JNCIP-ENT Class OF Service — 15 Questions

Question 1

An administrator is troubleshooting a multi-field classifier configuration on an MX router. The classifier is failing to match traffic based on Layer 4 ports. What is the most likely cause?

Accepted Answer

Layer 4 headers are encrypted by IPsec and cannot be inspected by standard classifiers. Multi-field classifiers in Junos are implemented using firewall filters with then forwarding-class actions, requiring correct term ordering and matching fields.

Answer

Layer 4 classification requires an explicit forwarding-class map under system services

Answer

Multi-field classifiers do not support TCP ports in Junos

Answer

Layer 4 matching is only supported on ingress CoS rewrite rules

Question 2

You are configuring a Junos device and need to classify incoming traffic based on the IP precedence bits within the IPv3 header. Which classifier type should you select under the class-of-service configuration hierarchy?

Accepted Answer

ip-precedence. IP precedence uses the three most significant bits of the IPv3 Type of Service (ToS) byte. In Junos, the correct classifier type for this is ip-precedence.

Answer

ieee-802.1

Answer

exp

Answer

dscp

Question 3

When configuring a hierarchical scheduler map on an MX Series router, what is the primary purpose of the aggregate level shaping?

Accepted Answer

To limit the total traffic across all queues associated with the scheduler map. Aggregate level shaping controls the total bandwidth allocated to a group of queues or sub-interfaces, ensuring overall traffic limits are respected.

Answer

To dynamically assign forwarding classes based on IP headers

Answer

To ensure absolute priority for best-effort traffic

Answer

To rewrite CoS bits on egress frames

Question 4

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?

Accepted Answer

discard. A policer in Junos uses policer action statements like loss-priority or discard. To drop non-compliant traffic immediately, the action discard is configured.

Answer

forwarding-class

Answer

transmit

Answer

mark-action

Question 5

Which standard Junos forwarding class is reserved for control plane traffic such as routing protocols and management traffic?

Accepted Answer

network-control. In Junos, the network-control forwarding class is reserved for control plane traffic to ensure it receives priority handling.

Answer

expedited-forwarding

Answer

assured-forwarding

Answer

best-effort

Question 6

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?

Accepted Answer

priority high. In Junos, priority high or priority low is used to designate strict priority queuing for a forwarding class in a scheduler.

Answer

buffer-size

Answer

transmit-rate

Answer

drop-profile

Question 7

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?

Accepted Answer

[edit class-of-service interfaces]. Rewrite rules in Junos are applied on egress interfaces under the interfaces statement within the class-of-service configuration hierarchy.

Answer

[edit firewall]

Answer

[edit routing-options]

Answer

[edit class-of-service classifiers]

Question 8

Which command is used to verify the operational state of forwarding classes and their assigned queues on a specific interface on Junos?

Accepted Answer

show class-of-service interface. The show class-of-service interface command displays operational CoS details, including queue mappings, for specified interfaces.

Answer

show route forwarding-table

Answer

show interfaces extensive

Answer

show firewall

Question 9

Which configuration stanza is used to define custom forwarding classes in Junos?

Accepted Answer

[edit class-of-service forwarding-classes]. Custom forwarding classes are defined under the [edit class-of-service forwarding-classes] hierarchy.

Answer

[edit class-of-service drop-profiles]

Answer

[edit class-of-service classifiers]

Answer

[edit class-of-service schedulers]

Question 10

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?

Accepted Answer

transmit-rate. The transmit-rate parameter in a scheduler definition guarantees a minimum bandwidth allocation for a queue even during congestion.

Answer

drop-profile-map

Answer

buffer-size temporal

Answer

priority high

Question 11

When configuring a hierarchical policer on a Juniper MX Series router, what is the significance of the logical-interface-policer setting?

Accepted Answer

It shares a single policer bucket across all forwarding classes on the logical interface. A logical-interface-policer applies a single policer instance shared across all queues or forwarding classes on that specific logical interface.

Answer

It separates policing actions strictly for Layer 2 control frames

Answer

It disables hardware-based policing and falls back to RE software rate-limiting

Answer

It applies the policer globally across all physical interfaces on the Routing Engine

Question 12

Which statement describes the function of a drop-profile in Junos CoS?

Accepted Answer

It determines when and how many packets to drop based on buffer occupancy. A drop-profile defines the packet drop probability as a function of buffer occupancy to implement RED or WRED congestion avoidance.

Answer

It maps incoming CoS bits to internal forwarding classes

Answer

It defines how traffic is shaped to a specific peak rate

Answer

It rewrites outer VLAN tags on trunk interfaces

Question 13

You need to configure a classifier that inspects the Multiprotocol Label Switching (MPLS) experimental (EXP) bits. Which classifier type must you specify?

Accepted Answer

exp. The MPLS EXP bits are classified using the exp classifier type in Junos CoS configurations.

Answer

fec

Answer

ieee-802.1

Answer

dscp

Question 14

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?

Accepted Answer

The commit fails because the sum of percentage transmit-rates exceeds 100%. 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.

Answer

Traffic exceeding 100% is dropped by the Routing Engine

Answer

Junos normalizes the percentages proportionally during commit time

Answer

The excess 10% is automatically assigned to the best-effort queue

Question 15

Which command allows you to view the configured drop-profile statistics and monitor packet drop counts on an MX Series router?

Accepted Answer

show class-of-service drop-profile. The show class-of-service drop-profile command displays drop-profile utilization and drop statistics.

Answer

show firewall log

Answer

show interfaces drop-rate

Answer

show system drop-stats