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Cisco 3750, VLAN, IP Phones, DHCP

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  • Cisco 3750, VLAN, IP Phones, DHCP

    Hello, This might be an "over the top" in questions, but I thought it would not hurt to ask. Have (3) Cisco 3750's that will be used for standard computer equipment and IP phones in a LAN. Was told needed to set up the IP phones on a separate VLAN than the rest of the equipment on the switches. Even thought some PC's will be connecting to the network thru a network port on the IP phone itself. All the PC's that connect thru the IP phone port use dhcp for their IP address and all the phones use a static IP. My concern is with the VLAN's on different IP subnets, what will be the proper way to set up the switchs so that the two vlan's can communicate. The equipment not on the vlan with the IP phones would be mainly file servers that will require authentication. I'm assuming that the dhcp serve would have to be on the same subnet as the IP phones, otherwise the PC's connected thru the IP phones would not receive an address. Can anyone tell me if my basic logic is correct or do I need to go back to the books. I appreciate any help. Thanks in advance,
    Tommy

  • #2
    Re: Cisco 3750, VLAN, IP Phones, DHCP

    Cisco's switches actually work wonders with voip phones. The voice vlan feature allows you to connect a computer to the port on the IP phone and have the computer be on the regular data vlan, thus keeping the two separate.

    interface f1/0/1
    switchport mode access
    switchport voice vlan x
    switchport access vlan x


    The switch/phone knows how to differentiate between phone traffic and computer traffic and will tag packets accordingly. Anything from the computer will be on it's typical data vlan and the computers can pull addresses and whatever else. All voice traffic from the phone will be on the voice vlan.

    I would also implement autoqos on the switchports. I'm pretty sure CDP must be enabled for this to work.

    in interface config mode:
    auto qos voip cisco-phone

    As far as your DHCP server goes....It can be anywhere in your network. For devices that aren't in the same subnet as the server, you just use the "ip helper-address x.x.x.x"" command to redirect DHCP requests to the server. I wouldn't statically assign anything. Everything should be DHCP. The server should have a scope for the phones and a scope for the computers, or multiple scopes. Addresses will be assigned correctly if you use the voice vlan command. Also....what is the default gateway for these switches? Do you already have a plan for how the switches uplinks will work?

    Hope this helps...

    If any of this doesn't make sense or you don't know how to configure the actual vlan interface let me know. Sometimes I suck at explaining...
    Last edited by kornface13; 30th December 2008, 20:17.
    -Tim

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    • #3
      Re: Cisco 3750, VLAN, IP Phones, DHCP

      Thank you for your reply. So, if I understand correctly, I can set the ip phone ports on the switch to a seperate vlan and the phone/switch will mark the packets coming thru from/to the data port on the phones differently for the pc's and allow the servers (file and dhcp) to comunicate thru the phone data port, even though they (PC & IP Phone) share wiring to communicate with the switch. Yes, I also plan to implement qos and I beleive you are correct about cdp. Thanks for pointing that out. I feel overwhelmed with all of the info I'm trying to learn about this and will continue with my reading. Thanks again for your help.
      Tommy

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      • #4
        Re: Cisco 3750, VLAN, IP Phones, DHCP

        Hey Tim , me again. For some reason did not see your whole reply when I read your post. The problem with the dhcp server is it is handle by a different agency that shares the lan and gateway with me. I was trying not to dump my phones into his dhcp server if I could help it. Was planning on using a static ip that matched the phone extension number so phone ext .803 would be ip x.x.x.03 and so on. Have less than 30 phones to keep up with so hope it would not be too big of a headache. All the pc's that connect thru the ip phones are laptops that travel. Did not want to try and set up my own dhcp server at this point in time. Maybe a project for 09. I might post back as I get deeprer into this. Thanks again.
        Tommy

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        • #5
          Re: Cisco 3750, VLAN, IP Phones, DHCP

          Yes, you are exactly right. Mostly....you don't set the port to a different vlan, you just add the "switchport voice vlan x" so it knows which vlan to use for the phone and which to use for the data. I think the phone actually does the vlan tagging, but I can't remember exactly how that works. Go give it a try. It works like a dream. This is exactly how we setup our network when we first rolled out voip phones.

          OK, well not having access to the DHCP server is a different deal. I now understand why you want to use static IPs. I just think it's additional management overhead that could be avoided. No big deal though.

          Don't forget to work some extensions for call parking, conferencing, and any shared lines or whatever. These won't really need IP addresses so it might make your IP plan a little different.
          -Tim

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