Class of Service Flashcards

How is CoS applied to interfaces in Junos?

Answer: By attaching classifiers, rewrite rules, and scheduler-maps to interfaces under [edit class-of-service interfaces]

Junos CoS components (classifiers, scheduler maps, shapers, rewrite rules) must be explicitly applied to each interface under the [edit class-of-service interfaces] hierarchy to take effect.

What is the difference between 'strict-high' and 'high' priority in a Junos CoS scheduler?

Answer: strict-high is always served before all other queues, regardless of remaining bandwidth; high uses weighted queuing and can be starved by strict-high

'Strict-high' priority in a Junos scheduler implements absolute priority (Priority Queuing) — the queue is always drained first before any other queue is served. 'High' uses weighted fair queuing and shares bandwidth proportionally.

In Junos CoS, what is an 'interface-set' used for?

Answer: To apply a single CoS configuration to multiple interfaces simultaneously, simplifying management of shared CoS policies

Interface-sets in Junos CoS allow operators to define a named group of interfaces that share the same CoS configuration, applying classifiers, scheduler maps, or rewrite rules to all members at once.

What CoS marking does the Expedited Forwarding (EF) per-hop behavior specify?

Answer: DSCP value 46 (101110 binary) — designed for low-latency, low-jitter, low-loss traffic like VoIP

EF (Expedited Forwarding) is defined with DSCP value 46 (binary 101110). It is designed for traffic requiring low latency, low jitter, and low packet loss, making it ideal for VoIP and real-time video.

How does Junos CoS handle traffic that exceeds a scheduler's 'transmit-rate'?

Answer: Excess traffic is queued in the delay buffer and transmitted when bandwidth is available, or dropped if the buffer overflows

Traffic exceeding the guaranteed transmit-rate is held in the forwarding class's delay buffer. If excess bandwidth is available from other queues, it is used. If the buffer overflows, packets are dropped according to the drop profile.

What is the function of 'loss priority' in Junos CoS?

Answer: Indicates how likely a packet is to be dropped during congestion — high loss priority packets are dropped first

Loss priority in Junos CoS indicates a packet's susceptibility to discard during congestion. High loss priority (HL) packets are dropped preferentially before low loss priority (LL) packets in WRED or tail-drop.