DHCP Lease times and IP Phones

We currently have a relatively small network, 20 servers, 2 DC's. 1 DC is handing out DHCP. We are implementing IP phones. Since we do not have 2 data drops at each location the Workstation will be plugging in to the phones, and the phones plugging in to the network.

So the problem is phones need DHCP and workstations need DHCP. Once the phone attains a DHCP address from our DC it is then flagged as VLAN traffic and gets routed to the IP phone server at which point it gets a DHCP assigned address on a different network. So essentially phones need an address on our network when they first boot up, but then they are transferred to a secondary network.

I wanted to figure a way of handling this without running out of DHCP addresses if the power goes out. Basically i didnt want to have 230 devices looking for an address when only 200 are available in the DHCP scope.

That being said. I could set my lease time very low in the DHCP scope, but I am not sure what the consequences of that would be. Say a lease time of like 5 minutes. I am really wondering how much that will slow down our LAN.

Any recomendations?
Atomic75Asked:
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WMIFCommented:
what kind of equipment are you running?  brands, models, etc.  i have my users pcs plugged into our phones and have the switches with different vlans for the networks.  when my phones boot, the switch directs the voice traffic to the voice vlan, and when the pc comes up, it goes on the data lan.  the phones never get an address on the data lan.

i am running cisco equipment.  call manager, 3560 switches, 7961 phones.
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Atomic75Author Commented:
It is shoretel equipment, with Extreme 24 Port switches. We do not currently have the phones installed. I am just going through the planing stages right now. The technician who installed the switches is the one who told me the only way to do it was to have the phones get an address from existing network so the traffic could then be tagged as VLAN and get routed to the correct location.

Perhaps this is due to the way their equipment runs.
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WMIFCommented:
maybe someone else can jump in here with shoretel experience.  that doesnt make sense to me why it would have to jump onto the other network in order to get tagged with a different vlan.  

can you setup a test lab to see if what he says is true?  i have become very cautious about what consultants have told me.  many times they tell me completely wrong.
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KiyamKadirCommented:
Dear,
basically i would recommend you using a 2 hours lease time if your users are 20% of the time at the same location and 80% of the time roaming.

for the reverse i do recommend you 1 day but not longer.


DHCP Traffic is very low in terms of bandwidth but you should be careful of your server's OS and performance as we have faced many issues with MS Win 2k3 Enterprise where as the DHCP service gets hit when we receive more than 180 simultaneous requests for address renewal.

Also watch out for the performance of your IP PBX server because they have to register the new ip addresses and the mac and directory number association again.

i recommended 2 hours because i have 500 users roaming all time and there is no problem so far.

We are purely running Cisco and Juniper gear.

i hope that it helped and you will have success.

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KiyamKadirCommented:
if you are running cisco use the IP HELPER-ADDRESS command to convert all your BROADCAST style DHCP REQUESTS into UNICAST style requests to decrease bandwidth consumption on your network.

Also for FIXED users use 4 days of lease time and prevent using NO LIMITED LEASE time since they cause DB inconsistencies if the DHCP service might crash.

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Atomic75Author Commented:
Thanks for the help everyone
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