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What are bridge and passthrough modes?

When you set up a gateway at home, you may opt to enable a bridge or pass-through mode. Depending on how you want to configure your network, you may configure the gateway to bridge over passthrough. So, what is the bridge mode, and what is the pass-through mode?

Before we discuss bridge vs pass-through mode, we need to understand what gateway is. In our previous article, we discussed gateway vs router and the differences between the two. A gateway is a device your ISP provides to you so that you can connect to the Internet. A gateway is a modem and router combo, which connects ISPs WAN (wide area network) to your home.

  • A modem is a layer-2 device that converts the analog signal to digital (such as the cable modem and DSL) and delivers traffic to you without any filtering.
  • A router is a layer-3 device that examines destination IP address, and either passes on to the adjacent network or discard it if it isn't.

What is a bridge mode?

An ISP provides you with a gateway which is a bundle of modem and router, and there are times when a customer may want to use their own router instead of the router built into the gateway. To accomplish this, an ISP may configure the gateway to a "bridge" mode, and pass the traffic through the gateway without performing the routing functions. By enabling the bridge mode, we're essentially disabling the router function (layer 3) and make the gateway function as a modem (layer 2). Since you're making the gateway a layer 2 device, you cannot assign a static IP address to your gateway in bridge mode. The network before the bridged gateway and after the gateway is in the same network.

As you're not filtering any traffic, you cannot use NAT feature in a bridge mode. However, you may still use the DHCP server functionality provided in most routers. Also, because you're disabling NAT on your gateway, you won't have a double NAT issue with the home router you may have connected to your gateway. A double NAT occurs when you have two devices requiring to be in the same LAN, but belonging to two different subnetwork due to having two routers. This is one of the primary reasons why you may want to place the gateway in bridge mode to avoid double NAT issue.

Depending on the model of the gateway, you may be able to enable bridge mode yourself. Some ISPs choose to configure this themselves, and you may have to contact your ISP to make this change.

What is the IP pass-through mode?

IP pass-through works essentially the same as the bridged mode where customer can use their own router behind the ISP-provided gateway. However, in IP pass-through mode the signal is terminated (bridge mode signal is not terminated) at the gateway and allows the ISP to connect to the gateway with its own IP. The traffic will still pass-though the gateway, and the ISP-provided public IP address will be assigned to the customer's router.

Some gateway vendors like 2wire call IP passthrough as the DMZ Plus mode. This is because the entire network is set up as the passthrough to a DMZ node, and the untrusted Internet will be connected to a DMZ network.

Conclusion

The bridge mode and IP passthrough mode both provide similar functionality where entire traffic is pass-through the gateway and the public IP is assigned to the customer's router behind the gateway. The bridge mode does not terminate the traffic at the gateway while the IP passthrough does terminate the traffic at the gateway. In order for ISPs to connect to the gateway, the IP passthrough mode should be used instead of the bridge mode.

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Comments (1)

  • David Ward Reply

    Good explanation.

    Still, if the ISP is terminating at the gateway, then the data will have to be resent back into the pipeline to your router. I would think there would have to be some type of addressing recognition in order to handle this. It may not be NAT, but there has to be something to separate the data that the ISP doesnt want to go to the customer.

    My system is a netgear "ATT" LTE router (MR-5100) that uses IP passthrough to my ASUS router. No matter how i configure it, the IP passthrough has NAT issues that slow the connection. When I use the LTE router instead of my ASUS, the connection is much faster and doesnt have NAT issues. Your explanation is the best I have seen on the internet, but still I would like to understand technically exactly what they are doing to the datastream when put into IP passthrough mode. It seems to be significantly different than a bridged device.

    Nov 27, 2021 at 10:24 AM

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