Cisco Cisco Prime Virtual Network Analysis Module (vNAM) 6.3 White Paper

Note that in newer data center QoS models, CoS 3 is reserved for lossless data (FCoE). However, in older 
WAN/campus QoS services models, CoS 3 is used for voice over IP (VoIP) signaling. The table assumes 
that FCOE traffic is localized to UCS and Ethernet-attached storage systems, enabling the use of CoS 3 
for VoIP signaling traffic in the data center QoS domain. Classification values may need to be tweaked 
per traffic characteristics; for example, CoS 4 could potentially be used for VoIP call control if video 
streams are not deployed.

It is a general best practice to mark traffic at the source-end system, or as close to the traffic source as 
possible, to simplify network design. However, if the end system cannot mark or cannot be trusted, 
marking can be performed on network ingress. In the VMDC QoS framework, the cloud data center 
represents a single QoS domain, with the Nexus 1000V forming the "southern" access edge, and the ASR 
9000 or ASR 1000 forming the "northern" DC PE/WAN edge. These QoS domain edge devices mark 
traffic, and these markings are trusted at nodes in the data center infrastructure. In other words, they use 
simple classification based on the markings received from the edge devices. Note that where VM-FEX 
adapters are used, marking is implemented on UCS Fabric Interconnects; in contrast to the Nexus 1000V 
implementation, there is no ability to conditionally mark-down CoS in the event of congestion.

In VMDC, the assumption is that DSCP values are not altered. Intermediate nodes would ideally support 
QoS transparency, so that CoS values would not need to be re-marked. That said, if QoS transparency is 
not supported on a particular node within the QoS domain, it will be necessary to workaround this gap 
by re-marking.

In VMDC VSA 1.0, VPX SLB does not support QoS transparency. The insertion of CSR as the tenant 
virtual private cloud represents a trust boundary. In these cases, it is necessary to classify and remark at 
the CSR.

In a router or switch, the packet scheduler applies policy to decide which packet to dequeue and send 
next, and when to do it. Schedulers service queues in different orders. The most frequently used are:

First in, first out (FIFO)

Priority scheduling (also called priority queuing)

Weighted bandwidth