QOS newbie question

Unanswered Question
Oct 28th, 2007


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?

I have this problem too.
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aamercado Sun, 10/28/2007 - 23:10

post the config on one of the interfaces

I believe duplex becomes hardcoded and if so you can auto qos on one of the interfaces and set the duplex back to auto.


rasmusan1 Sun, 10/28/2007 - 23:17

interface GigabitEthernet6/27

switchport mode access

switchport voice vlan 10

qos trust cos

qos trust device cisco-phone

auto qos voip cisco-phone

tx-queue 3

bandwidth percent 33

priority high

shape percent 33

spanning-tree portfast

service-policy output autoqos-voip-policy

aamercado Mon, 10/29/2007 - 10:27

On a "sh interface counters detail module" are you dropping frames on one of the qos queues?

rasmusan1 Mon, 10/29/2007 - 11:30

When I do a "sh interface counters detail", I have a lot of tx drops in the "Tx-Drops-Queue-1".

allan.thomas Mon, 10/29/2007 - 15:03

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:-

interface GigabitEthernet6/27

tx-queue 1

bandwidth 500 mbps

tx-queue 2

bandwidth 200 mbps

tx-queue 3

bandwidth 50 mbps

priority high

shape 50 mbps



Please remember to rate posts if you find this information helpful.

aamercado Mon, 10/29/2007 - 21:25

You can try upgrading the 7941 firmware to rule out any bugs


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