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QoS Solutions for 2960X L2 Switches LAN Based

I have 2 PBX systems, both running with analog and digital phones. They are separated with a series of L2 switches. The PBXs have a VoIP NIC which connects to the network at access VLAN 10. It runs at SIP and RTP voice protocols. Phone end points from PBX1 and PBX2 can call each other during the testing, however there are times that they cannot. Is it possible this is a QoS problem?


Voice traffic is not the only traffic existing in the network.


I have applied QoS on the access ports connecting the PBXs, and on the trunks that connects the L2 switches. These are my QoS configurations on the interfaces:


(config-if)#mls qos trust dscp

(config-if)#priority-queue out


I found out that I forgot to enable mls qos globally, I am just wondering if this is the cause of the PBXs phone end points  sometimes cannot call each other?

VIP Advisor

Re: QoS Solutions for 2960X L2 Switches LAN Based

Is pbx1 connected to 2 via a wan cconnection or lan? What exactly happens when it doesnt work?  I dont think its very likely a qos problem. Do you ever suffer from voice bekng broken up or echo on the call?

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Re: QoS Solutions for 2960X L2 Switches LAN Based

PBX1 and PBX2 are connected via LAN (a series of L2 Switches, 3 switches to be exact). They are on the same subnet and VLAN. No experiene of drop calls so far, voice not breaking down, no echo too.


What it experiences is sometimes its ok (phones of PBX1 could call phones of PBX2 and vice versa). But with no specific time and/or time frame evident yet, below scenarios happen:

1. Phones of PBX1 could call phones of PBX2, but phones of PBX2 could not call phones of PBX1 (one-way)

2. Sometimes, the vice versa of 1 happens

3. Sometimes, both ways PBX1-to-PBX2 and PBX2-to-PBX1 cannot call each other.


A System Alarm on the PBXs (NEC SV9100) is found which says IP Collision Errors (which from the programming manual is an IP Conflict).


I checked also the #show interfaces gigabitethernet 1/0/48 ---> interface connecting PBX2 and found collisions and late collisions. By forcing the interface to have speed/duplex=1000Mbps/Full, and also forcing SV9100s VoIP NIC to have 1000Mbps/Full, collisions and late collisions on the interface stopped. But IP Collision Errors on the SV9100 PBX2 is still occurring. 


And even though the IP Collision Error Alarms on the SV9100 are occurring almost every 1-2 minutes on the logs, the calls work most of the time. But as I have said, the fault has no specific time/time frame evident yet when it occurs.


PBX2 was added recently on the network, including the switch it is connected. It is an expansion to a new and different building. The new switch is connected via fiber optic cable to the existing switch on the old and existing building. So far, only voice (PBX) and data (PC end points) are connected on the new switch. They are on different subnet/VLANs.



VIP Advisor

Re: QoS Solutions for 2960X L2 Switches LAN Based

It sounds a bit like you have another device on your network (at the times you are having your issues) that has the same IP address as one of your PABXes.   Perhaps as a troubleshooting step, you could change the IP addresses of both of the PABXes to new ones that you know are not being used anywhere else.

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Cisco Employee

Re: QoS Solutions for 2960X L2 Switches LAN Based

Sounds like a routing issue to me.
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