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VoIP QOS on a 4506 switch with Avaya PBX - need help.

Please help confirm that QOS for VoIP is setup correctly on my

4506 which my main Avaya PBX is connected to. It has a Sup II-Plus

IOS=12.2(31)SGA with a WS-X4524-GB-RJ45V line card.

My Avaya PBX Medpros and Clan cards will be connected at 100MB to the line card

and I expect to have upwards of 40MB of voice traffic (DSCP-46) going through

some of those ports. The Avaya PBX marks voice packets with DSCP-46 and

control packets with DSCP-24.

I've configed auto qos and trust dscp on all ports and this the result

along with a typical port config:

qos dbl

qos map dscp 24 25 26 27 28 29 30 31 to tx-queue 4

qos map dscp 32 33 34 35 36 37 38 39 to tx-queue 4

qos map cos 3 to dscp 26

qos map cos 5 to dscp 46


policy-map autoqos-voip-policy

class class-default


interface GigabitEthernet1/2

switchport access vlan 126

switchport mode access

qos trust dscp

auto qos voip trust

tx-queue 3

bandwidth percent 33

priority high

shape percent 33

service-policy output autoqos-voip-policy

Is this auto qos config adequate for what I need - to insure that upwards of

40MB of voice traffic (~400 G-711 calls on some of the ports) will be serviced

by the priority queue?

Community Member

Re: VoIP QOS on a 4506 switch with Avaya PBX - need help.

Re: VoIP QOS on a 4506 switch with Avaya PBX - need help.


There can be an issue because of the shape and bandwidth command (33%). The shape command is used to set an upper limit for the respective queue. From the link in the previous post:

"Strict Priority / Low Latency Queueing

You can configure transmit queue 3 on each port with higher priority using the priority high tx-queue configuration command in the interface configuration mode. When transmit queue 3 is configured with higher priority, packets in transmit queue 3 are scheduled ahead of packets in other queues.

When transmit queue 3 is configured at a higher priority, the packets are scheduled for transmission before the other transmit queues only if it has not met the allocated bandwidth sharing configuration. Any traffic that exceeds the configured shape rate will be queued and transmitted at the configured rate. If the burst of traffic, exceeds the size of the queue, packets will be dropped to maintain transmission at the configured shape rate."

So if you connect your PBX directly to the port at 100 Mbps speed, I would remove the shape command. On a PBX port I would not pose any limit to voice traffic.

Hope this helps!

Regards, Martin

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