L3Out QoS can be configured using Contracts applied at the external EPG level. Starting with Release 4.0(1), L3Out QoS can also be configured directly on the L3Out interfaces.

Note	If you are running Cisco APIC Release 4.0(1) or later, we recommend using the custom QoS policies applied directly to the L3Out to configure QoS for L3Outs.

Packets are classified using the ingress DSCP or CoS value so it is possible to use custom QoS policies to classify the incoming traffic into Cisco ACI QoS queues. A custom QoS policy contains a table mapping the DSCP/CoS values to the user queue and to the new DSCP/CoS value (in case of marking). If there is no mapping for a specific DSCP/CoS value, the user queue is selected by the QoS priority setting of the ingress L3Out interface if configured.

When traffic enters the Cisco ACI fabric, each packet's priority is mapped to a Cisco ACI QoS level. These QoS levels are then stored in the CoS field and DE bit of the packet's outer header while the original headers are discarded.

If you want to preserve the original CoS values of the ingressing packets and restore it when the packet leaves the fabric, you can enable the 802.1p Class of Service (CoS) preservation using a global fabric QoS policy as described in this section.

The CoS preservation is supported in single pod and multipod topologies, however in multipod topologies, CoS preservation can be used only when you are not concerned with preserving the settings in the IPN between pods. To preserve the CoS values of the packets as they are transiting the IPN, use the DSCP translation policy as described in Multipod QoS and DSCP Translation Policy.

When traffic is sent and received within the Cisco ACI fabric, the QoS Level is determined based on the CoS value of the VXLAN packet's outer header. In multipod topologies, where devices that are not under Cisco APIC's management may modify the CoS values in the transiting packets, you can preserve the QoS Level setting by creating a mapping between the Cisco ACI and the DSCP value within the packet.

If you are not concerned with preserving the QoS settings in the IPN traffic between pods, but would like to preserve the original CoS values of the packets ingressing and egressing the fabric, see Class of Service (CoS) Preservation for Ingress and Egress Traffic instead.

As illustrated in this figure, traffic between pods in a multipod topology passes through an IPN, which may contain devices that are not under Cisco APIC's management. When a network packet is sent from a spine or a leaf switch in POD1, the devices in the IPN may modify the 802.1p value in the packet. In this case, when the frame reaches a spine or a leaf switch in POD2, it would have an 802.1p value that was assigned by the IPN device, instead of the Cisco ACI QoS Level value assigned at the source in POD1.

In order to preserve the proper QoS Level of the packet and avoid high priority packets from being delayed or dropped, you can use a DSCP translation policy for traffic that goes between multiple PODs connected by an IPN. When a DSCP translation policy is enabled, Cisco APIC converts the QoS Level value (represented by the CoS value of the VXLAN packet) to a DSCP value according to the mapping rules you specify. When a packet sent from POD1 reaches POD2, the mapped DSCP value is translated back into the original CoS value for the appropriate QoS Level.

Cisco APIC enables translating the DSCP and CoS values of the ingressing traffic to a QoS Level to be used inside the Cisco ACI fabric. Translation is supported only if the DSCP values are present in the IP packet and CoS values are present in the Ethernet frames.

For example, this functionality allows the Cisco ACI fabric to classify the traffic for devices that classify the traffic based only on the CoS value, such as Layer-2 packets, which do not have an IP header.