Class of Service Flashcards

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

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

Class of Service (CoS) in Junos enables network operators to prioritize and manage traffic based on its type or application, ensuring critical applications receive appropriate bandwidth and low latency.

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

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

Forwarding classes in Junos CoS are logical names that map to hardware output queues (typically 0-7). Classifiers assign packets to forwarding classes, and schedulers define service parameters for each class.

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

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

DSCP is a 6-bit field in the IP header's DSCP/ToS byte that marks packets with their desired service class. Junos uses DSCP values to classify incoming packets into forwarding classes.

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

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

A Junos CoS scheduler defines how each forwarding class queue is serviced: how much bandwidth it gets, its transmission priority (strict-high, high, medium, low), delay buffer, and congestion drop behavior.

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

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

Rewrite rules in Junos CoS modify the DSCP, 802.1p, or MPLS EXP code points in outgoing packet headers based on the packet's forwarding class and loss priority, propagating CoS markings to downstream devices.

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

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

WRED proactively drops packets based on queue depth and loss priority, causing TCP senders to reduce rates before queues are completely full, preventing the global synchronization problem of tail-drop.