Hi Brian:
Have a look here under the section titled "QoS Considerations" in the Multipod Whitepaper.
https://www.cisco.com/c/en/us/solutions/collateral/data-center-virtualization/application-centric-infrastructure/white-paper-c11-737855.html#_Toc521671377
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"Each class of service is identified with a specific 802.1p (CoS) value in the outer Layer 2 header of the VXLAN encapsulated traffic routed inside the Pod. This information allows the spine and leaf nodes inside the Pod to perform proper traffic classification and prioritization.
In an ACI Multi-Pod deployment, two important considerations arise when discussing end-to-end QoS behavior:
1. First, since the IPN devices are external to the ACI fabric and are hence not managed by APIC, in many cases it may not be possible to assume that the 802.1p values are properly preserved across the IPN network. By default traffic received by the spines on the interfaces connecting to the IPN devices is classified to one of the classes of traffic shown in Fig. 6 based on the CoS value in the outer IP header of inter-pod iVXLAN traffic. This may lead to unexpected handling inside the fabric for traffic flows received from the IPN. As a consequence, the recommendation is to configure a proper CoS-to-DSCP mapping on APIC to ensure that traffic received on the spine nodes of a remote Pod can be reassigned to its proper class of service before being injected into the Pod based on the DSCP value in the outer IP header of inter-pod iVXLAN traffic, as shown in Figure 7 below."
2. The DSCP values set by the spine nodes before sending the traffic into the IPN network can then be used to differentiate and prioritize the different types of traffic. In the example above, Policy Plane Traffic (that is, communication between APIC nodes deployed in separate Pods) is marked as Expedited Forwarding (EF), whereas Control Plane Traffic (that is, OSPF and MP-BGP packets) is marked as CS4. The IPN devices can be configured to prioritize those two types of traffic to ensure that the policy and control plane remains stable also in scenarios where a large amount of east-west user traffic is required across Pods."
