Cisco 3550 resetting TOS bits?

In article , wrote: :As part of some product testing that I am doing, I am sending traffice :between two end nodes where the some of the traffic has the :Minimize-delay TOS bit set and some has the Maximize-throughput bit :set. If I sniff the packets on the originating side, the IP header is :what I expect. However on the other end (after passing through a 3550 :switch) I see that the TOS bits have all been reset to 0x00.

:Is this normal operation? Is there a way to disable this?

As I recall, the TOS bits are one of the criteria used for QoS classification on the 3550. The output 0x00 could represent the result of QoS -- that is, everything is likely being mapped to the same default priority.

I have read a little about QoS but never played with it, so I do not know whether this could be disabled. -Possibly- there is a method involving mapping the various interesting ToS bits into the corresponding binary-number priority. But the 3550 does not have 8 different output queues, so you might be forced to map more than one point to the same output ToS I guess. (The 3750 has a greater number of output queues if I recall correctly.)

It is normal. By default, your equipment will not accept incoming QOS - for good reason. On the incoming interface type:

mls qos trust cos

that will tell the interface that your equipment trusts the QOS coming in.

Hope that helps,

Jim

Thank you so much! (I actually had to use "mls qos trust cos pass-through dscp").