qos ports

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Nov 22nd, 2007
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Hi all, If I have access switches plugged into my core layer 3 switch, if I prioritize my traffic at the edge using vlan, what do I need to do on the port that connects to the layer 3 switch (uplink)

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royalblues Thu, 11/22/2007 - 03:24
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Make sure that you trust the QoS settings on the uplink ports. By default when QoS is enabled the device rewrites the QoS values

So if you are doing QoS prioritization using DSCP, then make sure you trust DSCP on the uplink port. Additionally you can also configure priority queuing on these interfaces as well based on the DSCP value



carl_townshend Thu, 11/22/2007 - 04:01
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what excactly are we trusting? packetes coming into the switch, or leaving the switch here ?

what happens also if we dont configure the priority queing, does it still prioritize? what happens when the bandwidth limit is reached without no priority queing

voiper_99 Fri, 11/23/2007 - 02:55
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Tursting the QoS means that your core switch will "trust" whatever packet markings your access layer switches have set. If you don't set your core switch to trust the DSCP values then it will re write/re classify the packets which will waste CPU and RAM resources and would defeat the purpose of marking the packets at the access layer in the first place.

Priority queuing is used to ensure that certain packets will be transmitted instantly, regardless of whether there are other packets in the queue or not. However, you need to be careful with priority queuing because if you prioritise a set of traffic that is bandwidth hungry then none of your other traffic will be able to be sent until your priority queue is completely empty.

Without priority queuing your packets can still be prioritised - I know that sounds confusing so I shall explain myself :)

Priority queuing is what I have explained above, prioritisation on the other hand is the practice of marking important packets with a better DSCP value and less important packets with a lesser DSCP value. In short, packets with a higher DSCP priority will be sent quicker and dropped less (in times of congestion) and packets with a loser DSCP value will be sent slower than that of the higher priority packets and will be dropped more often in times of congestion.

To answer your question about a bandwidth limit, this is where priority queuing (when used correctly) is a good idea. i.e if you have a slow WAN link and you want to use VoIP, you should use the priority queue to ensure a smooth VoIP experience.

Also, with protocols like CBWFQ you can dedicate DSCP values a certain amount of bandwidth to ensure they will not be bandwidth starved and also to ensure they do not hog the bandwidth either.

By the way, another thing I should mention is that it all depends on what sort of devices you are using for the QoS and what IOS version they are running. i.e some devices will support CBWFQ others will support WRR, and some may not support either, or may support both.

I hope this answers your questions.


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