35 questions · Class OF Service · All types, answers revealed
Which standard Junos forwarding class is reserved for control plane traffic such as routing protocols and management traffic?
network-control is the default class for high-priority control plane traffic.
Why this answer
In Junos, the network-control forwarding class is reserved for control plane traffic to ensure it receives priority handling.
You are troubleshooting a scenario where multi-field classifiers and behavior aggregate classifiers are both configured on the same ingress interface. In what order does Junos process classification?
BA classifies based on CoS bits; MF filters can subsequently override the class.
Why this answer
Junos processes Behavior Aggregate (BA) classification first, followed by Multi-Field (MF) classification which can override the BA result.
What is the default number of forwarding classes available on Juniper MX Series routers equipped with Modular Port Concentrators (MPCs)?
Junos supports a maximum of 64 forwarding classes on MPC hardware.
Why this answer
Junos supports up to 64 forwarding classes on modern MPC-based hardware platforms, though default configurations typically use a subset.
You need to configure a classifier that inspects the Multiprotocol Label Switching (MPLS) experimental (EXP) bits. Which classifier type must you specify?
exp specifies the MPLS experimental bits classifier.
Why this answer
The MPLS EXP bits are classified using the exp classifier type in Junos CoS configurations.
In Junos CoS architecture, what role does a forwarding class play?
Forwarding classes group packets for queue scheduling and drop profiles.
Why this answer
Forwarding classes act as internal labels used by the router to group traffic into specific queues for scheduling and drop treatment.
When applying a rewrite rule on an interface, you notice that packets are being remarked incorrectly. You realize that both a DSCP rewrite rule and an IEEE 802.1p rewrite rule are configured on the same physical interface. What is the rule regarding multiple rewrite rules on a single interface in Junos?
Multiple distinct header rewrite rules can coexist on an interface.
Why this answer
Junos allows only one rewrite rule per packet header type (e.g., one DSCP, one 802.1p) per logical interface, but you can have different header types applied simultaneously if they map to different packet layers.
An administrator needs to implement a traffic conditioning profile on a Juniper MX Series router to limit traffic to a specific burst size and drop packets exceeding the rate. Which policing action parameter is required to ensure that exceeding traffic is dropped immediately without coloring?
The discard action drops the packet immediately.
Why this answer
A policer in Junos uses policer action statements like loss-priority or discard. To drop non-compliant traffic immediately, the action discard is configured.
Which statement describes the function of a drop-profile in Junos CoS?
Drop-profiles implement Random Early Detection (RED) based on queue fill levels.
Why this answer
A drop-profile defines the packet drop probability as a function of buffer occupancy to implement RED or WRED congestion avoidance.
When configuring a hierarchical scheduler map on an MX Series router, what is the primary purpose of the aggregate level shaping?
Aggregate shaping controls the sum of all traffic belonging to the child schedulers.
Why this answer
Aggregate level shaping controls the total bandwidth allocated to a group of queues or sub-interfaces, ensuring overall traffic limits are respected.
Which command allows you to view the configured drop-profile statistics and monitor packet drop counts on an MX Series router?
This command shows drop-profile states and drop counters.
Why this answer
The show class-of-service drop-profile command displays drop-profile utilization and drop statistics.
An engineer configures a two-color policer on an MX router. What are the two states or outcomes for evaluated packets in a standard two-color policer?
Two-color policers classify traffic into conforming (green) and non-conforming (red).
Why this answer
A two-color policer evaluates packets as either conforming (within the rate limit) or non-conforming (exceeding the rate limit).
Which TWO parameters can be configured within a Junos scheduler definition? (Choose two)
transmit-rate defines the minimum bandwidth guarantee.
Why this answer
Schedulers include parameters such as transmit-rate, buffer-size, priority, and drop-profile-map.
You are configuring traffic shaping on an egress interface and need to ensure that the shaper accounts for Layer 2 framing overhead (such as Ethernet headers and preambles). Which configuration statement achieves this?
layer-2-overhead ensures framing bytes are included in shaping calculations.
Why this answer
The explicit specification of accounting for layer 2 overhead in Junos CoS shaping is done via the explicit-tracking or layer-2-overhead options under the traffic-control-profile or scheduler.
Which TWO statements describe the characteristics of Random Early Detection (RED) and drop profiles in Junos? (Choose two)
The graph maps drop probability versus fill level percentage.
Why this answer
Drop profiles define drop probability against fill level, helping prevent TCP global synchronization.
Which THREE types of classifiers are natively supported in Junos Class of Service? (Choose three)
exp is used for MPLS experimental bits classification.
Why this answer
Junos supports multiple classifier types including dscp, exp, and ieee-802.1.
Which THREE components are required to fully configure a custom Class of Service scheme on an MX Series router? (Choose three)
Schedulers define queue scheduling and drop profiles.
Why this answer
Implementing custom CoS requires forwarding classes, classifiers, and schedulers.
Which TWO statements are correct regarding hierarchical scheduling (H-QoS) on Juniper MX Series routers? (Choose two)
Subscriber access relies heavily on H-QoS.
Why this answer
Hierarchical schedulers allow multi-level bandwidth allocation and tenant sub-rate shaping.
What is the primary function of a behavior aggregate (BA) classifier in Junos CoS?
BA classifiers use header bits to assign forwarding classes.
Why this answer
A Behavior Aggregate (BA) classifier classifies packets based solely on the CoS bits in the packet header (such as DSCP, EXP, or IEEE 802.1p).
Which TWO statements are correct regarding the behavior of policers in Junos? (Choose two)
Exceeding traffic can have its loss priority remarked rather than being dropped.
Why this answer
Policers can drop traffic or mark loss priority, and they operate based on token bucket algorithms.
You want to map three loss priorities (low, medium-low, high) to different drop profiles within a specific forwarding class. Which statement should you use inside the scheduler configuration?
drop-profile-map links loss priorities to drop profiles.
Why this answer
In Junos scheduler configurations, drop-profile-map associates a loss priority and forwarding class with a specific drop profile.
Which configuration stanza is used to define custom forwarding classes in Junos?
Forwarding classes are created under this hierarchy.
Why this answer
Custom forwarding classes are defined under the [edit class-of-service forwarding-classes] hierarchy.
You are configuring a policer and want to specify the rate in terms of bits per second. Which keyword is used to define the bandwidth limit in the policer configuration?
bandwidth-limit specifies the committed rate in bits per second.
Why this answer
Policer bandwidth limits in Junos are defined using the bandwidth-limit or burst-size-limit keywords.
Which component of a scheduler configuration determines how memory resources are allocated among different queues?
buffer-size allocates memory queue space.
Why this answer
The buffer-size parameter determines memory buffer allocation for each queue in a scheduler configuration.
Which THREE methods are valid for marking packet headers using CoS rewrite rules in Junos? (Choose three)
exp rewrite rules modify MPLS EXP bits.
Why this answer
Rewrite rules can modify DSCP, EXP, and IEEE 802.1p fields.
Which command displays the configured scheduler maps and their association with forwarding classes on a Junos device?
This command displays scheduler map configurations.
Why this answer
The show class-of-service scheduler-map command displays details of scheduler maps and queue associations.
Which TWO statements are true regarding traffic shapers versus policers in Junos? (Choose two)
Shaping is an egress traffic conditioning mechanism.
Why this answer
Shapers buffer excess traffic to smooth out bursts, whereas policers drop or mark exceeding traffic immediately.
When configuring a hierarchical policer on a Juniper MX Series router, what is the significance of the logical-interface-policer setting?
It enforces rate limits across the entire logical interface aggregate.
Why this answer
A logical-interface-policer applies a single policer instance shared across all queues or forwarding classes on that specific logical interface.
Which command is used to verify the operational state of forwarding classes and their assigned queues on a specific interface on Junos?
This command displays CoS settings and queue mappings for interfaces.
Why this answer
The show class-of-service interface command displays operational CoS details, including queue mappings, for specified interfaces.
Which THREE actions can be executed by a firewall filter policer when traffic exceeds the specified rate? (Choose three)
discard drops non-conforming packets.
Why this answer
Junos policers can discard packets, forward packets with changed loss priority, or transmit them without modification.
You need to ensure that delay-sensitive voice traffic is serviced before all other queues on an egress interface. Which scheduler property should you configure for the voice forwarding class?
Setting priority to high enables strict priority queuing for the queue.
Why this answer
In Junos, priority high or priority low is used to designate strict priority queuing for a forwarding class in a scheduler.
Which THREE types of information can be examined using the command 'show class-of-service interface' on a Junos device? (Choose three)
Shows scheduler maps bound to the interface.
Why this answer
The show class-of-service interface command displays assigned scheduler maps, rewrite rules, and active classifiers on the interface.
You have configured a custom scheduler map, but traffic in the best-effort queue is being starved during periods of heavy congestion. Which scheduler parameter should you adjust to guarantee a minimum bandwidth for this queue?
Transmit-rate guarantees a minimum bandwidth allocation.
Why this answer
The transmit-rate parameter in a scheduler definition guarantees a minimum bandwidth allocation for a queue even during congestion.
Which TWO conditions must be met when configuring strict priority queuing for a forwarding class in Junos? (Choose two)
Shaping priority traffic is critical to prevent starvation.
Why this answer
Strict priority queues require a priority setting (high) and typically a shaping rate or limit to prevent starvation of other queues.
You want to apply a rewrite rule on an outgoing interface to remark the DSCP bits of packets leaving your Juniper router. Where must this rewrite rule be applied within the CoS hierarchy?
Rewrite rules are applied on egress interfaces.
Why this answer
Rewrite rules in Junos are applied on egress interfaces under the interfaces statement within the class-of-service configuration hierarchy.
An administrator configures a scheduler map with a transmit-rate of percent 20 for a queue, but the total sum of transmit-rates in the map equals 110%. How does Junos handle this configuration?
Percentage transmit rates must total 100% or less.
Why this answer
Junos requires the sum of transmit-rates defined as percentages in a scheduler map to not exceed 100%. If it exceeds 100%, the configuration commit will fail.
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