Cisco Systems 4500 MQC-Based QoS on the Supervisor Engine 6-E and 6L-E

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Software Configuration Guide—Release 15.0(2)SG
OL-23818-01
Chapter 37 Configuring Quality of Service
Configuring QoS on Supervisor Engine 6-E, Supervisor Engine 6L-E, Catalyst 4900M, and Catalyst 4948E
by default. Only when the trusted boundary feature is enabled on an interface can the port enter untrusted
mode. In this mode, the switch marks the DSCP value of an IP packet and the CoS value of the VLAN
tag on the Ethernet frame as 0.
MQC-Based QoS on the Supervisor Engine 6-E and 6L-E
The operation of Catalyst 4900M, Catalyst 4948E, Supervisor Engine 6-E, and Supervisor Engine 6L-E
follow the QoS deployment outlined in RFCs 2597, 2598, 2474, and 2475. A high-level QoS flow is as
follows:
1. The incoming packet is classified (based on different packet fields, receive port and/or VLAN) to
belong to a traffic class.
2. Depending on the traffic class, the packet is rate-limited/policed and its priority is optionally marked
(typically at the edge of the network) so that lower priority packets are dropped or marked with
lower priority in the packet fields (DSCP and CoS).
3. After the packet has been marked, it is looked up for forwarding. This action obtains the transmit
port and VLAN to transmit the packet.
4. The packet is classified in the output direction based on the transmit port and/or VLAN. The
classification considerrs any marking of the packet by input QoS.
5. Depending on the output classification, the packet is policed, its priority is optionally (re-)marked,
and the transmit queue for the packet is determined depending on the traffic class.
6. The transmit queue state is dynamically monitored by using the AQM (Active Queue Management)
algorithm and drop threshold configuration to determine whether the packet should be dropped or
enqueued for transmission.
7. If eligible for transmission, the packet is enqueued to a transmit queue. The transmit queue is
selected based on output QoS classification criteria. The selected queue provides the desired
behavior in terms of latency and bandwidth.
Figure 37-1 illustrates a high-level flow of QoS function on Catalyst 4900M, Catalyst 4948E, Supervisor
Engine 6-E, and Supervisor Engine 6L-E.
Figure 37-7 QoS Packet Processing
Packet
Reception
Input
Output
Input QoS
Classification
Input
Policing
and Marking
Forwarding
Lookup
Output QoS
Classification
Output
Policing
and Marking
Active Queue
Management
Port/Queue
Scheduling
(Sharing/
Shaping)
Packet
Transmission
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