Within a layer 2 switch environment exists a broadcast domain. Any broadcast traffic on a switch will be forwarded out all ports with the exception of the port the broadcast packet arrived on. Broadcasts are contained in the same layer 2 segment, as they do not traverse past a layer 3 boundary.
Large layer 2 broadcast domains can be susceptible to certain unintended problems, such as broadcast storms, which have the ability to cause network outages.
Also, it may be preferable to separate certain clients into different broadcast domains for security and policy reasons. This is when it becomes useful to configure VLANs. A layer 2 switch can assign VLANs to specific switch ports, which in turn are in different layer 3 subnets, and therefore in different broadcast domains.
VLANs allow for greater flexibility by allowing different layer 3 networks to be sharing the same layer 2 infrastructure. The image below shows an example of a multi-VLAN environment on a layer 2 switch:. This is where a layer 3 switch can be utilized. A Layer 3 switch is basically a switch that can perform routing functions in addition to switching.
A client computer requires a default gateway for layer 3 connectivity to remote subnets. When the computer sends traffic to another subnet, the destination MAC address in the packet will be that of the default gateway, which will then accept the packet at layer 2, and proceed to route the traffic to the appropriate destination based on its routing table. It then rewrites the appropriate destination MAC address and forwards the packet back out the layer 2 segment: As previously mentioned, deciding whether you need a Layer 3 switch or a Layer 2 switch is much more dependant on your specific requirements than a like-for-like comparison.
It would be like asking whether a flat-head or a Phillips-head screwdriver is better — the answer is always dependent on the job at hand. Much like the difference between Layer 3 and Layer 2 in the OSI, the main difference between a Layer 2 switch and a Layer 3 switch is the routing function.
Layer 2 switching
A Layer 3 switch, on the other hand, can also do static routing and dynamic routing, which includes IP and virtual local area network VLAN communications. This dual-layer functionality is why a Layer 3 switch is also known as a multilayer switch. In practical terms, this means if your network runs on a Layer 2 domain then you would get no benefit from a Layer 3 switch. On the surface, routers and Layer 3 switches share a lot of the same attributes.
Both options offer the same routing protocols by accessing incoming data packets and making dynamic routing decisions based on the relevant source and destination addresses.
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The main difference between them is in the hardware, as a Layer 3 switch offers all of the capabilities of a switch but only some of the facilities of a router. Although this may sound like a negative, the fact is that Layer 3 switches were built for improved performance in LANs by ditching some of the complexities routers are held to.
As such, a Layer 3 switch can offer faster data transfers and can avoid network congestion within a LAN better than a router by skipping some steps in data verification. Similar to the decision between a Layer 2 or a Layer 3 switch, the question comes down to the application. If you are a large organisation with multiple offices, data centres and WANs you will require a router to allow those infrastructures to communicate. For example, a Layer 3 switch-based infrastructure needs routers to be installed on all sites, whereas a Layer 2 system can use Ethernet switches with router functionality as a subset.
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The data portion of this frame will contain the complete IP packet where the destination IP address remains unchanged and is sent out the appropriate interface. This process will continue at each router along the way until the information reaches a router connected to the destination network. It will see that the packet is addressed to a host that's on a directly connected network the closest match you can get for an address, short of the packet being addressed to you. Now then, I did slightly gloss over 1 part in the above explanation and that's the part about the router finding out the hardware address for the next hop.
I just didn't want to disturb the flow with entering into that there. In some cases, this will be a hard set value like a frame relay pvc within the configuration of the router. In some cases, you don't even need a hardware address like any point to point connection, there's only 1 possible host you could send it to , in those cases the router will just create a data link frame appropriate for the connection and it won't even need to be addressed.
Layer 2 vs Layer 3 Switch: Which One Do You Need?
This is why the OSI model is good. It's layered so that any layer can change and as long as it takes in information in a standard way the way the layer above wants to send it and spits out information in a standard way the way the layer below wants to receive it , then it's all good. When Frame Relay came along nothing changed with the way you had to address IP packets, all of the changes took place at the data link and physical layers.
If some new type of connection comes along in the future, only the data link and physical layers will likely change. When we go to IPv6, only the network layer should change it probably won't but that's more to do with how the layers tend to blur, but if it were truly layered that would be the case.
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Anyways, since I feel like doing an example here's one showing the whole process. Your computer has an address of Your default gateway is a router router a which has an address of That router is connected to the The network 2 is also an ethernet network. Also connected to network 2 is another router router b which has the address for the interface connected to network 2 at least of Router b is also connected to network 3 Router b's interface on network 3 has the address of Now then, your computer on network 1 with an address of We'll assume that none of the info in already cached in an ARP table on any of the machines or routers a big assumption but it's mine to make!
Your computer will create an IP packet addressed to That packet will be sent to the data link layer where it needs a MAC address. Based on the subnet mask, your computer will know that the destination computer isn't on the same local network. Router a will receive this frame and send the data portion up to the network layer layer 3. At the network layer, router a will see that the packet which is addressed to Router a will look in it's routing table to find out where to send the packet. The routing table will show that network 3 the closest match to The routing table will also show the IP address for the next hop is Router a will send out an ARP request onto network 2 asking for router b's MAC address well at least for the interface connected to network 2.
On receiving this, router a will send the IP packet still addressed to
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