QoS: Prioritizing Critical Enterprise Traffic

QoS (Quality of Service) is essential for meeting the SLAs of critical applications such as voice, video, and ERP on a shared network. Without QoS, a 100 GB backup can saturate the WAN and disrupt your Teams calls. This guide covers traffic classes, DSCP marking, queues, policing, and shaping. 2026.

DiffServ Model

An IETF standard based on the 6-bit DSCP field in the IP header (values 0-63). Each class receives defined treatment:

• EF (46): Expedited Forwarding — VoIP, with guaranteed low latency
• AF41-AF43 (34-38): interactive video
• AF31-AF33 (26-30): video streaming
• AF21-AF23 (18-22): ERP, CRM
• AF11-AF13 (10-14): bulk data
• CS6 (48): routing control plane (OSPF, BGP)
• CS0/DSCP 0: best effort (default)

Enterprise Traffic Classes (Cisco Recommendations)

• Voice (EF): VoIP RTP, <150ms latency, <30ms jitter, <1% loss
• Video (AF41): Teams, Zoom, WebEx
• Call Signaling (CS3): SIP, H.323
• Transactional (AF21): Salesforce, SAP
• Bulk (AF11): backups, file transfers
• Scavenger (CS1): P2P, personal traffic
• Best Effort (DSCP 0): web browsing, email

Steps in a QoS Policy

1. Classification

Identify traffic using L3/L4 information (ACL, received DSCP), L7 inspection (Cisco NBAR2), or ports. Cisco example:

• class-map match-any VOICE
• match ip dscp ef
• match protocol rtp audio

2. Marking

Apply DSCP markings to packets. Trust boundary: the access switch generally marks traffic according to the endpoint:

• policy-map MARK-VOICE
• class VOICE ; set dscp ef
• class VIDEO ; set dscp af41
• class class-default ; set dscp default

3. Policing (Input)

Limit the inbound data rate. Excess traffic is dropped or re-marked.

4. Shaping (Output)

Smooth traffic to a target data rate, reducing packet drops.

5. Queuing (Output)

Schedule packets according to their class:

• Priority Queue (PQ): voice traffic receives strict priority processing
• LLQ (Low-Latency Queuing): PQ with guaranteed bandwidth
• CBWFQ (Class-Based Weighted Fair Queuing): bandwidth allocated by class
• 8 hardware queues on Catalyst 9300

Cisco IOS Configuration (LLQ)

• class-map match-all VOICE ; match ip dscp ef
• class-map match-all VIDEO ; match ip dscp af41
• policy-map WAN-OUT
• class VOICE ; priority percent 10
• class VIDEO ; bandwidth percent 30
• class TRANSACTIONAL ; bandwidth percent 25
• class class-default ; bandwidth percent 25 ; random-detect dscp-based
• interface GigabitEthernet0/0/0 ; service-policy output WAN-OUT

WRED (Weighted Random Early Detection)

Proactively drops packets before saturation to prevent TCP global synchronization. Configuration:

• random-detect dscp-based: DSCP-based dropping
• random-detect dscp 46 40 50 1: minimal EF dropping
• random-detect dscp 0 20 40 10: aggressive BE dropping

WiFi QoS (WMM + 802.11e)

4 access categories mapped to DSCP:

• AC_VO (Voice): DSCP EF
• AC_VI (Video): DSCP AF41
• AC_BE (Best Effort): DSCP 0
• AC_BK (Background): DSCP CS1

End-to-End QoS

A common pitfall: QoS on the LAN but not the WAN is ineffective. Complete workflow:

• The endpoint marks traffic (the IP phone pre-marks it as EF)
• The access switch trusts the marking or re-marks the traffic
• The distribution/core network preserves DSCP markings
• The WAN edge (FortiGate, ISR) applies policing/shaping
• The MPLS carrier honors the classes according to the CoS agreement

Application-Aware SD-WAN

Modern SD-WAN (FortiGate, Cisco Viptela) uses SLA-based path selection:

• Link 1 (MPLS): 20ms latency → VoIP
• Link 2 (FTTH Internet): 30ms → video, SaaS
• Link 3 (4G): 50ms, backup
• Real-time health checks and automatic failover in <100ms

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