How To | Understand network configuration goals with Cisco Nexus 9K, CAT6500, and CAT4500 switches

Learn how to achieve network system objectives using Cisco Nexus 9K, CAT6500, and CAT4500 switches.

Updated at October 13th, 2023


Network Goals for Q-SYS

  • Transport of Q-SYS traffic (PTPv2 and Q-SYS Audio) across the network must be within 280.8 μs for PTPv2 and 288.8 μs for Q-SYS audio.
  • Switches can typically consume between 10 μs and 50 μs per hop.
  • Using 10 GbE links for transport between switches cuts down considerably on the serialization time—for example from 12 μs to 1.2 μs.
  • Maximum jitter allowable for PTPv2 clock steering is ±30 µs. Jitter exceeding this will cause PTPv2 problems.

Q-SYS Networking Tips

  • Q-SYS uses IEEE PTPv2 protocol, which is very sensitive to latency, jitter, and packet loss. Q-SYS audio data is tied to the PTPv2 clock sync mechanism, so it, too, is sensitive to latency, jitter, and packet loss.
  • Review the Q-SYS Technical Notes Q-LAN Networking Overview
  • Q-SYS PTPV2 traffic has a QoS value of EF (46) and must be placed in the highest priority queue, with Strict Priority Queueing enabled.
  • Q-SYS audio traffic has a QoS value of AF41 (34) and must be placed in the second highest priority queue, also with Strict Priority Queueing.
  • Items to consider since they can cause issues for Q-SYS on the network:
  • Do not enable jumbo frames or jumbo packets on any switch handling Q-SYS traffic, even on ports that carry no Q-SYS traffic. On some switches this is a per-port setting and others it is a global setting. Jumbo frames or packets may cause problems because of excessive latency.
  • Other traffic given equal or greater priority than Q-SYS PTPv2 (EF) or Q-SYS audio (AF41) through the same switch may cause problems.
  • Q-SYS traffic has very strict network requirements regarding latency. Interfaces need to be 1 GbE or faster.
  • Network interfaces must have zero errors (or near zero).
  • FCoE passing through the same interface and/or backplane queuing resources may cause problems.
  • Misconfigured multicast IGMP snooping V2 and/or PIM-Sparse Mode (PIM-SM) will cause problems.
  • Changes in spanning tree protocol (STP) on a network, even if it is done on different switches and/or different VLANs can sometimes cause PTPv2 clock problems. We recommend STP pruning to keep things as tight as possible. Turn off STP on any interfaces that connect to Q-SYS devices.


  • Cisco Nexus switches may have FEX (Fabric Extenders) to reduce per-port costs. As a general rule Q-SYS traffic should not run on FEXs (e.g., Nexus 2K) because of their performance constraints (limited QoS, oversubscription, and switching is not performed locally).
  • The only issue we have seen with Cisco Nexus switches and Q-SYS has been with a Nexus 3K because of a Broadcom bmc-shell ACL which blocked PTPv2 traffic through the switch. A Q-SYS network administrator who sees such a problem needs to open a Cisco TAC case for the Broadcom bcm-shell ACL to be changed; these would be low level changes. The Broadcom T2 chipset (and other chipsets from Broadcom) are shared between the Nexus 3K and 9K families
  • The CAT4500 series are unique as the only Cisco series that uses the DBL (Dynamic Buffer Limiting) algorithm. DBL was created by a student working on a PhD thesis on an algorithm that would offer better QoS than other methods, but it has still not found its way into any other than the CAT4500 series.
  • Find more information at the Cisco Network Switches page.