Question 1

Which of the following CANNOT be used as external marking bits?

Accepted Answer

C

Explanation: IP Precedence (IPv4 ToS field), Dot1p (802.1Q PCP field), and EXP bits (MPLS TC field) are all standardized fields within packet headers used to carry Quality of Service (QoS) information across a network. These fields are "external" because they are visible to and interpreted by different network devices. In contrast, a Forwarding Class (FC) is an internal construct within the Nokia Service Router Operating System (SR OS). After a packet is classified based on its external markings or other criteria, it is mapped to an internal FC. This FC then determines the packet's per-hop behavior, such as which queue it enters. The FC itself is not a field in the packet header and is not transmitted to other routers.

Question 2

Which of the following egress points does NOT support egress reclassification?

Accepted Answer

D

Explanation: Egress reclassification is a feature of service-level Quality of Service (QoS) policies. These policies are applied at the egress points of a service, such as Service Access Points (SAPs) and Service Distribution Points (SDPs). Spoke-SDPs, whether bound to an IES or a VPRN, function as service egress points and support egress QoS policies that include reclassification rules. Network interfaces, which operate in the global routing context (the "core"), do not support service-level egress reclassification. They use network QoS policies, which are designed to manage queuing and marking (e.g., Forwarding Class to MPLS EXP bits) for traffic transiting the service provider's core network, not for re-evaluating traffic based on service-specific criteria as it exits a service.

Question 3

Which of the following statements is a characteristic of a CFHP arbiter?

Accepted Answer

C

Explanation: A Class Fair Hierarchical Policing (CFHP) arbiter is a logical entity within a hierarchical QoS structure whose primary function is to manage and distribute bandwidth among its children. These children can be either policers or other arbiters. The arbiter is configured with its own rate limit and uses a weighting system to fairly allocate its available bandwidth to its children based on their demand. This dynamic allocation process effectively adjusts the bandwidth assigned to each child, ensuring that the collective traffic conforms to the arbiter's rate while maintaining fairness among the subordinate elements.

Question 4

Which of the following statements about behavior aggregate and multi-field classification in a Nokia 7750 SR is TRUE?

Accepted Answer

D

Explanation: In a Nokia 7750 SR, a SAP-ingress Quality of Service (QoS) policy performing Behavior Aggregate (BA) classification must have an unambiguous source for Layer 2 priority. The 802.1p (dot1p) bits in a VLAN tag and the MPLS EXP (lsp-exp) bits in a label are both considered Layer 2 markings. To prevent classification conflicts on packets that could potentially contain both (e.g., VPLS over MPLS), the system enforces a mutual exclusivity rule. Therefore, a single SAP-ingress QoS policy cannot be configured to trust both dot1p and lsp-exp simultaneously. This is a fundamental constraint of the QoS implementation on the platform.

Question 5

In a Nokia 7750 SR, which of the following QoS policies CANNOT include packet classification criteria?

Accepted Answer

B

Explanation: In Nokia's Service Router Operating System (SR OS), Quality of Service (QoS) policies are specialized for different functions and application points. A SAP-egress policy is applied to traffic leaving a Service Access Point (SAP) towards the customer. Its primary function is to manage the scheduling and shaping of traffic that has already been classified into Forwarding Classes (FCs) by an ingress policy. The SAP-egress policy defines the egress queues, their parameters (e.g., CIR, PIR), and maps the pre-determined FCs to these queues. It does not contain any criteria for inspecting packet headers (L2/L3/L4) to perform classification; that function is handled at ingress.

Question 6

Which of the following policies does NOT exist by default on a network port?

Accepted Answer

A

Explanation: Network policies are applied to Layer 3 network interfaces (IP interfaces), not directly to the Layer 2 network port itself. While a system-wide default network policy (ID 1) exists, it is only associated with a port after a network interface has been explicitly configured on that port. In contrast, default network queue policies and slope policies are fundamental to the hardware's traffic handling structure. A default network queue policy is applied to the MDA on ingress and the port on egress, and a default slope policy is applied to the ingress queues, all without requiring the configuration of a higher-level logical entity like an IP interface.

Question 7

Which of the following about the process of queuing self-generated traffic in the Nokia 7750 SR is TRUE?

Accepted Answer

B

Explanation: Self-generated traffic originates from the Control Plane Module (CPM) and is considered egress traffic from the control plane's perspective. This traffic is sent to an Input/Output Module (IOM) where it is placed into egress hardware queues for scheduling and transmission out of a physical port. Conversely, traffic from the network destined for the control plane is considered ingress traffic. While this traffic is first policed by hardware mechanisms on the ingress IOM to protect the CPU, it is ultimately forwarded to the CPM. Upon arrival at the CPM, the traffic is placed into software queues for processing by the relevant protocol or management application. Thus, ingress traffic is stored in CPM queues and egress traffic in IOM queues.

Question 8

Which action should be taken to ensure that packets do not use the shared buffer space when put into a queue?

Accepted Answer

B

Explanation: In Nokia's SR OS, the Maximum Burst Size (MBS) defines the total buffer space a queue can occupy, while the Committed Burst Size (CBS) defines the portion of that space that is reserved exclusively for the queue. The buffer space between the CBS and MBS values (MBS - CBS) is drawn from a shared buffer pool common to other queues. To ensure a queue uses only its reserved (committed) buffer space and does not draw from the shared pool, its maximum allowable burst (MBS) must be configured to be equal to its committed burst (CBS). This effectively sets the allowable shared buffer portion to zero.

Question 9

What feature makes a traffic stream real-time?

Accepted Answer

C

Explanation: A traffic stream is classified as real-time based on its sensitivity to Quality of Service (QoS) parameters, specifically latency and jitter. Real-time applications, such as Voice over IP (VoIP) and video conferencing, require a low and consistent end-to-end delay (latency) to function correctly for interactive communication. Furthermore, they are sensitive to jitter, which is the variation in packet delay. Maintaining a constant interval between packets is crucial for smooth playback and to avoid artifacts. Therefore, the defining characteristics are minimal delay and consistent packet timing.

Question 10

Which of the following statements about scheduling on the Nokia 7750 SR is FALSE?

Accepted Answer

A

Explanation: The statement is false because when a queue operates in priority-mode, the egress profile is re-determined based on the scheduling loop that serviced the packet, not the ingress profile. Low-priority traffic is explicitly directed to the above-CIR (or PIR) scheduling loop. Any packet serviced from the above-CIR loop is re-marked as out-of-profile at egress. Therefore, the claim that low-priority traffic scheduled in the above-CIR loop will be considered in-profile at egress is incorrect; it will be considered out-of-profile.

Question 11

According to the SAP-egress policy configured and applied, which queue will AF traffic use?

Accepted Answer

A

Explanation: In the Nokia Service Router Operating System (SR OS), when both a SAP-egress QoS policy and a queue-group template are applied to a Service Access Point (SAP), the configurations within the SAP-egress policy take precedence for forwarding class (FC) to queue mapping. The provided configuration shows that SAP-egress policy 10 explicitly maps FC 'af' to queue 3 (fc af create queue 3). Although the queue-group template "test" maps FC 'af' to queue 2, this mapping is overridden by the more specific definition in the applied SAP-egress policy. Therefore, all traffic classified as 'af' will be directed to queue 3.

Question 12

Which of the following statements about the token buckets of a policer is TRUE?

Accepted Answer

A

Explanation: In the Nokia SR OS dual token bucket policer model, an arriving packet is first checked against the Peak Information Rate (PIR) bucket. This bucket has a capacity defined by the Maximum Burst Size (MBS). If there are insufficient tokens in the PIR bucket to accommodate the size of the arriving packet, the packet is considered non-conforming to the peak rate. Such packets are colored "red" and are immediately discarded. Therefore, the PIR bucket enforces the absolute maximum rate, and any traffic exceeding this limit is dropped.

Question 13

Click the exhibit button below. A network operator has configured a network-queue policy to map
forwarding classes to queues, as shown in the exhibit below. Based on the default scheduling
behavior of the Nokia 7750 SR, in which order will packets be serviced?
4A0-107 practice exam questions

Accepted Answer

D

Explanation: The Nokia 7750 SR platform utilizes a default two-tier hierarchical scheduling mechanism. The first tier provides strict priority to expedited queues, to which the Expedited Forwarding (EF) class is mapped. Consequently, all traffic in the EF queue is serviced first. The second tier handles best-effort queues (H2, AF, BE) and is profile-aware. The scheduler first services all in-profile traffic (packets conforming to the Committed Information Rate, CIR) from these queues in a weighted manner. Only after the in-profile demand is met does the scheduler service the out-of-profile traffic (packets exceeding CIR but within the Peak Information Rate, PIR). This results in the servicing order described in option D.

Question 14

Click the exhibit button below. Given this scheduler-policy configuration, which of the following can be said about the scheduler called "high"? (Choose two) https://kxbjsyuhceggsyvxdkof.supabase.co/storage/v1/object/public/file-images/NOKIA_4A0-107/page_22_img_1.jpg

Accepted Answer

A, D

Explanation: The provided configuration defines a three-tier scheduler hierarchy. The statement tier 2 scheduler "low" parent "high" explicitly defines scheduler "high" as the parent of scheduler "low". Therefore, "high" is a parent scheduler. In the Nokia SR OS scheduling model, any scheduler defined within a scheduler-policy is part of a larger hierarchy on the physical port. The Tier 1 scheduler ("high" in this case) is a child of the port's root scheduler (or a virtual port scheduler). Thus, "high" is also a child scheduler in the context of the overall port scheduling structure.

Question 15

Which of the following statements describes a queue’s weight parameter that is used in the hierarchical scheduling?

Accepted Answer

D

Explanation: In Nokia's hierarchical scheduling, a two-pass process is used to allocate bandwidth. The first pass satisfies the Committed Information Rate (CIR) for queues and schedulers, typically based on strict priority. The weight parameter is not used in this pass. The second pass distributes any remaining bandwidth, known as above-CIR bandwidth. The weight parameter is used in this second pass to determine the proportion of this available above-CIR bandwidth that a queue (or scheduler) receives relative to other queues at the same hierarchical level and priority. A higher weight results in a larger share of the excess bandwidth, up to the queue's Peak Information Rate (PIR).

Free Nokia 4A0-107 Actual Exam Questions