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.
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
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
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
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.