QoS (Quality of Service) for Network Switches — Guide

DSCP (Differentiated Services Code Point) is a 6-bit field in the Layer 3 IP header, defined by RFC 2474, offering 64 possible values. It survives routing across subnets because it travels within the IP packet. 802.1p CoS is a 3-bit Priority Code Point inside the Layer 2 802.1Q VLAN tag, providing eight priority levels (0-7). CoS is stripped when a frame crosses a Layer 3 boundary unless the router explicitly re-maps it. In a typical campus network, switches trust DSCP on routed interfaces and CoS on Layer 2 trunks, maintaining consistent priority across both layers.

VoIP codecs such as G.711 and G.729 are exacting about latency (below 150 ms one-way) and jitter (below 30 ms). Weighted fair queuing distributes bandwidth proportionally but cannot guarantee that voice frames are dequeued before all other traffic. A strict-priority queue — also called a low-latency queue — services voice frames immediately, irrespective of other queue depths. The trade-off is that strict-priority must be policed: without a bandwidth cap, a misbehaving device could monopolise the scheduler and starve every other queue.

A trust boundary is the precise point in the network where DSCP or CoS markings are accepted as legitimate. On an access port connected to an IP phone, the switch trusts DSCP because the phone is a verified, managed endpoint. On a port facing a PC or guest device, the switch overrides any incoming markings and resets the frame to DSCP 0 or CoS 0. This prevents an end user from self-marking traffic as EF to gain unfair priority. Cisco implements this through the 'mls qos trust' command combined with CDP or LLDP-MED device detection; Aruba uses comparable per-port trust policies.

No. QoS is a congestion-management discipline, not a performance accelerator. When link utilisation is well below capacity, all frames are forwarded at wire speed regardless of their marking. QoS becomes operative only when a queue begins to fill — at that point, classification and scheduling determine which frames are forwarded first and which are delayed or dropped. This is precisely why QoS must be designed in advance: it provides no benefit until the moment congestion occurs, at which point a network without QoS will degrade voice and video indiscriminately.

Cisco Auto-QoS is a macro that generates a complete set of class maps, policy maps, and queue configurations when you issue the 'auto qos voip cisco-phone' or 'auto qos video' command on an interface. It configures trust boundaries, DSCP-based classification for voice (EF), video (AF41), signalling (CS3), and best-effort traffic, and assigns appropriate egress queue weights. Aruba switch platforms generally have no one-line macro equivalent to Cisco's Auto-QoS; administrators configure QoS through manual QoS policies, LLDP-MED trust settings, and per-port priority assignments, and the available shortcuts differ by model and OS version, so check your platform's documentation. Both approaches can reach the same outcome — the macro is quicker to deploy, while manual policies give finer control.

Call +971 55 651 7843, email [email protected], or complete the quotation request form. Provide the number of switches, the types of traffic requiring prioritisation (VoIP, CCTV, video conferencing), your current VLAN layout, and any existing QoS policies. Emirtech will conduct a site assessment at your Abu Dhabi or Al Ain premises and deliver a documented QoS design with DSCP-to-queue mapping tables, trust-boundary definitions, and a commissioning test plan.