Cisco APIC Layer 3 Networking Configuration Guide, Release 6.2(x)

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Cisco APIC Layer 3 Networking Configuration Guide, Release 6.2(x)

L3Out QoS guidelines and limitations

Updated: April 17, 2026

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Describes requirements and limitations for configuring L3Out QoS, including VRF enforcement settings, contract placement, custom policies, and switch-generation support for QoS levels.

L3Out QoS guidelines and limitations are operational requirements that

  • define policy enforcement through contracts and VRF settings,

  • identify switch-generation support for QoS levels, and

  • specify where to configure QoS classes and target DSCP values.

Configuration and operational guidelines

The following guidelines and limitations apply to L3Out QoS:

  • Layer 3 multicast traffic: A custom QoS policy does not support Layer 3 multicast traffic that originates outside the Cisco Application Policy Infrastructure Controller ( Cisco ACI ) fabric.

  • Contract-based enforcement: To enforce a contract-based QoS policy on the border leaf switch that hosts the L3Out, set Policy Control Enforcement Direction for the VRF to Egress.

  • Direct custom QoS: A custom QoS policy that is configured directly on an L3Out applies to traffic from the border leaf switch and does not require the VRF to use egress enforcement.

  • Policy enforcement: To enforce the QoS policy, set VRF Policy Control Enforcement Preference to Enforced.

  • Contract configuration: In the contract or contract subject that controls communication between the L3Out and other EPGs, configure the QoS class or target DSCP value.

  • Contract-subject priority: Select a QoS priority level when you create a contract subject. Do not select Unspecified.

  • Interface configuration: Apply a QoS class or custom QoS policy to an L3Out interface.

Configure the QoS class or target DSCP value in the contract, not in the external EPG (l3extInstP).

Note

A custom QoS policy is an exception. It sets the DSCP or CoS value even when the QoS class is Unspecified. When the QoS level is unspecified, the default level is 3.

Generation 2 switches support QoS levels 4, 5, and 6 in global policies, EPGs, L3Outs, custom QoS policies, and contracts. The following limitations apply:

  • Strict priority: Generation 2 switches support up to five classes that are configured with strict priority.

  • Switch generation: QoS levels 4, 5, and 6 are supported only on generation 2 switches.

  • Traffic between switch generations: Traffic between generation 1 and generation 2 switches uses QoS level 3.

  • FEX communication: Traffic that uses the new classes and communicates with a FEX carries a Layer 2 CoS value of 0.

Generation 1 switch model names do not end in EX, FX, FX2, GX, or a later suffix. Generation 2 and later switch model names end in one of these suffixes or a later suffix.