DHCP; IP Helper-address; CME; GW router

Answered Question
Jun 17th, 2008
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We are having issues with our phones not pulling down DHCP info. My question is this. On a single campus network (all behind one router), do i need the ip helper-address command. If so, do I apply it on the sub-interface of the router, or the virtual interface of the switch, or both. Also, does my switch need a virtual interface at all.


The DHCP pool is located on the GW router.


If I manually configure the IP information on the phone, it will come up. I then can go back to DHCP and the phone works fine.

Correct Answer by paolo bevilacqua about 8 years 10 months ago

Hi,


your config is fine. Can you check with "debug ip dhcp server packet" and "term mon", what is going on. Also, which phones are you using and which FW on the.

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thisisshanky Tue, 06/17/2008 - 10:22
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Helper is only required if your DHCP server is not in same vlan as the end devices are.If you have DHCP set on a Cisco IOS router or switch that is directly connected to the end devices or if the DHCP server (MS,third party etc), then you dont need helper.


In your case its the latter. Is the router act like a "router on a stick" config ? How many sub ints do you have ? Do you have a DHCP pool configured for each sub interface/subnet?

dphills18 Tue, 06/17/2008 - 10:40
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That's what I was thinking. That's why it's so confusing. I do have multiple subinterfaces, but i am only doing DHCP for the one VoIP network. I posted a modified config below of what I have on the router.


interface GigabitEthernet0/0.123

description ***Primary VoIP Interface***

encapsulation dot1Q 3

ip address 192.168.123.1 255.255.255.0

ip broadcast-address 192.1168.123.255

no snmp trap link-status

h323-gateway voip interface

h323-gateway voip bind srcaddr 192.168.123.1

!

!

!

ip cef

no ip dhcp use vrf connected

ip dhcp excluded-address 192.168.123.1

!

ip dhcp pool VoIP

network 192.168.123.0 255.255.255.0

option 150 ip 192.168.123.1

default-router 192.168.123.1

For the switch, I have no virtual interface referencing the VoIP network. I do have the VLAN and the switchport voice command under the interface where the phone resides.


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paolo bevilacqua Tue, 06/17/2008 - 10:45
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Hi,


your config is fine. Can you check with "debug ip dhcp server packet" and "term mon", what is going on. Also, which phones are you using and which FW on the.

dphills18 Wed, 06/18/2008 - 06:21
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okay. i got it to work by going ahead and inserting the ip helper-address. i wanted to try that first, because i didn't want to risk the debug bringing down the router and/or switch. i figured the ip helper-address couldn't harm anything.


all and all, i still can't understand why i needed to put it in. however, it is working right now. i will investigate it further, because it is really going to bug me.


thanks for your help.

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