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Layer 3 Switching Engine QoS Packet Flow

The PFC has the capability to classify, mark, and police traffic based on both Layer 2 and Layer 3 elements. The PFC also introduces the concept of QoS ACLs, which can be used for granular control over QoS configurations. Figure 10-9 illustrates the flow of a frame or packet through a PFC.

As the packet arrives at the PFC, the ingress interface or the ingress VLAN is examined to see whether an ACL is applied. If an ACL is not applied to either the ingress port or the ingress VLAN, the packet uses the default ACL. The default IP ACL consists of a single ACE that has a configurable marking rule and configurable policing rules. If the default ACL has not been configured, the ACE in the default ACL applies a DSCP value of 0 to all traffic passed from ingress ports configured with the untrusted port keyword. If an ACL is applied to the port or VLAN, the ACL examines the ACEs to see whether any entries match the arriving packet.

The PFC uses a DSCP setting for each IP packet that it processes. The PFC asks itself, “Do we trust the DSCP, IP precedence, or CoS values on the frame?” If these values are not trusted, the DSCP value specified in the default ACL is used. If these values are trusted, the switch uses the DSCP values received. For IP precedence and CoS values, the PFC uses an internal DSCP value generated from the IP precedence-to-DSCP or the CoS-to-DSCP mappings, respectively.

If policing has been enabled, the PFC determines the current traffic rate and decides to drop the packet, re-mark the packet, or place the packet in the egress queue unchanged.

If the packet is destined for the egress queue, the CoS value is derived from the DSCP value before the packet is delivered.

Egress Queue Scheduling

Egress queue scheduling defines the treatment that a frame or packet receives as it exits the switch. Figure 10-10 shows the treatment received by the frame or packet in the egress queue.

As the frame arrives at the egress queue, the CoS value is examined to determine the proper queue and threshold for the egress interface type. In this example, the switch has three different queues that a frame can be placed in: 2q2t, 1p2q2t, and 1p3q1t. The CoS value determines the proper egress queue. After the proper queue and threshold has been identified, the question is asked “Is there room in the egress queue for this frame or has the drop threshold for this CoS value been met?” If the threshold has been met, the frame is dropped; otherwise the frame is placed in a queue to await transport.

712 Chapter 10: LAN QoS

Figure 10-10 Ethernet Egress Port Treatment

From Switching

Engine or MSFC

Ethernet Egress port Scheduling, Congestion Avoidance, and Marking

(Default Values Shown)

Transmit

Frame