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few question about qos

i have few questions about QOS

1) Can qos policy only be applied at incoming frame/packets or it can also be applied at outgoing frames?

2)Is it correct that qos policy for incoming frames , will decide the queue the frames be at placed ingress port?

3) Is it correct that qos policy for outgoing frames,decides the queue the frames be placed at egress port?

4) Is it possible to trust the Cos/dscp in frame/packet and have qos policy for outgoing frames ?

5) Scenario:

When f0/1 is in trunk mode:

sw1(config)# int f0/1

sw1(cofig-if)mls qos trust dscp

when switch receives the frame,will it drive the cos value from dscp-cos map and place the frame in appropiate queue?

When f0/1 is not access mode:

When a switch receives the frame , Would switch drive the cos value from DScp-cos map even though the port is in access mode and place the frame in appropiate queue at ingress port?

thanks a lot!

Hall of Fame Super Blue

Re: few question about qos


1) Depends what you mean by QOS policy. Assuming you mean QOS policy to include




congestion avoidance



then yes a lot of these can happen to packets leaving an interface as well as entering an interface.

2) Depends if the device supports multiple ingress queues. Switches such as the 3560/3750/4500/6500 do support multiple ingress queues and you can configure your QOS policy to place certain packets in certain queues.

3) Yes although again this depends on how many egress queues there are.

4) Not sure i entirely follow. You can trust the marking received in the packet and still have a policy that decides what to do with that specific marking.

5) As far as i know the DSCP-cos map is used for egress queueing only.

6) As 5.


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