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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
Classification criteria for the policy map on the physical member ports cannot be based on a combination
of fields. This restriction ensures that if the EtherChannel policy is marking down DSCP or CoS, the
marked (modified) value-based classification can be implemented in hardware.
Auto-QoS is not supported on EtherChannel or its member ports. A physical port configured with
Auto-QoS is not allowed to become a member of a physical port.
Software QoS
At the highest level, there are two types of locally sourced traffic (such as control protocol packets,
pings, and Telnet) from the switch: high priority traffic (typically the control protocol packets such as
OSPF Hellos and STP) and low priority packets (all other packet types).
The QoS treatment for locally sourced packets differs for the two types.
Catalyst 4900M, Catalyst 4948E, Supervisor Engine 6-E, and Supervisor Engine 6L-E provide a way to
apply QoS to packets processed in the software path. The packets that get this QoS treatment in software
can be classified into two types: software-switched packets and software-generated packets.
On reception, software-switched packets are sent to the CPU that in turn sends them out of another
interface. For such packets, input software QoS provides input marking and output software QoS
provides output marking and queue selection.
The software-generated packets are the ones locally sourced by the switch. The type of output software
QoS processing applied to these packets is the same as the one applied to software switched packets. The
only difference in the two is that the software-switched packets take input marking of the packet into
account for output classification purpose.
As per CSCtf40934, the following change is implemented beginning with Cisco IOS
Release 12.2(54)SG:
• All locally generated v4 or v6 packets that leave the switch tagged reflect precedence value in the
CoS bits.
• For all other packets, PAK_PRIORITY is used to determine CoS as explained in the following
section.

High Priority Packets

High priority packets are marked as one of the following:
• Internally with PAK_PRIORITY
• With IP Precedence of 6 (for IP packets)
• With CoS of 6 (for VLAN tagged packets)
These packets operate as follows:
• They are not dropped because of any policing, AQM, drop thresholds (or any feature that can drop
a packet) configured as per the egress service policy. However, they might be dropped because of
hardware resource constraints (packet buffers, queue full).
• They are classified and marked as per the marking configuration of the egress service policy that
could be a port or VLAN. Refer to the “Policy Associations” section on page 37-85.
• These high priority packets are enqueued to queue on the egress port based on the following criteria:
–
If there is no egress queuing policy on the port, the packet is queued to a control packet queue
that is set up separately from the default queue and has 5 percent of the link bandwidth reserved
for it.