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Community Member

837 and DHCP

I am using SDM on my 837, which is configured with static IP from the telco. We have 5 statics. I want to use DHCP for the rest of the computers on my LAN. I am trying to configure DHCP with a different subnet from my statics. The only interface I can use is a loopback interface that I create with 192.168.1.0/255.255.255.0. It won't work, so I don't think a loopback interface will do this. Can I set up the 837 as a DHCP server for a different subnet, or do I have to use DHCP Relay? Thanks in advance. Jim

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Accepted Solutions
Community Member

Re: 837 and DHCP

Have you configured Network Address Translation (NAT)? The private addresses you are using through DHCP will need to be translated into public addresses using NAT. I am fairly sure that the SDM lets you configure NAT. If not, you can always configure NAT by telneting to the router.

Here is an example on how to configure NAT: http://www.cisco.com/en/US/products/sw/iosswrel/ps1839/products_feature_guide09186a0080087bae.html

Hope that helps.

Andy

7 REPLIES
Community Member

Re: 837 and DHCP

Can you not telnet to the router and add a secondary address to the inside interface?

For example:

interface FastEthernet0/0

ip address 2.2.2.2 255.255.255.0 secondary

ip address 1.1.1.1 255.255.255.0

DHCP does work on secondary interfaces I think.

I don't know if the SDM gives the capability to set up a secondary address or not.

Andy

Community Member

Re: 837 and DHCP

Thanks for the help, Andy.

1) I set up a secondary IP address on Ethernet0. I was unable to use 172.16.0.0 because it would tell me 255.255.255.0 (or anything else) was an invalid subnet. I used 172.16.0.1.

2) I configured DHCP. My running config is this:

ip dhcp pool mms

import all

network 172.16.0.0 255.255.255.0

dns-server 111.111.111.11 111.111.111.13

default-router 222.222.222.81 255.255.255.248

lease infinite

!

!

3) DHCP is working, all the settings in the client computer appear to be correct, but the client computer is unable to get Internet. Internet is all OK, my statics work fine.

Any thoughts?

Thanks again,

Jim

Community Member

Re: 837 and DHCP

Have you configured Network Address Translation (NAT)? The private addresses you are using through DHCP will need to be translated into public addresses using NAT. I am fairly sure that the SDM lets you configure NAT. If not, you can always configure NAT by telneting to the router.

Here is an example on how to configure NAT: http://www.cisco.com/en/US/products/sw/iosswrel/ps1839/products_feature_guide09186a0080087bae.html

Hope that helps.

Andy

Community Member

Re: 837 and DHCP

Hi Andy,

I've got NAT going, outside access to my servers, and my inside subnet going. Thanks for the help. Another important thing - I was at early version of SDM and updated to 2.2. That added important features including NAT wizard, which helped.

I still have an issue, though. I cannot hit my servers from an inside computer unless I replace the domain name in the address with the inside IP address of the server. This is a problem because we have a lot of stuff built around the domain name that we use inside and customers use outside. I've tried various approaches in NAT but I can't resolve it.

Any further thoughts?

Thanks again for your help.

Jim

Re: 837 and DHCP

Hi

As per the supporting doc if you configure secondary ip address and a pool associated with that secondary ip address it will allocate from that secondary pool once the primary pool is exhausted.

"If the client is directly connected (the giaddr field is zero), the DHCP Server matches the DHCPDISCOVER with DHCP pool(s) that contain the subnet(s) configured on the receiving interface.

If the interface has secondary IP addresses, the subnets associated with the secondary IP addresses are examined for possible allocation only after the subnet associated with the primary IP address (on the interface) is exhausted. "

regds

Community Member

Re: 837 and DHCP

Thanks for the reply, spremkumar. I've been working on it over the weekend. Please see my post in reply to Andy. I made progress -- have DHCP working -- but cannot get to the Internet on the 172.16.0.0 subnet. Note that the secondary IP address I have entered is 172.16.0.1 -- it wouldn't let me use 172.16.0.0 as the secondary. When I set the network parameter for DHCP server, it goes to .0 automatically.

I'll appreaciate any further thoughts!

Thanks again,

Jim

Re: 837 and DHCP

You cannot use 172.16.0.0/24 as your secondary address on the interface because it's the network address. First available address for use in this subnet is 172.16.0.1/24 and you are already using that address.

Change the default-router ip address under your dhcp pool mms to 172.16.0.1. This is the gateway your dhcp clients will use to get to any address not local to their subnet.

Ofcourse, you have to configure this interface as your ip nat inside interface and the interface to your ISP as your ip nat outside interface. Create a nat pool and use the public IP assigned by your ISP as your PAT address for the dhcp clients.

Good Luck!!

HTH,

Sundar

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