JNCIA Class of Service 2 — Questions and Answers

Question 1: How is CoS applied to interfaces in Junos?

By attaching classifiers, rewrite rules, and scheduler-maps to interfaces under [edit class-of-service interfaces] (Correct answer)
By adding CoS configuration directly in the routing protocols
By using firewall filters with CoS match conditions
By configuring CoS globally without per-interface assignment

Correct 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.

Under [edit class-of-service interfaces interface-name]: 'unit N classifiers dscp classifier-name' applies a DSCP classifier for input classification; 'unit N rewrite-rules dscp rewrite-name' applies egress DSCP rewriting; 'scheduler-map map-name' applies the scheduler map (linking forwarding classes to schedulers) for output queuing. Shapers are applied via 'traffic-control-profiles'. This interface-level application gives operators per-interface CoS control. Some platforms support hierarchical CoS (H-CoS) with multiple scheduler levels.

Question 2: What is the difference between 'strict-high' and 'high' priority in a Junos CoS scheduler?

strict-high is always served before all other queues, regardless of remaining bandwidth; high uses weighted queuing and can be starved by strict-high (Correct answer)
strict-high and high are identical in behavior
high priority gets more bandwidth than strict-high
strict-high only applies to MPLS traffic

Correct 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.

Junos CoS priority levels (highest to lowest): strict-high → high → medium-high → medium-low → low. 'Strict-high' implements strict priority queuing: the PFE always services this queue before any others, irrespective of rates. This can starve lower-priority queues if strict-high traffic exceeds its allocation. Best practice: use strict-high only for low-volume, latency-sensitive traffic (e.g., VoIP at <30% of link) and police it to prevent starvation. 'High', 'medium', 'low' use deficit weighted round-robin (DWRR) sharing.

Question 3: In Junos CoS, what is an 'interface-set' used for?

To apply a single CoS configuration to multiple interfaces simultaneously, simplifying management of shared CoS policies (Correct answer)
To group interfaces for LACP link aggregation
To assign multiple IP addresses to a single interface
To create redundant interface groups for failover

Correct 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.

Interface-sets are configured under [edit class-of-service interface-set set-name]. Interfaces are added as members and then CoS components (scheduler-map, classifiers, rewrite-rules) are applied to the set rather than individual interfaces. This simplifies configuration when many interfaces share the same CoS policy (e.g., all customer-facing interfaces on a PE router). Changes to the interface-set automatically apply to all member interfaces. Useful in service provider environments with many similar customer interfaces.

Question 4: What CoS marking does the Expedited Forwarding (EF) per-hop behavior specify?

DSCP value 46 (101110 binary) — designed for low-latency, low-jitter, low-loss traffic like VoIP (Correct answer)
DSCP value 0 (best-effort)
DSCP value 10 (AF11)
DSCP value 32 (CS4)

Correct 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.

EF PHB (RFC 3246) defines a service where packets experience minimal queuing delay, jitter, and loss. DSCP 46 (101110) is the standard marking. To achieve EF behavior, EF-marked packets are placed in a strict-priority queue in Junos (mapped to strict-high priority scheduler). Police EF traffic to guarantee it doesn't exceed its allocated bandwidth. Typical EF usage: VoIP RTP streams, interactive video conferencing. Assured Forwarding (AF) classes (AF11-AF43) use DSCP values 10, 12, 14, 18, 20, 22, 26, 28, 30, 34, 36, 38.

Question 5: How does Junos CoS handle traffic that exceeds a scheduler's 'transmit-rate'?

Excess traffic is queued in the delay buffer and transmitted when bandwidth is available, or dropped if the buffer overflows (Correct answer)
Excess traffic is immediately discarded without buffering
Excess traffic is remarked to a lower DSCP class
Excess traffic is forwarded without queuing

Correct 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.

Junos CoS scheduling: the 'transmit-rate' (or 'bandwidth-limit') defines the guaranteed minimum bandwidth. Excess traffic from other queues can be borrowed if 'excess-rate' is configured. The 'buffer-size' (as temporal in microseconds or percentage of total buffer) determines how long packets can be buffered before being dropped. When buffers fill, the drop profile (RED or tail-drop) determines which packets are discarded. 'Shaping-rate' caps the maximum bandwidth for a queue, preventing it from using more than allocated even if other queues are idle.

Question 6: What is the function of 'loss priority' in Junos CoS?

Indicates how likely a packet is to be dropped during congestion — high loss priority packets are dropped first (Correct answer)
Sets the routing priority for a packet class
Defines the priority of the interface in STP
Determines which BGP route is preferred for a traffic class

Correct 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.

Junos CoS loss priority has four levels: low (LL), medium-low, medium-high, and high (HL). DSCP-based classifiers can set both forwarding class and loss priority. For example, AF11 (DSCP 10) = AF forwarding class + low loss priority; AF12 (DSCP 12) = AF forwarding class + medium loss priority; AF13 (DSCP 14) = AF forwarding class + high loss priority. WRED drop profiles use loss priority to determine drop probability curves — high loss priority traffic starts being dropped at lower queue depths. This allows flow-level differentiation within a forwarding class.

JNCIA Class of Service 2

How is CoS applied to interfaces in Junos?