Specifications

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The above command sets the following map:
CoS 0 1 2 3 4 5 6 7
DSCP 20 30 1 43 63 12 13 8
While it is very unlikely that the above map would be used in a real life network, it serves to give an idea of what can
be achieved using this command.
8.11.2. IP Precedence to DSCP Mapping
Like the CoS to DSCP map, a frame can have a DSCP value determined from the incoming packets IP Precedence
setting. This still only occurs if the port is set to trusted by the administrator, and they have used the mls qos trust-
ipprec keyword. This keyword is only supported on GE ports and 10/100 ports on the WS-X6548 line cards. For
10/100 ports o the WS-X6348 6148, and WS-X6248 line cards, ACL’s should be used to assign ip precedence trust
to incoming data.
When QoS is enabled, the switch creates a default map. Please refer to Table 3 for default settings. This map is
used to identify the DSCP value that will be set based on the IP Precedence value. Alternatively, the administrator
can set up a unique map. An example of this is shown below:
Cat6500(config)# mls qos map ip-prec-dscp 20 30 1 43 63 12 13 8
The above command sets the following map:
IP Precedence
0 1 2 3 4 5 6 7
DSCP 20 30 1 43 63 12 13 8
While it is very unlikely that the above map would be used in a real life network, it serves to give an idea of what can
be achieved using this command.
8.11.3. PFC Classification
When a frame is passed to the PFC, the process of classification can be performed to assign a new priority to an
incoming frame. The caveat here is that this can only be done when the frame is from an untrusted port, or the frame
has been classified as being untrusted.
A policy map class action can be used to
1. TRUST COS
2. TRUST IP-PRECEDENCE
3. TRUST DSCP
4. NO TRUST
The TRUST DSCP keyword assumes that the frame arriving into the PFC already has a DSCP value set prior to it
entering the switch. The switch will maintain this DSCP value.
With TRUST IP-PRECEDENCE, the PFC will derive a DSCP value from the existing IP Precedence value resident in
the ToS field. The PFC will use the IP Precedence to DSCP map to assign the correct DSCP. A default map is
created when QoS is enabled on the switch. Alternatively a map created by the administrator can be used to derive
the DSCP value.
Like TRUST IP-PRECEDENCE, the TRUS COS keyword instructs the PFC to derive a DSCP value from the CoS in
the frame header. There will also be a CoS to DSCP map (either a default one of an administrator assigned one) to
assist the PFC in deriving the DSCP.