Specifications

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© 2009 Cisco Systems, Inc. All rights reserved. This document is Cisco Public Information. Page 36 of 89
7.3 Preserving the Received ToS Byte (DSCP Transparency)
When a packet enters the switch, the switch will derive an internal DSCP value from the incoming priority bits (based
on the trust setting). This internal DSCP is used to write the ToS byte when the packet egresses the switch. This
action can thus change the ingress DSCP setting.
Those customers who would like to preserve the integrity of their DSCP can use this feature to avoid the PFC
rewriting the DSCP on egress. This feature is supported on the PFC3, PFC3B and PFC3BXL. It is worth noting that
only the PFC3B and PFC3BXL support this for MPLS frames and tunnelled traffic (i.e. GRE and IP-in-IP). This can
be achieved using the following command:
Cat6500(config)# no mls qos rewrite ip dscp
7.4 Port based vs. VLAN Based QoS
When a QoS policy is applied to a port, the PFC needs to know whether this policy is applicable to this port only or
whether this policy is part of a global policy that applies to all ports in a given VLAN. By default, policies applied to a
port are viewed as only being applicable for that port. If, however, it is required to apply a policy on a VLAN (which
implicitly means the policy should be applied to all ports in that VLAN), then the port needs to be marked indicating
its part of a VLAN QoS policy. Should this be a requirement, then the following command needs to be applied to all
ports in that VLAN.
Cat6500(config)# mls qos vlan-based
7.5 Setting the Switch to Queuing-Only Mode
Configuring trust requires QoS to be enabled on the Catalyst 6500. Enabling QoS also activates all of the QoS
features (Classification and Policing) on the PFC. It is possible a scenario might arise where trust is required, but
classification and policing are not. When this situation arises, the switch can be configured to operate in a mode
where PFC QoS functions are disabled while leaving other QoS functions untouched. This mode is referred to as
Queuing-only mode and can be enabled as follows:
Cat6500(config)# mls qos queueing-only
7.6 Input Classification and Setting Port Based CoS
On ingress to a switch port, an Ethernet frame can have its CoS changed if it meets one of the following two criteria:
1. The port is configured as untrusted, or
2. The Ethernet frame does not have an existing CoS value already set
Should you wish to re-configure the CoS of an incoming Ethernet frame, you should use the following command
Cat6500(config)# interface fastethernet 5/13
Cat6500(config-if)# mls qos cos 4
This command sets the CoS of incoming Ethernet frames on port 13 on module 5 to a value of 4 when an unmarked
frame arrives or if the port is set to un-trusted.
7.7 Applying CoS Mutation on 802.1Q Tunnel Ports
CoS mutation is a feature available on the PFC3x that allows a “mutated” CoS value to be used on 802.1Q trunk
ports for ingress queue threshold management. This feature maintains the CoS value of the incoming packet and
does not change it in the same way a CoS rewrite action might take. Enabling this feature creates a map that defines
what “mutated” CoS value each ingress CoS value will take. An example of a map is shown below: