[ACSA Education #19] Domain

In the last post, we looked at the basic commands of the AOS-CX switch.

This time, we'll explore design, deployment, configuration, and troubleshooting to deepen your understanding of Layer 2 concepts.
First of all Layer 2 domainLet's learn about the concept.

Collision Domain

Let's say Alice and Bob are having a conversation.

Even though Alice has something to say, she listens intently while Bob speaks and waits for her turn.
If Alice were to say what she wanted to say right away, it would be difficult to understand what each other was saying as it would be mixed with Bob's voice.

When the two of you are talking and realize that your voices are clashing, you will stop and say this:.

Bob: “I’m sorry. Please speak.”

Alice: “No. I’m sorry. Please continue speaking.”

In this way, both of you will step back and then start talking again.

The same goes for Ethernet networks. Only one Ethernet host can transmit data at a time on a given segment. This is due to an algorithm running on every NIC., CSMA/CD (Carrier Sense Multiple Access / Collision Domain)is controlled by . This works similarly to the Alice and Bob conversation example above.

When several people are in a room, we can speak whenever we want.
However, before we speak, we check to see if anyone else is speaking.
This is Ethernet Carrier Sense mechanismYou can think of it as being the same as .
Ethernet cards detect the state of “Carrier Sense”, which identifies which NIC is currently transmitting.

When two devices transmit simultaneously, the electrical signals collide and become corrupted. This is called “Collision“" he said.
When this condition is detected, each station will back off and try again.
It's the same situation as when Alice and Bob stop talking to each other after realizing what they were talking about.no see.

Ethernet collisions occur when a hub is used on a network. A hub is a Layer 1 device that receives digital signals 1 and 0 and immediately repeats them on all other ports. For this reason, hubs are also called multi-port repeaters.

All 16 devices connected to a 16-port hub are in the same collision domain. When one device transmits, the remaining 15 devices must wait. If two hosts transmit simultaneously, a collision occurs, causing everyone to back off, wait a few milliseconds (ms), and then try again.

Fortunately, hubs are no longer used today. In many places, Layer 1 hubs are being replaced by more intelligent Layer 2 switches. Under normal circumstances, properly deployed and configured switches should not cause conflicts.

※ Similarly, Wi-Fi also uses an algorithm called CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance). This is similar to Ethernet's CSMA/CD, but adds a separate mechanism to prevent collisions in advance.

All devices connected to the same wireless access point (AP) channel have the same Collision DomainBecause it is in , you can only transmit one at a time.

Collision Domain and Network Performance

Understanding Collision Domains can also help you optimize network performance.

Let's compare a room with two people versus eight, ten, or fifty. Obviously, conversations are faster, easier, and more efficient with two people. As more people join, multiple conversations arise, leading to distractions and increased chances of "collisions." To improve communication and prevent people from waiting too long, you can divide the room into separate rooms.

Similarly, let's say you have an 8-port hub. While one host transmits, the other seven hosts must wait. With a 48-port hub, 47 hosts must wait. In this situation, it would be beneficial to separate the traffic into multiple domains (collision domains).

Of course, as mentioned earlier, hubs, which are Layer 1 devices, are no longer used and are being replaced by Layer 2 switches.
Under normal circumstances, a collision will not occur.

Similarly, consider a single AP. Setting the power to maximum would expand the coverage area and save costs. A single AP would provide wireless service to all 75 nearby users. However, while one device transmits, the other 74 users would have to wait, and people would complain about the wireless connection being slow.

To solve the problem, buy more APs, set their power to low, and set each to its own channel.
Set each AP to only allow about 25 users to connect. Each AP represents a separate collision domain. Now, up to three users can transmit simultaneously, significantly improving wireless network performance.

Rather than blindly attaching as many users as possible to an AP, it is better to optimize performance by connecting only an appropriate number of users (10 to 25) to one AP.

Broadcast Domain

Broadcast Domainis available on all devices Broadcast frameA group of devices on the same network that can receive and respond.

As explained earlier, broadcast is a type of communication where one device connects to all other devices.

• In Layer 2 FF:FF:FF:FF:FF:FF
• In Layer 3 255.255.255.255

Use to deliver broadcast messages.

A Layer 2 switch forwards broadcast frames to all ports except the port that sent the broadcast (the ingress port).

In the above figure, Host A transmits the broadcast address.

Hello! Does anyone have the IP address 10.1.1.1?

Host A

Both Host B and Host C receive this message because the switch forwards it to all other ports. However, hosts in broadcast domain 2 cannot receive this message (because they are in different domains).
Similarly, if Host D sends a broadcast message, hosts in Broadcast Domain 1 cannot receive it.

Now let's connect these two networks into one internetwork.
What device will you use? Yes. router. The two networks are now connected. However, unlike Layer 2 switches, Layer 3 routers do not forward broadcasts.

That is, it defines the boundaries, or edges, of a broadcast domain. Therefore, even if two networks are connected, broadcast traffic will still flow as before. Hosts in Domain 1 will receive broadcast messages originating in Domain 1, but will not receive broadcast traffic from Domain 2. However, because the networks are connected, interdomain communication is possible via unicast and multicast traffic.

The relationship between broadcast domains and network performance

Why not eliminate the router and simplify the network? Connecting all hosts together through a single broadcast domain would eliminate the need for routers and save costs.

This ties into the issue of network performance.
As with Collision Domains, large broadcast domains cause performance issues.

For unicast traffic, when communicating from Host A to Host F, only two stations will process the packet.
On the other hand, broadcast frames require all stations to process the traffic. The switch must forward broadcast frames to all ports (except the ingress port). This increases switch utilization and load, as well as bandwidth usage, because all switch links must forward all broadcasts.

Hackers can exploit this by generating millions of broadcast packets through programs and flooding the network, depleting resources for essential network traffic. This is called a Denial of Service (DoS) attack. Therefore, a smaller broadcast domain improves network performance.

Okay, now that we understand domains, let's learn about VLANs.