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QoS reply behavior

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?


Re: QoS reply behavior

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