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Different Subnets on same Switch

Posted on 2010-08-23
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Last Modified: 2012-05-10
Hello All.

I have a client who has a VoIP phone system on the same subnet as all computers. Whenever music on hold is playing, the network slows to a crawl. The problem i am running into, is that there is one cat 5 jack in each office. Running off that jack is the phone as well as the computer. What would be the best way to configure a seperate subnet for the phones which would allow them to talk to the PBX (which is on the same subnet, but can/will be chagned to the new one).

Thanks
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Question by:jmbowles01
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by:ValleyENT
ID: 33504901
Well, we are doing something similar, we have a phone plugged into the wall and a computer plugged into the phone. You will need to find out which DHCP scope options are needed to set your specific phones IP addressing and configure your DHCP servers scope to match. We actually use the phone controller to control DHCP addressing for the phones, and my DHCP server for the computer/server systems.
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by:Dynamite-Web
ID: 33504923
Assuming that these network devices are cisco, you should be able to implement the auxiliary vlan feature so that the phones could be put on a different vlan than the PCs.  In that case, you'd want to re-ip the phones to another subnet and that should eliminate the problem you're having.

Some info on aux vlan w/ cisco:
http://www.cisco.com/en/US/docs/switches/lan/catalyst3550/software/release/12.1_19_ea1/configuration/guide/swvoip.html

If they are not cisco devices capable of aux vlan, then we'd have to look at other options.
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Jimmy Larsson, CISSP, CEH earned 2000 total points
ID: 33504933
Does your phones support 802.1q-trunks? If so you can run voip and data on 2 different vlans, separated from eachother but in the same cable. VoIP-systems like Cisco can do this because the phone contains a small switch so that the computer can be jacked into the phone that is jacked into the outlet. The switch then runs a 802.1q-trunk with voice+data vlan where quality of service is configured so that voice-traffic is prioritized.

/Kvistofta
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by:Dave_LaSalle
ID: 33505177
Having them on the same subnet is ok if your network switch(s) support either Class of Service sometimes referred to as Type of Service if older - or Quality of Service.  The phones always tag the data and the switch prioritizes it.  May have to enable it in a managed swtich.  After that it's a matter of bandwidth.  They're not by chance running off of hubs???  Traffic would be sent (copied) to every port.
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by:Jimmy Larsson, CISSP, CEH
ID: 33505237
Still there is a need for the phones to have built-in switches (or individual switches per room) since there is only one outlet per room.

Of course you can run on the same vlan, but it is definately good design and best practice to keep them in separate broadcast domains.

/kvistofta
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by:petelettin
ID: 33506021
voip traffic is unicast and therefore should only affect the 2 end devices not slow the whole network down.
voip traffic is only a few hundred kbits per second, a single switch should be able to cope with several connections without any slow down.

QOS - Quality of Service; usually give priority to voice traffic over data the QOS settings are usually set by the phone.

QOS settings on the PC can be altered by the phone it connects through and therefore may not have any effect when changed.

Having seperate vlans will not help if the switch is struggling to cope with the load. All the traffic has to be processed by the switch.

Music on hold is sent from the PBX server

The PBX is usually only involved in setup and tear down of calls the voice traffic is from phone to phone unless the PBX is also the gateway for external calls in which case it also acts as a router.


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