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Re: QoS for voice traffic

 
Inge_Suzzy
Frequent Advisor

QoS for voice traffic

How can i configure QoS without using voice vlan? not all of my switches support it.
It is simple?


I only have 2 vlans, my ports are hybrid, vlan assigned for ip telephony is tagged and vlan for data is untagged.
Lately we have problems of dropping calls, i don't think it's a qos factor, but my client think that's the problem

 

the models that i have 4800G, 4500G

____________________________________
Ing. Angélica Susana Hernández Vázquez
System and Field Engineer
12 REPLIES 12
Michael A. McKenney
Respected Contributor

Re: QoS for voice traffic

You could put the phones on their own switch off a separate port of a router or firewall.  You don't need a highend switch.  A simple 1 Gbps switch would do since most voice traffic is 56kbps.  

manuel.bitzi
Trusted Contributor

Re: QoS for voice traffic

The best way is to priorize the whole vlan. i.e.:

 

traffic classifier MatchAny operator and
 if-match any
#
traffic behavior SetVeryHigh
 remark dscp cs6
 remark dot1p 7
 remark local-precedence 7
qos policy PrioVeryHigh
 classifier MatchAny behavior SetVeryHigh
#
qos vlan-policy PrioVeryHigh vlan 9 inbound

 

br

Manuel

H3CSE, MASE Network Infrastructure [2011], Switzerland
Inge_Suzzy
Frequent Advisor

Re: QoS for voice traffic

thank you, I will try that configuration

 

And last question, I did an investigation regarding this fact of qos, I found that switches has a default qos treatment, I captured some traffic of a mirrored port on a switch, to see if the packets were prioritized, and I see that all packets of telephony says 0xb8 DSCP 0x2e and all traffic of data says 0x00 DSCP 0x00 and search this and found that it's the correct treatment for an IP Telephony scheme.

 

My question is this, When is necessary implement some technic of qos?

 

Is it necessary when you have a segmented LAN, without restrictions of bandwidth or some policy of use?

____________________________________
Ing. Angélica Susana Hernández Vázquez
System and Field Engineer
Michael A. McKenney
Respected Contributor

Re: QoS for voice traffic

You use QoS on Voice, when the phone calls are choppy or dropping due to connectivity issues.  On slow internet connections, I usually put the IP phones on their own switch.   A inexpensive switch is less expensive than wasting time on QoS rules or configuring an isolated VLAN. 

manuel.bitzi
Trusted Contributor

Re: QoS for voice traffic

Hi Inge

 

the voip packets are marked with DSCP 46 by the phone, but the switch will not care about this. activate "qos trust dscp" on each switchport. that will help.

 

br

Manuel

H3CSE, MASE Network Infrastructure [2011], Switzerland
Inge_Suzzy
Frequent Advisor

Re: QoS for voice traffic

But on the manual says that, by default mode 4800G and 4500G has 802.1p precedence trust mode, for tagged packets, its 802.1p value is used to map to local and drop precedence and for untagged packets, the port priority is used to map to local and drop precedence and the por priority is the default.

 

And it's extrange for me that calls are drop even when there are no other traffic, I do that test on night,

 

And this command has to be configure and all switches and ports,on the LAN, is it?

 

 I know  qos it's  required for voip but if i have switches that work on giga the end devices work at giga and a call consume 80kbps, and the data transfer is also over kbps, and the lan  it's not rate limiting and the switches have a very good througput, i do not understand very well when to use qos, I think it could be the problem if also the end users access youtube, share autocad doc's or something that consume a great amount bandwidth, but that doesn't happen.

 

Even the simplest network has to had qos and not only vlan segmentation, it doesn't matter the switch capacity, is it?

 

Why if it is so necessary, it is not enable by default =s

 

____________________________________
Ing. Angélica Susana Hernández Vázquez
System and Field Engineer
Michael A. McKenney
Respected Contributor

Re: QoS for voice traffic

No traffic?  I bet a network scanner would find SQL, Email, DNS, and other traffic on your network.   On my A5800 switches, my 5 IP phones are in the trusted VLAN with my users and servers.  No issues.  Switch performance can handle all the traffic and not drop a packet. 

 

QoS and Voice VLAN are necessary because a drop packet on a computer is no issue.  It just resends.  A dropped packet on a phone is detected as a hang up.  At home, my Sprint Airave device for my cell phone has QoS priority 1 on my Dlink router so I don't drop my calls.  Airave is a mini cell tower that routes my calls over my internet connection back to a Sprint NOC. 

 

You have two choices. 

1.) Create a separate VLAN for phones only and put the network cable to your phones on this VLAN.  Keep it isolated from the other VLANs.  

2.) Install an inexpensive switch to handle the phones.

 

My phone vendor usually just puts in a separate switch for VIOP.  This way you just plug and play.  No need to program a VLAN or QoS.  It is isolated.   Any old switch would probably due. 

Vince-Whirlwind
Honored Contributor

Re: QoS for voice traffic

" calls are drop"

You mean the call ends? That's not a VoIP issue. VoIP uses a connectionless protocol so packets are fire-and-forget. If voice packets fail to reach the other end you get call quality issues but you do not get calls dropping.
If calls are dropping, that would indicate a signalling problem, and/or a problem with the call controller.

Vince-Whirlwind
Honored Contributor

Re: QoS for voice traffic


@michael a. McKenney wrote:

  A dropped packet on a phone is detected as a hang up.   


This is incorrect. A dropped voice packet has no effect whatsoever on call setup or tear-down.

Using a seperate switch is also not very good advice - some telephony providers do this because their knowledge of networking is very poor and they don't know how to manage two VLANs. The net effect is double the infrastrucutre, inefficient use of structured cabling, inefficient use of power and rack space.

The OP already has two VLANs, and the endpoints are already adding the correct DSCP value to the VLAN tags.
As per previous advice, just add the qos trust command to the interfaces.

Voice quality issues: check our your QoS.
Calls dropping: nothing to do with your network - flick the problem to the telephony provider.