How do I set up IP Pass-through on a Sprint U770 USB Modem?

Summary:
Starting with Sprint’s U770 USB modem, the IP address from the Sprint network is not immediately available to connected devices (by default). The Sprint dynamic IP address can be passed through the modem as well as through a capable CradlePoint router to connected devices. To do so, the following configuration changes are required.Resolution:
Sprint U770 Modem Configuration:By default, the Sprint U770 modem does not pass-through the Sprint network-assigned IP address. To enable IP Pass-through (IPPT), follow these U770 navigation steps:

  1. From an internet browser, type http://192.168.0.1
  2. Login to the CradlePoint router administration configuration pages.
  3. Next, browse to http://192.168.10.1/ to access U770 configuration pages.
  4. Login as Administrator. You find the password field in the upper right of page. The U770 default password is “admin”.
  5. Click on the Settings tab.
  6. Click on the Advanced Settings menu option.
  7. Click on the Manual Configuration option.
  8. Click on the IPPT Configuration option.
  9. Check the “Enable IPPT” box then click Save.                                                                                User-added image


CradlePoint Router Configuration:

A few CradlePoint routers support IP Pass-through (IPPT), allowing the modem’s IP address to be passed directly through the CradlePoint device to the CradlePoint LAN Ethernet port. To setup the CradlePoint device for IPPT mode, follow these steps:

CradlePoint Series 3 routers:
Applies to the CradlePoint MBR1400, CBA750B, CBR400, CBR450.  Note: CBA750B only: Verify the IPPT hardware switch on the device is set to “1” (IPPT enabled).

  1. Browse to http://192.168.0.1  (This is the default CradlePoint administration address.)
  2. Log into the CradlePoint router administration page using the administration password.  (The default password is the last 8 digits of the MAC address.)
  3. Click on the Network Settings tab.
  4. Select WiFi / Local Networks from the drop-down list.
    • On the CBA750B, this is Local Networks.
  5. Select the LAN network port that is to be the recipient of the modem’s IP address. This is usually the Primary LAN, but may be one created by the router administrator.
  6. Click the Edit button to open the Local Network Editor.
    • MBR1400 only: On the Interfaces tab, modify the interfaces that will be part of this network. For IPPT, these interfaces must all be of type Ethernet.
  7. In the IPv4 Settings tab, modify the Routing Mode to IP Passthrough.
  8. Click the Submit button at the bottom of the Editor.                                                             User-added image
    • NOTE: Generally, no other modifications are needed.
  9. OPTIONAL:  When the router was configured to IP Passthrough mode, the Ethernet LAN port was reset – which should trigger a DHCP lease request by the upstream device. However, some upstream CPE routers or devices (such as cameras) connected to CradlePoint’s IPPT LAN port do not always detect that the Ethernet connection was dropped. They continue to hang on to the DHCP address previously assigned (which is not the IPPT address). To work around this, you may modify the DHCP Lease Time in the CradlePoint router so to force the upstream device to renew the IP address:
  1. From the Local Network Editor, click the DHCP tab.
  2. Adjust the Lease Time from 720 mins (default) to a smaller value.
    • NOTE: To confirm this setting addresses the issue, set the time to 1 minute. The IPPT address should be seen by the upstream device in this time. Once this action is confirmed, adjust the value to a time that’s more acceptable to the system environment.
  3. Additionally, the first time you configure the system for IPPT mode, you may need to reset the upstream device’s Ethernet port to release the previous DHCP lease.
CradlePoint Series 2 routers:
Applies to the CradlePoint CBA750
  1. First, verify the IP Passthru switch on the CBA750 is set to “1” (IP Passthrough enabled).
  2. Browse to http://192.168.0.1  (This is the default CradlePoint administration address.)
    • NOTE: this is the default CradlePoint administration address.
  3. Log into the CradlePoint router administration page using the administration password.
    • NOTE: the default password is the last 6 digits of the MAC address.
  4. In the BASIC tab, click on IP Passthru and verify the settings are accurate for your system.   User-added image
  5. OPTIONAL:  When the router was configured to IP Pass-through mode, the Ethernet LAN port was disconnected – which should trigger a DHCP lease request by the upstream device. However, some upstream CPE routers or devices (such as cameras) connected to CradlePoint’s IPPT LAN port do not always detect the Ethernet connection was dropped. They continue to hang on to the DHCP address previously assigned (which is not the IPPT address). To work around this, you may modify the DHCP Lease Time in the CradlePoint router to force the upstream device to renew the IP address:
  1. From the BASIC tab, click DHCP
  2. Adjust the Lease Time from 1440 mins (default) to a smaller value.
    • NOTE: To confirm this setting addresses the issue, set the time to 1 minute. The IPPT address should be seen by the upstream device in this time. Once this is confirmed, adjust the value to a time that’s more acceptable to the system environment.
  3. Additionally, the first time you configure the system for IPPT mode, you may need to reset the upstream device’s Ethernet port to release the previous DHCP lease.User-added image



FINAL NOTE

The Sprint U770 provides a DCHP address with a /30 subnet. Some Sprint customers mistakenly believe this allows both 192.168.10.2 and 192.168.10.3 addresses to be assigned. This is not true. The subnet mask is not large enough for two additional IP addresses. In Series 3 CradlePoint routers only (MBR1400, CBA750B), the CradlePoint router can be configured to override this issue by forcing a /24 subnet. This CradlePoint /24 subnet feature is available in version 4.3 or later.

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