Multiple subnets on a single Voice VLAN

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Aug 2nd, 2007
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Due to a solution that has been given to us. We are left with looking at configuring multiple subnets on a single voice vlan. This goes against best practice but has anybody actually configured this in the past?

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eoinwhite Fri, 06/17/2011 - 07:56
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Found this old post while googling. Also interested to hear of anyone's experience of this. I might need to do this temporarily rather then supernetting.

How do we know what phone will get what IP address if we have two scopes on the DHCP server.

thisisshanky Fri, 06/17/2011 - 10:44
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whats the reason for doing this ? do you need more than 254 addresses ?

Abu Hadee Fri, 06/17/2011 - 20:00
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Hi Rob

This is very common thing in our lab where we do configure different ip subnet on same VLAN. But it's not a good idea to configure the same on production network. More over, you need to configure static IP address as you can't have DHCP.



eoinwhite Sat, 06/18/2011 - 11:20
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Thanks for the replies,

As stated this was only to be a temporary measure, the general concensus is that its a bad idea and I uderstand why from an administrative point of view why this would be the case. Obviusly a one for one match with one subnet to one vlan is tidier.

But what actually are the downsides of this method, does it impact the voice in someway and why can I not use DHCP?



billybjo1 Sat, 06/18/2011 - 12:13
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You can have DHCP, but it may not work the way you want. If you use an ip-helper, that will only work for the primary subnet. If you have local DHCP servers in the actual same VLAN offering multiple subnets (i.e. server or router), they too are going to assume a request is the primary subnet (i.e same subnet they exist on). The only way you could possibly have some control over DHCP offer would be with static assignments.

Of course you can still have multiple subnets without DHCP. But you really are not going to want to configure static IP addresses on phones, I assume.


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