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3550 qos

i have a question to ask....i have read many times about the mls qos trust {cos,ip precedence, dscp) command in the catalyst 3550...however until now i still not very understand when to use it or how to use it. is it that when i use trust it will straight ahead use the internal default mapping (for example cos-dscp mapping)? is it we only use the trust at the distribution layer n no need it at the qos edge switch? can you give me an example of an evironment where i need to use the trust command? is it when we use the trust command at the qos domain only?


Re: 3550 qos


This command is used to tell the switch whether or not frames arriving on the interface carry CoS, IP Precedence, or DSCP values that can be trusted. For instance, if a port on your 3550 is connected to an upstream switch that is marking all traffic as it enters the network, you can configure the port on the 3550 to trust those markings as they enter the switch (3550). On the other hand, ports that are configured as untrusted cause the 3550 to re-write the CoS value to zero.