Understanding VLANs

By default the Cisco switches do not break the broadcast domain and what VLAN does it breaks the one physical switch into multiple virtual switches to isolate the broadcast domain.
Broadcast slows the network traffic due to a lot of broadcast requests.

Any time your computer goes to talk only one person can talk on electrical wire at any given time. If two people talk at same time and electrical pulses hit each other and you get collision.
So before you talk you have to sense the wire whether there is data on wire or not and if there is data it is known as multiple access, and you wait for your turn but what if there are two persons waiting for the communication and they both send traffic after wire is empty there data will collide, and we detect this collision using collision detection.

Notice that after using Carrier sense multiple access/collision detection we can get a lot collision with one big giant network.


Consider this network where two laptops talk to each other too much and they rarely talk to server and I do not want the server to listen all the broadcasts generated by these laptops so what I can do is I can isolate this to separate VLANs.


Now we have created three different VLANs and if laptops generate broadcast now then this is only going be listened by the hosts that are part of this VLAN.



When we have two computers at different switches and we need to place them at same VLAN we use a link called uplink to connect those two switches together. It is highly recommended that you use SFP ports for this uplink because they drive the traffic at very good speed.

Why SFP ports?

If we have a 48 ports switch and all have 1Gb speed and we have 4 SFP ports with 1Gb speed as well so what is the difference they all have same speed?

The switches use a mechanism called back-plan and there are two types of back-plan, over-subscribed and non-blocking.

Non-blocking uses a strong back-plan and if there all ports are talking at same time with full speed then there will be no speed issue and you do not necessary need SFP ports in this case. This kind of switches are  expensive.

Over-subscribed uses a weak back-plan and if all the 48 ports start to talk at full speed then this will group the ports and will assign single unit speed to them, for instance we have 48 port switch and back-plan divides ports into 8 groups each group with 6 ports and assign 1Gb speed to 6 ports, this downgraded the switch, and if we used these ports for uplink then our entire network will be at slow speed, and there come the SFP ports which uses a non-blocking back-plane always and you will be using separate 1Gb speed for your uplink.


SFP ports are always separate from regular ports and use a special connector with copper or fiber cable where normal ports use RJ-45 cable and port.

Trunk Port

This port carries the data of multiple VLAN using single interface it is upgraded form of uplink where uplink are consider to carry the data of only one VLAN.

By default all switch ports are in VLAN 1 also called a default VLAN, we can say that by default any brand new switch have 5 VLANs.


We turned on the switch and ran the show vlan command to see the default VLANs, and here we have result.
act/unsup (active/unsupported) it means VLANs are active but not supported.

VLAN Configuration




Here we have created a manual VLAN and assigned a port to it.

VLANs information is stored in vlan.dat file, it is not stored in running or startup configuration.

We can assign multiple interfaces to a VLAN using range command.



Trunk Configuration


We have turned this mode to trunk but this port will not go to trunk mode unless the port is plugged in and turned up. So make sure you have connection to the port to make it trunk.

Prerequisites for 200-301

200-301 is a single exam, consisting of about 120 questions. It covers a wide range of topics, such as routing and switching, security, wireless networking, and even some programming concepts. As with other Cisco certifications, you can take it at any of the Pearson VUE certification centers.

The recommended training program that can be taken at a Cisco academy is called Implementing and Administering Cisco Solutions (CCNA). The successful completion of a training course will get you a training badge.

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