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.
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Table of Contents
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.