VLANs Explained | Cisco CCNA 200-301

Hey what's up guys. Welcome to CertBros! In this video we're going to be looking at vlans okay so vlan stands for virtual local area network using vlans we can virtually separate our LANs into smaller chunks but why would we want to do that? Well there are a few reasons why we might want to use VLANs one of the main reasons is something we've already spoken about and that is broadcast traffic. So let's remind ourselves about broadcasts. A LAN is a single area of a network and in this case, we have a single broadcast domain created by our switch if host A sends a broadcast then every device will receive it that's fine for our 4 computer network but let's imagine each computer represents an entire department very quickly broadcast traffic can start to slow down our networks and devices. So we need a way to manage all of this traffic. One way is to add a router. This would create separate broadcast domains per interface but it's an extra bit of hardware to purchase install cable and manage. Another, more radical option is to physically split the network into separate sections. This would create broadcast domains for each department. Traffic wouldn't be sent from one department to the other which drastically reduces our broadcast overhead. But this option still has the issue of needing to purchase and set up new equipment and neither one of these options is very scalable. Say we wanted to add a new department both options would require work to set up and we don't want to do that. This is where VLANs come in VLANs give us all of the benefits of physically separating our network but with the added bonus of being able to do this virtually. The traffic still behaves in the same way as if it was physically split. Traffic isn't forwarded to any other departments and it's almost like having four miniature switches inside our single one The way this works is by assigning interfaces to specific VLANs. Only interfaces in the same VLAN can communicate with each other in this example finance is yellow IT is red Sales is green and HRis purple. I'm using colors but in reality, you will probably use numbers such as VLAN 10 VLAN 20 etc If we wanted to add a new finance server then we just need to assign that interface to the finance VLAN and communication between those two interfaces is allowed because they're on the same VLAN If we wanted to add a brand new marketing department then we just create that VLAN and assign it to an interface As you can see VLANs make it very very easy to control broadcast domains It's also scalable. We don't need new equipment or to re-cable everything anytime you want to make a change okay, so that's the overview. Let's look at how this works in a bit more detail Straight out of the box Cisco and most other switches have a default VLAN called VLAN 1. Every interface is assigned the default VLAN. So this means every interface can talk to each other. From here we can start to make our own VLANs and split up our switch. we may want to make a VLAN called VLAN 10 and another called VLAN 20 You can add up to 4094 VLANs. This is the maximum number that is supported So now interfaces assigned to VLAN 10 can only communicate with other interfaces assigned to VLAN 10 and it's the same for VLAN 20. VLANs aren't restricted to just one switch We can have the same VLANs across multiple switches. This makes it very versatile and scalable But this does leave us with a problem. If interfaces can only communicate with other interfaces in the same VLAN, which VLAN do we assign to the link between our two switches? If we leave it as the default VLAN, VLAN 10 and VLAN 20 won't be able to send traffic. If we assign it to one of our new VLANs then only that VLAN can send traffic between the two switches. The solution is a special type of interface called a trunk So there are two types of ports on a switch, an access port and a trunk port. When a port is assigned to a VLAN like VLAN 10 and VLAN 20 here this is known as an access port An access port is designed for endpoint devices to access the network such as computers and laptops. Now a trunk port can send traffic from different VLANs They're used to send traffic between networking devices This leaves us with another problem though. How does the receiving switch know which VLAN the traffic belongs to? The whole point of a VLAN is to separate traffic. So when a trunk sends data that could be from VLAN 1, VLAN 10 or VLAN 20 it needs a way to identify which VLAN that traffic belongs to This is done by using something called a tag To understand tags we need to take a couple of steps back Now most devices including computers do not know what VLANs are Some devices are VLAN aware such as ip phones but most are not. So a computer generates a frame as normal. When that frame arrives at a switch it's the switch that handles the VLANs. then it's forwarded to the correct destination. The sending computer and the receiving computer have no idea that VLANs were used. Now when we have multiple switches connected with a trunk port there is an extra step. The computer sends the frame as usual but when the switch sends a frame over the trunk port it adds a new bit of information. This is called an 802.1q tag 802.1q is the IEEE standard. It can also be called Dot1q. This tag is 4 bytes and it contains a few bits of information TPID or tag protocol identifier. This is used to identify the frame as an 802.1q tagged frame. It could also be ISL but is rarely used today. TCI or tag control information this contains three bits of information. The priority DEI or drop eligible indicator and most importantly the VLAN id. This tag field is then read and removed by the receiving switch So to recap the frame is untagged when it's sent from the computer is tagged over the trunk port it's then sent to the destination computer untagged and our computers are again none the wiser. The next thing we need to talk about is something called native VLANs. A native VLAN is configured per trunk interface. It's the VLAN that the switch assumes the frame is on if it arrives on a trunk port with no tag By default the native VLAN is VLAN 1. So let's say both of our computers are assigned to the default vlan 1. Just like before, the frame is sent untagged to the switch Now if a switch is sending a frame across a trunk port that belongs to its native VLAN the frame is not tagged. When a switch receives a frame on its trunk port without a tag it will assume that frame belongs to its native VLAN which in this case is VLAN 1. It will then forward that frame to the destination, again untagged. So you might be wondering, why we use native VLANs. And there are a few reasons why Hubs can't read or write tags. All hubs can do is forward frames Let's say we have a hub in the middle connected to another host Using untagged native VLANs means we can send frames to our new host If we didn't use native VLANs and the frame was sent tagged then the host will just discard the frame because it doesn't understand VLAN tags. So a native VLAN is configured per trunk interface. This can lead to problems. For example let's say we change one side of our trunk to have the native VLAN of 20 instead of 1. We now have two switches with different native VLANs. Our computer will send the frame As before, the first switch will see the interface is assigned to its native VLAN and then forward that frame untagged across the trunk. Do you see the problem here? The second switch will assume the traffic should belong to its native VLAN, VLAN 20 and the frame will not be sent to the destination computer on VLAN 1. Luckily in real life you will be alerted to this type of configuration on the switch You'll probably see a message like this saying "native VLAN mismatch" This video is part of our full CCNA Course which can be found in the description. So please feel free to go and check that out. Don't forget to like, comment and subscribe. The support from you guys really helps this channel grow. other than that, thank you for watching! 👍

Channel: CertBros

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Keywords: ccna 200-301, ccna training, cisco, cisco ccna, cisco networking, computer networking, computer networking course, networking, virtual lan, vlan tagging, vlan, vlans, vlans explained, virtual local area network, what is vlan, vlan explained, vlan configuration, how to configure vlan, vlan switch, vlan trunk, vlan tutorial, 802.1q tagging, vlan trunking, vlan trunking explained, dot1q, native vlan, trunk port, ccna certification

Id: A9lMH0ye1HU

Length: 11min 15sec (675 seconds)

Published: Tue Oct 06 2020