Be careful when applying ingress QoS policies when they must interact with a fabric QoS policy. The fabric QoS policy overrides any traffic classification conducted by the ingress policy when determining which traffic should be placed in the priority, AF, or BE queues within the fabricq ASIC. In addition, the fabric QoS policy is used to determine which traffic is placed in the priority queue within the ingressq ASIC fabric queues and the switch fabric ASICs (S2 and S3 stages) rather than any
priority marking set by the ingress QoS policy. The
priority marking performed by the ingress QoS policy is still used when determining packet scheduling in the shape queues within the ingressq ASIC.
For example, if an ingress QoS policy were to classify and mark particular traffic types as being
priority and a fabric QoS policy were to be applied either marking alternative traffic as being
priority or not setting
priority at all, then the ingress policy
priority statement is effectively ignored in the fabricq, the S2 ASICs, and the S3 ASICs. Use caution to ensure that there is no conflict between the ingress QoS policy and the fabric QoS policy.
A very simplistic illustration would be if an ingress QoS policy uses a class-map to exclude MPLS experimental 3 from the
priority class, but the fabric QoS policy places MPLS experimental 3 traffic in the
priority class. In this case, the MPLS experimental 3 traffic is placed in the high-priority S2 and S3 queues and the fabricq high-priority port or queues.
If the ingress QoS policy remarks certain traffic with values that the fabric QoS policy class-maps are to match on, then the remarked traffic is matched and placed in the appropriate port or queues. This provides the ability for the ingress QoS policy and the fabric QoS policy to complement each other, rather than potentially conflicting.
As noted above, fabric QoS is constrained to a subset of the possible
match criteria that can be used in its class maps. If the ingress QoS policy were to set a
qos-group marking for all traffic that should be placed in the priority queue and another
qos-group marking for all traffic to be placed in the AF class, then if the fabric QoS policy class maps matches on the
qos-group values, the policy is honored end to end. This approach enables multiple ingress QoS policies to interact in the expected manner with a fabric QoS policy.
It is important to remember that if an ingress QoS policy is applied to an interface and the fabric QoS policy has been applied to the router, then the ingress MSC RX PSE is required to perform two classification cycles. This has an impact on the forwarding capacity of the line card or PLIM, reducing the performance to about 62.5 Mpps.
You can choose not to apply a specific fabric QoS policy, giving the ingress QoS policy the decision on which traffic is placed in the high-priority queues but removing the ability to differentiate between AF and BE classes in the fabricq ASIC.