Performance Optimization
Core concepts
Performance questions focus on QoS, buffering, and traffic prioritization. Know the QoS pipeline on AOS-CX: classification (trust DSCP or CoS, or ACL-based), marking, queuing, and scheduling (strict priority vs. weighted). Understand DSCP values (EF=46 for voice, AF classes for video/data), CoS-to-queue mapping, and how trust modes affect behavior. Also know link aggregation for bandwidth, jumbo frames, and flow control.
Traps to watch
- Trust boundaries: If a port trusts DSCP, an untrusted host can mark its own traffic EF. Questions test where trust should be enabled (uplinks, not access ports).
- Strict priority starvation: A strict-priority queue can starve lower queues under load - the exam asks when to use weighted scheduling instead.
- Jumbo frames end-to-end: Enabling jumbo frames on one link but not the path causes MTU black holes.
- Flow control (PAUSE): Can cause head-of-line blocking; not a substitute for proper buffering.
Worked example
A voice packet arrives with DSCP EF (46) on an access port configured to trust DSCP. It maps to the highest-priority queue and gets strict-priority scheduling. If the same port instead trusts CoS, the 802.1Q priority bits (typically 5 for voice) drive the queue. The exam may ask which config yields correct voice treatment - answer: trust on the uplink, mark at the edge, and map EF to the priority queue.
Rule of thumb
Classify and mark at the edge, trust only on trusted uplinks, and reserve strict priority for latency-sensitive voice only.