QoS reply behavior

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Apr 23rd, 2009
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Hi, I have a problem regarding QoS replies. I am simulating two 4500 switches (A and B) connected by a trunk link. The two switches have one laptop directly connected to each of them (LA for switch A and LB for switch B). I then set the ports connected to the laptops to a default dscp value of 5 (qos dscp 5). Then I performed a ping test from LA to LB.


Am I correct to assume that any packets from LA will be ALWAYS marked with dscp 5 and reach LB with the same mark?


I also noticed that if the request has a mark of dscp 5, the reply is always marked down to 0. Is this a normal behavior even though I configured its port to qos dscp 5 also?


If this is the case then is it safe to assume that a reply will always be marked with a value of 0?

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davy.timmermans Thu, 04/23/2009 - 21:31
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Hi Greg,


If you enable QoS globally, each port is untrusted by default. Untrusted means that all packets that comes in will we remarked to 0.


In your case:


1)you've to check if your packets are getting marked.

2)As mentioned above, the other switch will receive the packet and mark the packet down to 0 because by default it doesn't trust the traffic it receives.


You've to put 'qos trust dscp' on all interswitch links in order to maintain the marks

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