Use of virtual chassis Why we need virtual chassis in a network ? What is the use of it ? Why not have one switch instead of having 2 or more switches (Virtual chassis) in a network .

There are two main reasons for using stacking techniques like virtual chassis: redundancy and ease of management.

If you have just a single switch and it fails in some way, then your network is completely down. If you have a stack of switches and one node in the stack fails, then you have some form of connectivity left. When you design your network right, it can survive failure of any single device in the network.

As for the second reason: if you want to provide access for 400 endpoints, you would need a very large switch to provide for all those access ports. More likely, you would use 10 smaller switches because that's a lot cheaper usually. When you stack those 10 switches, you can manage them as a single device, making your job a lot easier.