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DHCP, trunks, vlans, and VoIP


I did not know exactly which forum to post this. I had a couple of problems last week with a cut over to a new network design. This design was supposed to logically segregate VoIP infrastructure from DATA. This means for example Vlan 10 is the DATA Vlan and Vlan 100 is the VoIP Vlan. There are no Layer 3 devices only Layer 2 devices. I have 3560E switches servicing Cisco IP phones with the built in switchport that services Windows PC's. Some ports on these switches are configured for endpoints that have both phones and PCs using trunking while others are configured for just one using switchport access vlanning. The VoIP Vlan and the Data Vlan both have a DHCP server respectively. However, when I brought up this network, the PC's and phones were having trouble getting DHCP IP addresses or correctly getting IP addresses. Especially there were problems on the Trunk ports where both a VoIP device and a DATA device were trying to get to their respective DHCP servers. As a result, I had phones get DATA IP addresses and PC's getting phone IP addreses. Some device did not even get IP addresses. I had to move to one DHCP server with 2 scopes, but this does not segregate my VOIP VLAN from my DATA VLAN as I would like. Question, Does DHCP initial broadcast from the phone (which is the trunk end point) broadcasted on both VLANs? This is what it appears to be doing, even though I have the command "switchport voice vlan 100". Is there a something I'm missing with either DHCP, trunking, or the VoIP Phones?

Any suggestions would be very appreciated!

Community Member

Re: DHCP, trunks, vlans, and VoIP

can you post your 3560 switch config.


Re: DHCP, trunks, vlans, and VoIP

Well, from your post, it seems you're trunking with your phones.

You really don't need to do that, fyi. All you have to do is specify the VLAN for voice and access, and you'll be good to go.

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