Multicast, QoS, and Traffic Management
Multicast delivery uses IGMP snooping to constrain group traffic to interested ports; without it, multicast floods like broadcast. Know IGMP versions, querier election, and PIM (sparse/dense) for L3 multicast routing. QoS covers classification/marking (DSCP, CoS), queuing/scheduling (strict priority, weighted), policing vs shaping, and congestion management. Trust boundaries matter: set DSCP trust at the edge and preserve it internally.
Traps: (1) IGMP snooping without a querier can stall group state-enable a querier or use PIM. (2) Policing drops excess; shaping buffers and delays-know the difference. (3) CoS (L2, 3 bits) vs DSCP (L3, 6 bits) mapping. (4) Strict priority can starve lower queues-use weighted for bulk. (5) Marking at the wrong boundary erases QoS intent.
Rule of thumb: classify and mark at the access edge, trust DSCP on uplinks, and reserve strict priority for latency-sensitive voice only.
Worked example: Voice uses EF (DSCP 46), bulk uses AF11. Configure an egress policy with a strict-priority queue for EF and weighted queues for the rest. Verify with per-queue counters that voice is not dropped during congestion.