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ISR 1941 as a DHCP client on Gi0/0

12 years 8 months ago #37594 by talk2sp
Dear Board members,

I setup a Cisco ISR1941 recently and prior to the network deployment the client told me that they will change their internet access module from 3G to something much more reasonable and that has a static IP config. After the deployment, the client insisted on the continuing usage of the 3G modem. Now knowing fully well that except I purchase the 3G interface card for the 1941, it will be impossible to use the 3G usb Modem on the 1941. In this same rack is a HP DL380G7 Proliant Server with 4intf and one iLo int. So I setup the usb 3G modem on the server and then shared the internet connection to one of the NIC and from the NIC a cable to Int Gi0/0 of the ISR router. But when I configure the router to receive its address automatically from the server, i wont be able to log onto the router again. For example. the router has 2Gi ports; Gi0/1 is for the DHCP which connects to the Gigabit port on the 2960 and provides all clients dhcp address. Gi0/0 is the router's WAN interface with IP now since I don't have a static IP I set the interface to auto but when i do that, I cant access the router either through telnet or through CCP. Only VIA serial.

My questions are:

What I am trying to achieve can it be possible?

How do I configure an ISP's link on a 1941? Do I just config the ISP's IP and DNS on Gi0/0?

Will all dhcp clients connected to int Gi0/1 be able to access the internet? Bcos I realized that from Gi0/1 (192.168.x.x) I can ping Gi0/0 with with reply.

I believe someone understands where I diving @.

I will appreciate a quick response.


c0de - 3
Take Responsibility! Don't let failures define you
12 years 8 months ago #37595 by Chris

Forgive me if I am being redundant, but I have not fully understand the current setup and what exactly you are trying to achieve.

So far, this is the setup I have understood:

Internet--3GUSB---HPServer---G0/0-ISR-G0/1--- LAN

You're trying to provide Internet access to the LAN - correct? You mention you can't telnet to the router, but from where? Do you mean you can't telnet to it via Internet or from G0/1's LAN ?

Is it possible to post a brief diagram and state exactly what problem you have?


Chris Partsenidis.
Founder & Editor-in-Chief
12 years 8 months ago #37596 by Arani
Yes. I think a schematics of the network layout with the interfaces indicated well, would help us understand the situation.


Picking pebbles on the shore of the networking ocean
12 years 8 months ago - 12 years 8 months ago #37597 by talk2sp

@Chris. The current setup is: ISR1941>Catalyst2960>LAN systems (and DL380G7 with static IP)
i.e. ISR1941 on port gi0/1 to 2960 on gigabit port.

Now i configured Gi0/1 as a dhcp server so that it supplies all LAN systems with an auto-ip.(working perfectly)

I was hoping that Gi0/0 will be the interface I will connect to the IDU (indoor Unit from the ISP), and configure a static ip on this interface. But the client changed his mind on using a bigger ISP cos of cost and decided to want to continue with the usage of his 3GUSB modem.

Before the Network rollout, the architecture was like this:
A PC which has the 3gUSB modem installed on it, the USB modem is shared to the NIC of the PC, the NIC to the internet port of a Linksys wireless N router (dhcp disabled) and from one of the four ports to a layer1.5 linksys switch.

Chris now that I am done with the Network roll out, this is what I want the network to look like:

Internet...IDU(Static IP)...(gi0/0)1941(gi0/1)...2960.....LAN

But the client insists on the usage of their 3G modem so this is how it will now look like:
3G...DL380(to do the internet sharing)...(gi0/0)1941(gi0/1)....2960.

The issues are as follows:
everything works fine from the 3G to the DL380 and to the 2960 (not involving the 1941) but if i do that, the purpose of the 1941 will be defeated. After I share the internet connection on the dl380 (running winserver 2008), from the interface it shared to, becomes ( as usual) Now from that interface, instead of plugging to the switch (so the server does DHCP, I plug to the router's Gi0/0 (with an existing IP of .. (Because I want the router to do the dhcp and as well make use of other routing features).

...........I hope you getting me..............

Using CiscoCP it sees the gi0/0 as the wan interface and specified that I should input the ip address from the service provider. But in this case there is no ip address since the 3G usb modem does not receive static ip, secondly the NIC to which the 3G was shared now has a static IP from which it is to serve other clients.

My challenge..

If I change the gi0/0 to receive its ip automatically, it does not get an ip from the server and worst of all i cant access the router via telnet or the CCP except via serial.
If i configure the interface with assuming I am placing a static dhcp on that interface, it still does not work, the systems connected to the other interface gi0/1 via 2960 will not be able to receive internet connection.

gi0/1 :
dhcp pool is from .10 to .50.

From any system lets say .11 i can ping that is interface gi0/0 but i cant telnet, and I cant use the ccp. Now since I can ping and get a reply that means if it is configured with the internet address, the client systems will be able to access the internet.

Chris I wish and hope you understood me at this point. From above you will realize that i have 2 major issues. First (which is more paramount) is the 3g to the server to the router issue and second, I cant access the gi0/1 interface which is (which i thought was possible) and also with this part of the problem, I cant access the gi0/0 web config (ccp) neither can I telnet to it whilst i am on 192.168.5.x.

Chris I took time to spell the issue out well. Please I will have sent in a diagram but presently I am on another project.

Thanks for your time.


c0de - 3
Take Responsibility! Don't let failures define you
Last edit: 12 years 8 months ago by talk2sp. Reason: typo error
12 years 8 months ago - 12 years 8 months ago #37598 by talk2sp
And may be to simplify the whole trouble, this is the setup I am trying to achieve:Internet--3GUSB---HPServer---G0/0-ISR-G0/1--- LAN.

May be you should just walk me through a new / proper configuration or something of sought.

Thanks Chris.


c0de - 3
Take Responsibility! Don't let failures define you
Last edit: 12 years 8 months ago by talk2sp. Reason: In addition
12 years 8 months ago - 12 years 8 months ago #37599 by Chris

I believe I understand your issue and it seems to be easily fixable.

Here's the final setup we are talking about:

3GUSB---HPServer---G0/0-ISR-G0/1--- LAN

1) Enable Internet sharing on the HPserver and check that its working.

2) On the NIC that connects the HP server with the G0/0 of your ISR router, connect a laptop and make the necessary network modifications to the laptop's nic so that you can confirm the internet sharing via the HP server is working. Note your laptop's settings.

3) Connect G0/0 of your ISR to the HP server. Configure the same settings you used on the laptop NIC, to the G0/0 interface.

4) From the ISR console, ping the HP server with a source IP of interface g0/0 to ensure the router ' see's ' the HP server. If the test passes, then perform the same test, but ping a public IP - logically, you should get a reply.

5) Configure NAT on your ISR. G0/0 is the 'outside nat', G0/1 is the 'inside nat'. Configure a static route on the ISR, so that it points to the HP server. See our Cisco Router NAT overload article for the commands. It's exactly the same scenario, but instead of a public dialer0 interface, you have gig0/0.

6) Configure your DHCP services on your ISR as needed. This is covered under the Cisco Routers section as well. All clients must use ISR G0/1 as their gateway.

You can check the NAT on your ISR by issuing the 'show ip nat translations' command.

Hope that helps - let us know of your progress.

Cheers mate,

p.s I suggested to use NAT on the ISR because I am not sure if your Internet Sharing will work properly on the HP server. If you don't want to use NAT on the ISR, you'll need to add a static router on your HP server so that it knows where the LAN ip network is (via ISR router) and also ensure Internet sharing is enabled for your LAN IP network.

Chris Partsenidis.
Founder & Editor-in-Chief
Last edit: 12 years 8 months ago by Chris.
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