YANG models to device-specific configurations (e.g., CLI, SNMP).

EF PHB is used for low-loss, low-latency traffic such as voice. The recommended DSCP value for EF is 46 (binary 101110).

B is correct because the Path Computation Element Protocol (PCEP), defined in RFC 5440 and extended for Segment Routing in RFC 8664, is the dedicated protocol used between a PCE and a PCC to request, report, and delegate SR-TE paths. PCEP carries explicit path computation requests (PCReq) and replies (PCRep), as well as state reporting (PCRpt) and delegation (PCUpd) messages, making it the standard control-plane protocol for SR-PCE architectures.

In the Modular QoS CLI (MQC), the class-map component is used to define traffic classes by matching packets based on criteria such as DSCP (Differentiated Services Code Point) or IP precedence values. The class-map uses match statements (e.g., match ip dscp, match ip precedence) to classify traffic, which is then applied within a policy-map for QoS actions. This makes class-map the correct MQC component for traffic matching.