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Why do we need Routers at all when we have VLAN ?

Vlan breaks the broadcast domain. Vlan along with the use of SVI can be used for routing.

So why do we need routers for in networking ?

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There are a couple of problems with your idea.

You cannot get traffic from one VLAN to another VLAN without routing. Either you have a dedicated router, or a layer-3 switch, which is really a router, too. A layer-2 switch with separate VLANs and SVIs still cannot let hosts on one VLAN communicate with hosts on a different VLAN. Frames can be switched at layer-2 on the same VLAN, but you need to strip the frames to get to the packets to route them between VLANs.

Switches are mostly limited to ethernet. Routers are used to connect different physical and data-link protocols, too. Some translating bridges can do some of that at layer-2. For example, WAPs translate layer-2 frames between ethernet and Wi-Fi, but only if they are on the same layer-2 LAN.

Even layer-3 switches usually have fewer resources (RAM, CPU, etc.) than dedicated routers do, and dedicated routers often have special hardware that you do not normally find in switches.

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