Arista EOS QoS Configuration: Class Maps to Strict Priority

Default Arista EOS QoS treats all unclassified traffic on a first-in, first-out basis, which means a microburst from a backup job or a vMotion event can drop voice packets and ruin call quality. This article shows three escalating approaches to QoS on Arista switches: trusting DSCP/CoS at the edge for quick wins, building class maps and policy maps with the Modular QoS CLI for explicit classification, and enforcing strict-priority queuing so critical traffic always leaves the switch first. Shallow-buffered platforms like the 7050X benefit most from these configurations.

Solution 1: Quick Fix — Trust the Edge

interface Ethernet12
   description UPLINK_TO_VOIP_VLAN
   qos trust dscp
   ! qos trust cos   ! for Layer 2 tagging

Only apply trust on ports where you control the device — an untrusted laptop can mark BitTorrent as EF (DSCP 46) and hog bandwidth.

Solution 2: MQC Class Maps and Policy Maps

! Step 1: Class map identifies voice traffic
class-map type qos match-any CLASS-VOICE
   match ip dscp 46
   match ip dscp 40

! Step 2: Policy map maps it to traffic class 5
policy-map type qos POLICY-EDGE-IN
   class CLASS-VOICE
      set traffic-class 5
   class class-default
      set traffic-class 0

! Step 3: Apply to the interface
interface Ethernet48
   description UPLINK_TO_CORE
   service-policy type qos input POLICY-EDGE-IN

Solution 3: Strict Priority Queuing

! Map Traffic Class 5 to output Queue 5
qos map tc 5 to mc-queue 5 uc-queue 5

! Enforce strict priority for voice
interface Ethernet48
   tx-queue 5
      priority strict
   ! Best-effort data gets round robin
   tx-queue 0
      bandwidth percent 50

This overrides the default weighted round-robin behavior so that class 5 traffic is serviced before anything else. It is the right choice for latency-sensitive voice and video on converged networks.

Hardware Considerations

Platforms like the 7050X series have shallow buffers and critically need QoS, while deep-buffered 7280R series are more forgiving but still benefit from proactive queue configuration. Monitor queue behavior with show qos interface Ethernet48 and show qos statistics after applying policies.

More Arista topics: MLAG configuration run book and the EOS configuration cheat sheet.

原文链接：https://techresolve.blog/2026/03/09/configuring-arista-qos