SG300-28P QoS Question

[bhackbarth]

Hello,  I have SG300-28Ps as the PSE's for my IP telephone system.  The phones are tagging their voice packets as DSCP 46 as directed by auto voice vlan. The QoS settings on the switch are at default - Basic Mode, Trust DSCP, strict priority, etc.

On the PBX itself, DB programming allows me to program the 'Type of Service' for the voice packets. The recommended value in the manual was 184 which makes sense, as this decimal value for ToS corresponds to DSCP 46, CoS 5, etc.

The question comes though, do I need to change the trust mode on the switch? I'm not real clear on the differences between them.

Regards,

-Brayton

[Tom Watts · Accepted solution]

Hi Brayton,the trust mode doesn't need to be changed. 802.1p specifies a 3 bit field called a  PCP within the etherner frame header when using tagged vlan frames. This will contain a class of service priority.

The CoS is able to map to DSCP values. The DSCP has a 6 bit field called diffserv (differentiated service). CoS values are able to be mapped to DSCP values. Video is generally CoS 4 while voice is generally CoS 5. Within the SX300 you are able to manually set the mapping to any value you'd like. With trust mode, the switch will basically accept and agree with whatever the tagged ethernet frame contains. Without trust mode, the switch will remark the packet based on the PCP and DiffServ value to fit in to the different categories.

-Tom
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