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QoS Trunk question

Hi all,


i have a question regarding QoS on non-trunk links.

If i have two switches connected via a normal uplink (no trunk) and a frame is sent from one switch to another no CoS information is included in this frame as it is not a tagged frame, but how does the other switch apply QoS to this frame then. Will it use the ToS byte?

So if there is no trunk connection in the beginning, e.g. a PC is connected to an access port of the switch and the PC sends a data packet to the switch, the switch apllies QoS to this frame(if configured but in this case lets assume the switch applies a higher DSCP value to the packets on the port). So the first switch uses DSCP, as it cant use CoS, and switches the packet with high priority. But as soon as the packet is send to the other switch, again no trunk connection, how will the other switch handle the QoS part? Will it just use the ToS part?

And if so how does it work as i thought at ingress the switch would check the CoS value to see in which ingress queue to put the frame.

But with the above mentioned scenario there is no CoS value to check, but still the frame has QoS applied to it but only on L3 (DSCP).

Or can the switch check also the ToS part on an ingress port to know where to put it?


As you said, there will be no CoS to check on the port, as it is an access and the CoS is only included in the 802.1q frames.

If left as default, your switch will overwrite any DSCP/ToS value with 0, so make sure you use the mls qos trust dscp command.

You can also set the DSCP to CoS maps if you need to send the packet over a trunk with COS.


Thanks for your answer.

What i dont understand is, if you have two switches connected via a uplink connection (no trunk) and on the first switch you connect an ip phone for example and apply CoS on the ingress port, the frame travels through the switch with high priority but as soon as it leaves the switch on the uplink the CoS is lost. So if the second switch receives the frame it does not see any CoS and treats is as any other frame, even though there is still a ToS part. Or is the ToS part also lost? I dont think so.

So my question would be, does the second switch use the ToS part to prioritize the frame and when no CoS is received by the second switch does it apply QoS only after the frame was switched to the forwarding enginge as the port cant use the CoS to check which queue it should use.


Its hard to make clear what i mean but hope one can understand it.


Thanks for any help!