Class of Service (CoS)
What the CoS domain tests
JNCIP-ENT tests classification, scheduling, shaping, policing, rewrite rules, and drop profiles in Junos. You must trace a packet forwarding class and loss priority from ingress to egress.
Core rules
- A classifier maps DSCP, 802.1p, or multifield match conditions to a forwarding class and loss priority.
- Schedulers define transmit rate, buffer, and priority; a scheduler map attaches them to an interface.
- Rewrite rules mark egress CoS bits; they do not classify ingress traffic.
- Policers enforce rate limits and can set loss priority; drop profiles manage congestion.
- CoS is applied at egress by default; ingress classification must be configured explicitly.
Worked example
Voice packets arrive with DSCP 46, but the interface has no DSCP classifier. The default classifier places them in best-effort, so the voice queue is empty while voice suffers. Applying a DSCP classifier to the ingress interface maps EF to the voice forwarding class. A scheduler map with strict priority for voice then gives it priority. The reasoning is that marking alone does not create a queue.
Traps
Do not expect an egress rewrite rule to fix ingress classification. Do not attach a scheduler map without checking the interface actual CoS configuration. Do not use strict priority for multiple classes without a bandwidth cap. Verify loss priority as well as forwarding class.