I'm new ro QOS, and is implementing a IP telephony solution for a customer. We have got a Catalyst 4510, where the phones are connected, and then the PC's are connected to the phones.
I have configured the "auto qos voip ciscophone" on the interfaces connected to the Cisco phones (7941's). However, we are experiencing a big performance descrease on the PC's (about 1/2 or 1/3 the performance or so), when the "auto qos voip ciscophone" is configured on the interfaces. When I remove the auto qos from the interfaces, everything works at normal speed.
Is it a bad idea to configure the "auto qos voip ciscophone" on the interfaces, when the PC's are connected to the phones?
Essentially there are four egress or TX queues per interface, and in this particular instance the transmit queue 1 is the queue for all other traffic which has not been marked or classified.
Therefore in order to guarantee the minimum bandwidth for tx-queue 3 (33 percent) which is the strict priority queue for RTP, then queues will be serviced accordingly. This means that packets in TX queue 1 will be the first to be dropped as you have seen.
The fact the minimum bandwidth for the tx-queue 3 is 1/3 of the interface would seem some what excessive, thus starving other queues of bandwidth. Similarly the shaping or maximum bandwidth for queue 3 is also set to 1/3 and I would not expect voice traffic to peak 300 mbps on a gigabit interface.
Typically queue 3 is for VoIP and queue 4 is used for signalling,routing, BPDU updates etc, this obviously depends on your dscp mappings.
Depending on your traffic profile, which seems that your traffic typically falls into queue 1, then I would divide the bandwidth accordingly.
I would recommend initially adjusting the tx-queues as follows, and monitor the situation. You can always adjust the minimum and shaped maximum levels if you feel that voice quality is being comprised:-
bandwidth 500 mbps
bandwidth 200 mbps
bandwidth 50 mbps
shape 50 mbps
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