JNCIA Class of Service

Question 1

What is Class of Service (CoS) in the context of Junos networking?

Accepted Answer

A set of mechanisms for differentiating network traffic and providing varying service levels (priority, bandwidth, delay) to different traffic types

Answer

A Junos feature for classifying user login privileges

Answer

A method for encrypting traffic based on sensitivity

Answer

A routing protocol that prioritizes certain prefixes

Question 2

In Junos CoS, what is a 'forwarding class'?

Accepted Answer

A named group that maps to a specific output queue, determining how packets are buffered and scheduled for transmission

Answer

A firewall filter action that forwards packets

Answer

A routing policy term that classifies routes

Answer

A VLAN used for priority traffic

Question 3

What does DSCP stand for and what is its role in Junos CoS?

Accepted Answer

Differentiated Services Code Point — a 6-bit field in the IP header used to mark packets for CoS classification

Answer

Dynamic Service Control Protocol — a protocol for QoS negotiation

Answer

Data Service Class Priority — a Juniper-proprietary marking field

Answer

Destination Service Code Point — identifies the destination queue

Question 4

In Junos CoS, what is a 'scheduler' responsible for?

Accepted Answer

Defining the transmission priority, guaranteed bandwidth percentage, delay buffer size, and drop profile for a specific forwarding class queue

Answer

Scheduling routing protocol updates

Answer

Managing interface physical layer timing

Answer

Scheduling configuration commits

Question 5

What is the purpose of a Junos CoS 'rewrite rule'?

Accepted Answer

To mark or re-mark DSCP/802.1p/MPLS EXP bits in packet headers as they egress an interface, propagating CoS information downstream

Answer

To rewrite the destination IP address for NAT

Answer

To modify BGP route attributes for traffic engineering

Answer

To change the CoS classifier on an interface

Question 6

In Junos, what is 'weighted random early detection' (WRED) used for?

Accepted Answer

To proactively drop packets from lower-priority traffic before queues fill completely, preventing TCP global synchronization

Answer

To randomly select routing paths for load balancing

Answer

To randomly assign packets to forwarding classes

Answer

To discard entire flows when bandwidth is exceeded