Juniper Exam JN0-664 Topic 1 Question 52 Discussion

When a Juniper device receives a packet on an interface with both a Behavior Aggregate (BA) classifier and a Multifield (MF) classifier, Junos OS follows a specific processing order to apply Class of Service (CoS).

Understanding the Classifiers in Junos CoS

1 Behavior Aggregate (BA) Classifier

Uses packet headers (DSCP, IP precedence, or MPLS EXP bits) to classify traffic into forwarding classes.

Applied at the ingress interface.

Example: A packet with DSCP 46 (Expedited Forwarding) is mapped to a high-priority queue.

2 Multifield (MF) Classifier

Uses match conditions (like source/destination IP, port numbers, protocol types) to classify traffic.

Typically used for more granular classification beyond what BA can provide.

Junos Processing Order:

When both BA and MF classifiers are configured on an interface, Junos first applies the BA classifier, then the MF classifier. MF classifier can override the BA classification if necessary.

Evaluating the Answer Choices

B. The packet will be processed by the BA classifier first, then the MF classifier.

Correct, because Junos first applies BA classification based on DSCP/MPLS EXP bits.

After BA classification, the MF classifier is applied, which can refine or override the BA classification.

A. The packet will be discarded.

Incorrect, because classification does not drop packets unless explicitly configured with a filter or policing action.

C. The packet will be forwarded with no classification changes.

Incorrect, because both classifiers are applied in a specific order, meaning classification changes will occur.

D. The packet will be processed by the MF classifier first, then the BA classifier.

Incorrect, because BA classification is always applied first, followed by MF classification.

Final Answer: B. The packet will be processed by the BA classifier first, then the MF classifier.

Official Juniper Reference: 'When both BA and MF classifiers are applied on an interface, Junos OS first classifies packets using the BA classifier before applying the MF classifier.'