“Double” DHCP Advanced Router Settings: Is it an issue?

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[Mike]

I decided to go with a new post on this one about DHCP and my odd setup.

Equipment and Setup

1. I have a T-Mobile Sagemcom Fast 5688W Gateway gateway (a dumbed down all-in-one modem/router combo).   This gateway doesn’t have a true guest account feature and it’s just a name for sharing the main account.  There is little you can do with the settings.
2. To increase security, and have a guest account, I decided to purchase a TP-Link router (AX55).
3. I hooked an ethernet cable from the Gateway’s LAN RJ45 port into the TP-Link router’s WAN RJ45 port.
4. Back at the Gateway, I was able to turn off the WiFi and just use the TP-Link’s WiFi.
5. Everything seemed to be working just fine for net connectivity.

Then I read somewhere that if both the Gateway and Router where “serving” DHCP IP, I might get conflicts.

1.  Since I can’t turn off the Gateway’s DHCP, I turned it off in the Router’s settings.
2.  After reboots for the Gateway, Router and all devices…….I found that most of the devices would not connect to the net.

I just barely know what’s going on, but something seems odd to me.

1.  If the DHCP server was turned off at the Modem, normally wouldn’t the DHCP IP’s from the Gateway just pass through the Modem?
2.  If I turn off the DHCP server at the Modem, are there some other settings I need to do?
3.  If everything is working with supposed “Double DHCP” maybe I don’t do anything?
4.  Is there a way to test to determine what is really going on?

I appreciate any ideas or help.

Mike

References

SagemCom Gateway:  https://www.t-mobile.com/support/public-files/attachments/hint-gateway/Sagemcom%205G%20Gateway%20User%20Guide_English.pdf

Archer AX55 Router: https://www.tp-link.com/us/home-networking/wifi-router/archer-ax55/#specifications

• This topic was modified 2 years, 2 months ago by Mike.
• This topic was modified 2 years, 2 months ago by Mike. Reason: Fixed Link

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Here’s a graphic  representation of a network with one Modem and one Router. In your case, they are combined. But here’s how it works.

You ISP issues you ONE IP address to your Modem (in my case 68.11.213.90. You have several devices. You cannot have multiple devices using the SAME IP address, so the Router uses the one address that your ISP provides on the WAN side using the ISPs assigned series of addresses.

What the Router does is use a DIFFERENT series of IP addresses (192.168.1.1 to 192.168.1.254 = DHCP = dynamic addressing) on the LAN side to provide multiple addresses for the multiple devices. It also keeps up with which LOCAL device has which IP address so it can ROUTE the data to and from the Internet to the correct device. The router provides the GATEWAY for the data traffic through the ONE IP address provided by the ISP.

You complicate matters by adding a second Router. If you take one of the wired devices in the image and use it duplicate the router’s function, you create another series of IP addresses and that is the “double DHCP” that is being referred to.

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Mike

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Then I read somewhere that if both the Gateway and Router where “serving” DHCP IP, I might get conflicts.

• – – – – –

This is true. It depends on the LAN side subnet created by each device (your TMobile gateway and your tplink router).

If they both use 192.168.1.x (for example) you are in trouble.
If one uses 192.168.1.x and the other uses 192.168.2.x all is well.
In and of itself, double DHCP is not a problem.

Look in the tplink router. What it sees as the WAN IP address tells you the subnet used by the T-Mobile gateway.

 

Get up to speed on router security at RouterSecurity.org and Defensive Computing at DefensiveComputingChecklist.com

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Mike

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Look in the tplink router. What it sees as the WAN IP address tells you the subnet used by the T-Mobile gateway.

So in my case:

T-Mobile Gateway=192.168.12.1

TP Link Router=192.168.01 (this is also what ipconfig reports)

(see details on next post  IP Settings in AX55 TP Link Router)

So if those IP’s don’t change ?   then I should be okay.  I’m still baffled by “if I turn off DHCP server on the TP-Link router then devices lose connectivity.

Thanks PKCano for the great visual help with a summary of the issue.

Mike

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IP Settings in AX55 TP Link Router

Internet Connection Type:  Dynamic IP
IP Address: 192.168.12.147
Subnet Mask: 255.255.255.0
Default Gateway: 192.168.12.1
Primary DNS: 192.168.12.1
Secondary DNS: 0.0.0.0

LAN
MAC Address: 6C-5A-B0-8E-29-1C
IP Address: 192.168.0.1
Subnet Mask: 255.255.255.0

DHCP Server: Enabled
IP Address Pool: 192.168.0.2-192.168.0.253

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Do not disable DCHP on anything.
On your new router this provides the main and guest access.

Using the same IP range should not be an issue because the router will manage the routes (it’s called a router for a reason).
As you have discovered, plug it in and go.

The only issue that may arise is if you game over the internet, because you have Double NAT (from the 2 routers). Game makers know about this so it should not be a problem.

cheers, Paul

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