2 dhcp on network and voice and data vlan

[John Cheetley]

Hi Cisco engineers,

Am aware that you can't have 2 DHCP's on network for varied reasons including allocating of wrong IP addresses being picked up, but when you have data and voice vlan, and the 2 DHCP ranges associated to this,how can you "null" 1 dhcp network range( data) to just use the voice ip range to be sent to the phones?

If this discussion is in the wrong section, please let me know. If this question has been asked already, please send me the link

Thanks in advance again.

[mattjones03]

Hi,

Just to clarify;

Do you have a voice and data VLAN that you would like to establish a dedicated DHCP range within.

I assume at this stage, the phones are in the voice VLAN and other devices (I.e. PC's) are in the data VLAN?

[pwwiddicombe]

There's nothing wrong with having 2 DHCP servers if you have thought it out - in fact, this can be a partial redundancy if one fails.  Just make sure the scopes either don't overlap, or that you have excluded the desired range of the "other" DHCP.

for instance, if you have 192.168.1.0/24, you could allocate .2 through .100 for one, and .101 through .200 for the other one.  The workstation will pick one (typically the first that replies) and confirms that one.

Other alternative is to allocate .2 thru .200 for both, but to EXCLUDE .2 through .100 on one and .101 thru .200 on the other.  This means that, if one fails and you run out of addresses, you can "delete" the exclusion on the other one easily.

for voice and workstations, they are typically on different vlans; and you have 2 scopes on the dhcp server in the desired subnet corresponding to the address ranges of each vlan.

[pwwiddicombe]

You will need to have multiple vlans if you expect voice and data to have different address ranges; and then you probably need to have routing configured on the switch that both are attached to. 

You only need a helper address if the DHCP server isn't on the subnet the device is on - helper makes sure the request on another VLAN can make it to the DHCP server, who THEN has to have a scope for each subnet it will be expected to issue leases for.

[John Cheetley]

Then I would need ip helper..but devices are getting 2 ip addresses each...from data and voice associated DHCP's.. I presume I can exclude all IP's in the Data DHCP and just use voice DHCP..?

[mattjones03]

Hi John

What handsets do you have?

[pwwiddicombe]

Off the top of my head...

ip routing

int vlan 10

desc data vlan

ip address 192.168.10.1  255.255.255.0

int valn 20

desc voice vlan

ip address 192.168.20.1 255.255.255.0

ip helper-address 192.168.10.5

int range fast 1/1-24

switchport

switchport access vlan 10

switchport voice vlan 20

So IF you have a cisco switch and IP phones, then a phone plugged into the port will be put into vlan 20, and send off a DHCP request to the DHCP server at 192.168.10.5, and will get an IP address in the range 192.168.20.x; assuming that range has been defined on the DHCP server.  A PC plugged into the data port on the cisco phone (or plugged directly into the port) will get an address in vlan10, which I have assumed here is the vlan the DHCP server sits in, and has a scope for 192.168.10.x.  Because the DHCP server is in the data vlan, no helper address is necessary for VLAN 10.

[John Cheetley]

HI pwwiddicombe,

I never heard of EXCLUDE..

[John Cheetley]

Hi pwwiddicombe, 

Sorry for taking so long to get back to you.

Some how I have the command " no ip dhcp use vrf connected" atop of the ip address range excluded from being used.

I have tried to delete but system tells me no vrf is connected.

This is related to the initial DHCP issue.

[John Cheetley]

Hi mattjones03,

I have a voice and data vlan setup.

Phones are trying to get ip address range but getting 2 (2.xx and 1.xx)

I assume i need to use ip helper-address to tell the system which range to use to allocate ip correctly?

[paul driver]

Hello

It can also depend on how you've setup your dhcp scopes, Do you have multiple scopes or just one large range to accommodate both subnets?

You'll only require helper addresses if the dhcp server in not on the same network as the clients.

res
Paul

[mattjones03]

Hi John,

Thanks for the clarification.

Keen to understand the handsets you are using as this can sometimes depend on the configuration used.