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What’s a Network Bridge?

A network bridge reduces Ethernet collisions by dividing a network into segments and using a MAC address table to control where frames are forwarded. It operates at Layer 2, making forwarding decisions based on MAC addresses rather than broadcasting all traffic indiscriminately.

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About this video

Ethernet hubs broadcast every incoming frame out all active ports simultaneously, which means two frames arriving at the same time will collide. Both transmitting devices must then wait through a backoff algorithm before retransmitting. While the Ethernet standard accommodates a certain level of collisions, the problem scales with network size and activity, making collision reduction a practical necessity in environments with many communicating nodes. A network bridge was developed to address this limitation by introducing segmentation into what remains a single logical network. By dividing the network into two segments, a bridge ensures that a collision on one side has no effect on the other, and that devices on opposite segments cannot collide with each other at all. This segmentation alone significantly reduces the collision domain without requiring the network to be split into entirely separate networks. The bridge achieves intelligent traffic control through two core functions: learning and forwarding. During learning, the bridge observes incoming frames and records the source MAC address of each device along with the port on which that device was detected, building a MAC address table. During forwarding, the bridge consults that table when a frame arrives destined for a known MAC address. If the destination is on the same port the frame arrived on, the bridge does not forward it to the other segment. If the destination is on the opposite port, the bridge forwards the frame across the segment boundary to reach the correct device. Because both of these functions require the bridge to inspect Layer 2 frame data, it is classified as a Layer 2 device on the OSI model.

What you'll learn

What's covered

Network Bridges

Aligned to

CompTIA Network+
1.2 Compare and contrast networking appliances, applications, and functions.
2.2 Given a scenario, configure switching technologies and features.
1.1 Explain concepts related to the Open Systems Interconnection (OSI) reference model.
Cisco CCNA
1.13 Describe switching concepts
Cisco CCST Networking
4.5 Explain basic switching concepts

Key terms

Bridge
A network device that connects two or more network segments and filters traffic based on MAC addresses.
Collision Domain
A network segment where data packets can collide with each other when transmitted simultaneously.
Media Access Control
MAC
A sublayer of the Data Link layer that controls how devices on a network gain access to a medium and transmit data.
Data Link Layer
Layer 2 of the OSI model responsible for node-to-node data transfer and error detection on a single network segment.
MAC Address Table
A table maintained by a switch (also called the CAM table) that maps source MAC addresses to the switch ports on which those devices were learned. When a frame arrives, the switch looks up the destination MAC in this table to forward it out only the correct port rather than flooding all ports.
Frame
The unit of data used at Layer 2 (Data Link layer) of the OSI model, containing source and destination MAC addresses, a payload (the encapsulated Layer 3 packet), and a frame check sequence for error detection. Frames are how switches and other Layer 2 devices think about and forward data.

Transcript

The Problem With Hubs

One of the problems with an Ethernet hub is that it allows collisions to happen on our network, and this can be problematic. One of the ways that we can reduce the amount of collisions on our network is by having a network bridge segment our network. We don't use a lot of network bridges nowadays, but we're going to build off of this in future lessons.

One of the problems with an Ethernet hub is that when a frame comes into the hub, it gets resent out all other live ports. Now imagine when another frame comes in at the same time that tries to get sent out at the same time as well. This causes a big problem, because now you've got two frames that are trying to be communicated at once, and that just isn't going to be well communicated. It's not going to work. So this is what we call a collision. And both machines that tried to speak at the same time will have to go through a backoff algorithm before they speak again.

Now, a little bit of this is okay. In fact, it's built into Ethernet and how Ethernet is supposed to function. But it becomes problematic the more and more communication that you have on your network. So more nodes and more active computers are going to have problems, and nowadays computers are very active. So we need to avoid collisions if we can.

How a Bridge Segments the Network

A bridge will help reduce the amount of collisions that we have. What it'll do is create some segmentation within our network to reduce the amount of collisions. It does this in two different ways.

Number one is it splits this network into essentially two. It's not two networks, it's still all the same network, but it creates two segments out of it. And you can have a collision in this segment, and you can have a collision in this segment, but you won't have a collision between those segments. So what that means is that a collision on this side isn't going to affect the other side. Not to mention, when this computer speaks on this side of the network, it's never going to collide with a computer on this side of the network.

The Bridging Table (Learning)

It does this by creating a bridging table, also known as a MAC address table. A bridging table because it's a bridge that creates the table; a MAC address table because it actually is recording MAC addresses. What it's doing is recording the MAC address and what port that MAC address is on.

So essentially what happens is when this device communicates on this side of the network, that gets sent out all of the ports from this hub right here. What hears that is this bridge on this port. This is port one right here, and it records that. It does that for all of the devices. So you can see this is on port one as well. This device is on port two, and this device is on port two. And we might have many more machines that are connected to this, but essentially it tracks all of those different ports. So now it can separate out the two segments.

The process that a bridge uses to create this MAC address table right here is called learning. So that is one of the functions of a bridge, is to learn the MAC addresses and the ports they're on.

Forwarding

The second function is forwarding. So what will happen is that this device right here, let's say, is trying to communicate to this device right here. It will send out a frame destined for this MAC address, the hub will send that out all other live ports, and this bridge will receive that. It will notice that this is on port one, which is what it came in on. And since it came in on that port where it associates with this MAC address, it will not send that to the other side of the network.

However, let's say now that this machine right here is communicating to this machine. It's sending out a frame destined for this MAC address, that gets sent out all of the ports on this hub right here. And then this bridge right here sees that it's destined for this MAC address over here. So what it's going to do is forward that out port two, because that's what's in its MAC address table, that it's on port two. So it'll forward it out port two, it'll get sent out to this segment of the network, and this device will receive that. So what happens is that this is going to act as a buffer between those two sides.

A Layer 2 Device

We consider a network bridge to be a layer 2 device. And we consider it a layer 2 device because it actually has to take a look at that layer 2 information to do both the learning and the forwarding of this frame.

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