Setting up QoS on a 3750 switch and am a tad confused with the mapping tables and when/how packets are re-marked.
As I understand it the mapping tables are used to generate a internal QoS label (DSCP) that is used to process the packet. The IP Precedence or DSCP of the packet is not changed as it traverses the switch. There are two exceptions to this; the DSCP mutation map and the police-dscp map. These mapping table can change the IP Precedence/DSCP value of the packet.
The IP Precedence/DSCP of a packet can also be changed on ingress by applying a service policy that re-marks the packet.
Is this correct?
Also when a port is set in the no trust state the default DSCP value for the port is used. Does this re-mark the IP packet or is it only used to generate the internal QoS label?
Phil:The IP Precedence/DSCP of a packet can also be changed on ingress by applying a service policy that re-marks the packet.
Is this correct?
A: Yes, and also, if the port have trust ip prece or trust cos then the DSCP will change according to the internal map. remember, ip precedence and CoS are only bits while DSCP is 6 bits so it's not one-to-one mapping. for example: packet with CoS 5 and IP field have DSCP value of 44 comes in, the CoS is trusted and mapped to DSCP 40 per default internal mapping hence the DSCP is changed to 40 from 44.
Phil:Also when a port is set in the no trust state the default DSCP value for the port is used. Does this re-mark the IP packet or is it only used to generate the internal QoS label?
A: Both, it is used for internal qos and that same DSCP is used when the packet leaves the switch, so it's re-written.
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