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IP Passthrough Best Practices

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Series 3: IP Passthrough Best Practices

Products Supported: AER3100, AER2100, AER1600, MBR1400v2, CBA850, IBR11x0, IBR6x0, IBR6x0B, IBR900. Click here to identify your router.

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This document is intended to assist users in configuring a Series 3 Cradlepoint in IP Passthrough Mode to act as a transparent bridge and provide the carrier’s IP address to an internal router, firewall (Cisco, Juniper, etc…) or computer.


Configuration Difficulty: Novice
  • Step 1: Log into the router's Setup Page. For help with logging in please click here for the Netcloud OS prior to 6.0, or here for post 6.0 NCOS.
  • Step 2: For Instructions on configuring the Cradlepoint for IP Passthrough, please consult this article for  pre 6.0 NCOS or this article for post 6.0 NCOS.
  • Step 3: Ensure the placement of the Cradlepoint gives the optimal signal strength and clarity. For more information on how to determine signal strength please consult this NCOS 6.0 article or this article for pre 6.0 NCOS. For more information on signal strength values, please consult this article.
  • Step 4: For 6.0 NCOS, verify the Internet connection is working by going to System > Diagnostics, then using the PING TEST and run the ping test to a URL (e.g.
    • For any NCOS prior to 6.0 navigate to SYSTEM SETTINGS > SYSTEM CONTROL to find the ping test.

Subnet Selection Mode

This option overrides the subnet mask that is assigned to the modem from the carrier. In some cases, the assigned static IP addresses are in the same network at multiple locations, or a subnet mask that would designate that the IP address is a non-usable network or broadcast address and cause the internal router to reject it. In these cases, it is necessary to force a different subnet mask.

  • Force 24 Subnet” – This is the recommended setting. This setting will alleviate the issue where an are assigned IP addresses in the same network at multiple locations, or an IP address and subnet mask combination that designates the IP address as a non-usable network or broadcast address and is rejected by an internal router.

  • Force 31 Subnet” – this setting is used when two IP addresses in the same network are connected to the same internal router, which would cause overlapping routes. This option also requires that the internal router supports the /31 subnet mask.

  • Custom Subnet” – this setting is used to specify a custom subnet, such as a /30 in a scenario where the internal router doesn't support the /31 subnet mask.

Ethernet Cycle Time

This option will disable the Ethernet ports on the Cradlepoint by the specified interval (10 seconds is default) when the internet IP address is obtained. This will usually cause the end device to request a new IP address because the physical interface goes down. When the Cradlepoint first boots up a private IP address will be handed out via DHCP (if enabled). For some devices it could be important to set the DHCP lease time to 2 minutes when receiving a dynamic IP address from the Carrier or ISP. If there is only one internet source with a static IP address that the Cradlepoint is receiving, this option can be disabled and then statically set the IP address and Gateway on the end device.

Note: Cisco devices usually will not release the IP address when the interface goes down and is important to lower the DHCP renewal interval as to properly obtain the correct IP from the Cradlepoint.


If after following the above instructions and are unable to maintain an internet connection, please review the following information.

Cradlepoint Routers operating in IP Passthrough mode utilize a MAC binding feature to restrict access to the passed through IP address to a single client device. When moving the ethernet cable between clients a power cycle is required for the Cradlepoint, to ensure full functionality is provided to the new client.

LTE spec defines the MTU at 1428. As a best practice try and avoid excessive fragmentation for efficient application data traffic flow. It is recommended to follow the MTU\MSS design guide to avoid fragmentation:

MTU/MSS Design Considerations

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Published Date: 06/03/2016



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