The MLS QOS trust commands are used to determine how an internal DSCP
value is derived which will be used in physical queue assignments. You
either 1) Trust the incomming L2 COS value with the "mls qos trust cos"
command which intern uses the COS-DSCP map to derive the internal DSCP
value, or 2) you Trust the incomming DSCP value with the "mls qos
trust dscp" command which intern uses the incomming DSCP value as the
internal DSCP value. There is also a 3rd option to not trust anything
incomming but use your own class maps to classify, policy maps to mark
and this will be used to derive the internal DSCP value which determines
the physical queue the packet gets put in to.
Another note is that the mls qos trust cos/dscp are mutually exclusive,
meaning it is one or the other (not both). If both are specified in the
config the one listed last in the config wins.
Also, I've seen in many configurations the mls qos trust command
accompanied with Service policy command that has it own class-maps (for
classification) and policy-maps for marking, in this case the mls qos
trust statements are ignored and the class-maps/policy-maps from the
service policy command are used.
In summary your thinking is correct, you will use either the "cos-dscp"
maps or trust the incoming DSCP value or don't trust anything, to derive
the internal dscp value (which is everything).
This is covered in more detail in the " Switch Configuration Guide" for
each IOS version of each switch. These maps are either defined per port
(as in the 3550) or globally via the queue set command (as on the 2960,
2560, 3750 switches).
Brian
On 5/8/2011 4:47 PM, Emin Guliyev wrote:
Hey guys,
Just wanted to clear this up. So, If I use "mls qos
trust cos" on an interface, from then on all the queue map, threshold
map will be based on Cos-Dscp table? That is also the same for " mls
qos trust dscp" dscp-cos tables will be used to convert layer2
markings to layer3 and vice versa respectively?
MQC will be also based on the either "trust" or "set" statement?
I might not have been clear with my question. Let me know if you need
me to re-word it.
Thanks,
Emin
_______________________________________________
For more information regarding industry leading CCIE Lab training, please visit
www.ipexpert.com
Are you a CCNP or CCIE and looking for a job? Check out
www.PlatinumPlacement.com
_______________________________________________
For more information regarding industry leading CCIE Lab training, please visit
www.ipexpert.com
Are you a CCNP or CCIE and looking for a job? Check out
www.PlatinumPlacement.com
