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QOS 6509 v12.2(17d)SXB10

6509 has WS-X6724-SFP, WS-X6748-GE-TX (RcvQ-1Q8T, xmtQ-1P3Q8T) and Sup720. I have questions about the command: mls qos rewrite ip dscp

Q.1. When is the received TOS rewritten ?

Q.2 Why would I want to use: no mls qos rewrite ip dscp ' to preserve TOS byte ?? At 6509, I am 'trusting dscp' at trunk ports, Can TOS byte still be overwritten ?

1 REPLY
Purple

Re: QOS 6509 v12.2(17d)SXB10

Hi,

1. The TOS is rewritten on egress

2. You seem to be right. When you set a port to trust DSCP, you are getting the switch to not reset the DSCP to 0 for incoming packets. The switch will use whatever DSCP the packet came with. On egress, the switch will use the DSCP to CoS map to select the egress queue. Finally, when sending the frame out, the switch will set the CoS value to whatever CoS value the DSCP was mapped to. However, the DSCP value will not be overwritten, since that is supposed to be trusted. Therefore, the 'no mls qos rewrite ip dscp' appears to be redundant in this case. However, the difference is that this command is a global command and will result in ToS-preservation for packets received on all interfaces and not just those set to trust DSCP

Hope that helps - pls rate the post if it does.

Regards,

Paresh.

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