JNCIE-ENT: Class of Service (CoS)

This domain tests how Junos classifies, queues, schedules, and marks traffic. JNCIE-ENT expects a complete path: classify on ingress, assign a forwarding class and loss priority, schedule the queue, drop with a profile, and rewrite on egress.

Core rules

• Behavior aggregate (BA) classifiers map DSCP, IP precedence, or 802.1p code points to a forwarding class and loss priority. Multifield classifiers use firewall filter actions such as forwarding-class and loss-priority.
• Schedulers define queue treatment: transmit-rate, buffer-size, priority strict-high or low, and drop-profile maps. A scheduler map binds forwarding classes to schedulers.
• Drop profiles implement WRED-style tail dropping by fill level and drop probability; apply them per loss priority.
• Rewrite rules mark code points on egress. Classification is ingress; rewriting is egress.
• Keep the forwarding-class-to-queue mapping consistent across the topology.

Worked example

Map DSCP EF (binary 101110) to expedited forwarding and AF21 (010010) to assured forwarding: set class-of-service classifiers dscp DSCP-CLASS forwarding-class expedited-forwarding loss-priority low code-points 101110 set class-of-service classifiers dscp DSCP-CLASS forwarding-class assured-forwarding loss-priority low code-points 010010 Give EF a strict-high scheduler and AF a 30 percent scheduler with a drop profile. Apply the classifier to the ingress interface and the scheduler map to the egress interface. EF gets priority service; AF gets bounded bandwidth and early drop as its queue fills.

Traps

DSCP code points are binary, not decimal; 46 is 101110. A classifier must be applied on ingress, and a rewrite rule on egress, or markings are not what you expect. Strict-high queues can starve others. Transmit rates above 100 percent are rejected or scaled. A drop profile attached to the wrong loss priority has no effect.