Cisco WS-X6K-SUP1A-PFC= Software Guide - Page 717

Chapter 41 Configuring QoS Understanding How QoS Works Marking at Untrusted Ports QoS marks all frames received through untrusted ports with the port CoS value (the default is zero). QoS does not implement ingress port congestion avoidance on untrusted ports: the traffic goes directly to the switching engine. Marking at Trusted Ports When an ISL frame enters the switch through a trusted port, QoS accepts the three least significant bits in the User field as a CoS value. When an 802.1Q frame enters the switch through a trusted port, QoS accepts the User Priority bits as a CoS value. QoS marks all traffic received in other frame types with the port CoS value. The port CoS value is configurable for each Ethernet port (for more information, see the "Configuring the CoS Value for a Port" section on page 41-33). Ethernet Ingress Port Scheduling and Congestion Avoidance QoS does not implement ingress port congestion avoidance on ports configured with the untrusted, trust-ipprec, or trust-dscp keywords: the traffic goes directly to the switching engine. QoS uses CoS-value-based receive-queue drop thresholds to avoid congestion in traffic entering the switch through a port configured with the trust-cos keyword (for more information, see the "Configuring the Trust State of a Port" section on page 41-32). Receive Queues Enter a show port capabilities command to see the queue structure of a port. The command displays one of the following: • rx-(1p1q4t)-one strict-priority queue and one standard queue with four thresholds • rx-(1q4t)-one standard queue with four thresholds • rx-(1p1q0t)-one strict-priority queue and one standard queue with no configurable thresholds Strict-priority queues are serviced in preference to other queues. QoS services traffic in a strict-priority queue before servicing the standard queue. When QoS services the standard queue, after receiving a packet, it checks for traffic in the strict-priority queue. If QoS detects traffic in the strict-priority queue, it suspends its service of the standard queue and completes service of all traffic in the strict-priority queue before returning to the standard queue. Ingress Scheduling QoS schedules traffic through the receive queues based on CoS values. In the 1p1q4t and 1p1q0t default configurations, QoS assigns all traffic with CoS 5 to the strict-priority queue; QoS assigns all other traffic to the standard queue. In the 1q4t default configuration, QoS assigns all traffic to the standard queue. Ingress Congestion Avoidance If a port is configured with the trust-cos keyword, QoS implements CoS-value-based receive-drop thresholds to avoid congestion in received traffic. 78-13315-02 Catalyst 6000 Family Software Configuration Guide-Releases 6.3 and 6.4 41-11