Egress forwarding class override provides additional QoS flexibility by allowing the use of a different forwarding class at egress than was used at ingress.
The ingress QoS processing classifies traffic into a forwarding class (or subclass) and by default the same forwarding class is used for this traffic at the access or network egress. The ingress forwarding class or subclass can be overridden so that the traffic uses a different forwarding class at the egress. This can be configured for the main forwarding classes and for subclasses, allowing each to use a different forwarding class at the egress.
The buffering, queuing, policing, and remarking operation at the ingress and egress remain unchanged. Egress reclassification is possible. The profile processing is completely unaffected by overriding the forwarding class.
When used in conjunction with QoS Policy Propagation Using BGP (QPPB), a QPPB assigned forwarding class takes precedence over both the normal ingress forwarding class classification rules and any egress forwarding class overrides.
Figure: Egress forwarding class override shows the ingress service 1 using forwarding classes AF and L1 that are overridden to L1 for the network egress, while it also shows ingress service 2 using forwarding classes L1, AF, and L2 that are overridden to AF for the network egress.