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Basic Network Fundamental - Multilayer or Layer 3 Switch and Usage of Layer 3 switch instead of Router, Switch Virtual Interfaces

1) What is the Multilayer or Layer 3 switch?



• A multilayer switch is capable of both Switching and Routing. It is Layer 3 aware.

• A regular layer 2 switch is not layer 3 aware, it doesn’t think at all about IP addresses, it only cares about Layer 2 information like MAC addresses.

• You can assign IP addresses to its interfaces, like a router. And you can configure routed ports which function like an interface on a router.

• You can create virtual interfaces for the each VLAN and assign IP addresses to those interfaces.


2) Why we use Multilayer or Layer 3 switch instead of router?



• It can be used for inter-VLAN routing.

• Inter-VLAN routing, it is using one connection for each VLAN between the router and switch.

• But if you have many VLANs you probably won’t’ have enough interfaces on your router.

• The router on a stick, which uses a single trunk connection which carries traffic from all VLANs between the switch and router for inter-VLAN routing.

• This is efficient in terms of the number of interfaces, just one, but in a busy network all the traffic going to the router and back to the switch can cause network congestion.

• So, in large networks, a multilayer switch is the preferred method of inter-VLAN routing.


3) What is Router on a Stick (ROAS)?


• ROAS is used to route between multiple VLANs using a single interface on the router and switch.

• The switch interface is configured as a regular trunk. The router interface is configured using subinterfaces.

• You configure the VLAN tag and IP address on each subinterface.


4) What is the Switch Virtual Interfaces (SVI)?


• SVIs (or Switch Virtual Interfaces) are the virtual interfaces you can assign IP addresses to in a multilayer switch.

• Configure each PC to use the SVI (not the router) as their gateway address. When using router on a stick, the router was used as the PC’s gateway. This time we will use the switch’s SVIs instead.

• To send traffic to different subnet/VLANs, the PCs will send traffic to the switch and the switch will route the traffic.

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