layer3 switching vs routing

hi

i want to know the difference between the layer3 switching and routing. which is better. could u explain what is layer3 switching and routing in detail.

if possible with an example

Hi Anwar,

While the question your asking is on our "to do" list, meaning that we plan to cover the topic in detail, I'll briefly give you a run up in order to help you clear a few things.

Firstly, let's explain what we mean by LayerX switching, where the "X" part can be substituited by 2 or 3 in most cases.

When a device is say to "switch" on a specific layer, it basically shows us how inteligent this device, since higher layer switching requires smarter devices.

I assume your familair with the OSI model so I won't get into too much details on its layers. Layer 2 of the osi model is where the MAC addresses exist, the so called "physical" address, while layer 3 of the OSI model contains the network addressing information or if you like, the "logical" addresses e.g an IP Address.

So, when someone tells us a device is capable of layer 2 switching, we understand that the device is able to distingish between different MAC addresses, which are at the begining of the frame, but is not smart enough to understand any further information.

A layer 3 switching device understands the information contained in the 3rd layer of the OSI model, meaning, it can read and understand IP Address, which as we said, exist in the 3rd layer.

While routers work on the 3rd layer of the OSI model, routing packets between various network, switches work on the 2nd layer of the OSI model, switching packets based on their MAC addresses which is what's normally required within a LAN.

Now, in the past few years, vendors have managed to make most higher model switches become layer 3 switching devices by adding additional functionality to them, in simple terms .. making them smarter.

So I surpose you might be asking "So what more can a layer 3 switch device offer from a layer 2 device ?"

One answer are VLANs (Virtual Local Area Networks), which allow you to split a network in to multiple broadcast domains (see relevant section in our Networking topics).
When you split one network into 2 VLANs using the same switch, in other words, you assign e.g ports 1,2,3,4 to VLAN 1 and ports 5,6, and 8 to VLAN 2, then the result is the same as physically splitting the same network using a router. Packets on one network are not seen by the other cause the switch isolates them between each other.

This is one simple function that a Layer 3 switch has over a Layer 2 switch.

As I said, we plan to analyse this subject in great depth, and be sure all questions you have, will be answered!

Cheers,

Just to add a little.

Some people think of a Layer 3 switch as a router. It isn't. As Chris mentions it adds VLAN capabilities to it. But you still need to go through a router to go between subnets.

I'm sure there are some devices that will allow you to route and switch, but then you would be slowing the device down. You want your switch to be fast and if you add Routing to it, you are making it work harder. This may be OK in a small environment, but would be a killer in a large one.

Tom.

To further complicate the issue
A layer three switch network implementing VLANs needs a router to communicate between VLANs


There is an article out there which explained layer3 switching in depth.. i just can't find it !!


cheers,
Sahir.