DHCP - stacked catalyst 3750's

Unanswered Question
Mar 8th, 2007

i know this is quite possible but i am unsure if the switches i will be using can do this alone.

ive got IP endpoints that will be piggy backed to PC's at the desks. they will both be on seperate VLAN's. i am curious if they can pull addresses from two different DHCP scopes.

i can code the VLAN tag into the phone and am wondering if that will be sufficient to pull the address from the second scope, and if the PC's without the tag will pull from the first. thanks in advance.

I have this problem too.
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allenelson Thu, 03/08/2007 - 20:29

i forgot to mention, the phones being piggy backed to the pc's means all switch ports are going to be trunk ports.

yimkwokchung Thu, 03/08/2007 - 22:41

Hi Allen,

The phones is Cisco IP Phone? If yes, you can config switchport as access port and use voice vlan command. The IP Phone will use the voice vlan and the PC will use the data vlan.


interface FastEthernet1/0/1

switchport access vlan 10

switchport mode access

switchport voice vlan 20

For DHCP scope issue, you can define two dhcp scope in the c3750 switches, c3750 will lease different ip address to PC and IP Phone according to different vlan setting.


ip dhcp pool voice



option 150 ip

ip dhcp pool data



allenelson Fri, 03/09/2007 - 07:13

ah ok. i didn't know you could configure two VLAN's using an access port. much appreciated, i'll test it out as soon as i get to stage the devices. thanks again.

allenelson Fri, 03/09/2007 - 07:15

er, i'm sorry it's too early to think. the phones aren't cisco end points, they're inter-tel's. but they support almost every VLAN feature an end point can, so i don't think that'll be an issue.. once again i'll test it out soon and use the good advice given. ;)

cratejockey Mon, 03/12/2007 - 05:17

Your switch should support two vlans per interface onc being a data VLAN and the other tagged as VOICE VLAN#. We have done this in quite a few places. The one thing I can't help out with is the non-Cisco phone site. Like the other poster stated Cisco Switches do support this.

Good Luck.


frederic.nilsso... Fri, 03/16/2007 - 12:55

Hello, if the phone supports that you choose an VLAN for the phone and another VLAN for the PC switchport on the IPT, it should logical make that PC-port an untagged port and then the phone itself would tag the packet with the VLAN tag of our choice. And then you have a trunk connecting the phone to the swtich. Then you are back to assigning the DHCP scope on the switch or using an IP-helper on the router to point at an microsoft/other brand server.


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