An Introduction to Layer 3 Switches

In today's complex business networks that comprise many virtual LANs and subnets, a Layer 3 switch plays an important role in many systems. But do you need this on your network? Let's see.

What is a Layer 3 switch?

Simply put, a Layer 3 switch combines the functionality of a switch and a router. It acts as a switch to connect devices that are on the same subnet or virtual LAN at lightning speed and has built-in IP routing intelligence to function as a router. It can support routing protocols, inspect incoming packets, and may even make routing decisions supporting source and destination addresses. This is often how a Cisco Layer 3 switch acts as both a switch and a router.

Often referred to as a multi-layer switch, a Layer 3 switch adds a lot of flexibility to a network.

Characteristics of a layer 3 switch

The characteristics of a Layer 3 switch are:

• It comes with 24 Ethernet ports, but no WAN interface.
• It acts as a switch to connect devices within the same subnet.
• The switching algorithm is simple and is the same for most routed protocols.
• It operates on two OSI layers: layer 2 and layer 3.

Purpose of a Layer 3 switch

There is a lot of confusion about using a Layer 3 switch because, in a traditional configuration, routers operate at Layer 3 of the OSI model while switches operate at Layer 2. So how does this Layer 3 switch fit into this model? Also, the name "layer 3 switch" confuses because switches normally operate from layer 2.

Layer 3 switches were originally designed to improve routing performance in large networks, especially corporate intranets. To understand the purpose, let's go back a bit in time to see how these switches evolved.

Layer 2 switches work well when there is low to medium traffic on VLANs. But these switches would hang when traffic increased. So, it became necessary to increase the functionality of Layer 2.

One option was to use a router instead of a switch, but then routers are slower than switches, so this could lead to slower performance.

To overcome this disadvantage, the researchers considered implementing a router within a switch. Although technically feasible, it was not the ideal choice because Layer 2 switches operate only on the Ethernet MAC frame while Layer 3 handles multiple routing protocols. The researchers felt this was too complicated, so they came up with the idea of a Layer 3 switch that would act like routers with fast-forwarding done through the underlying hardware.

This is why the most difference between Layer 3 switches and routers lies within the hardware. If you take a look at the hardware of a Layer 3 switch, you will see a mix of traditional switches and routers, except that the router software logic is replaced by integrated circuit hardware to improve performance.

Also, a Layer 3 switch router will not have WAN ports or other WAN features that you would normally see on a traditional router.

Benefits of a Layer 3 switch

From the discussion above, the purpose/benefits of a layer 3 change are:

• Routing support between virtual LANs.
• Improve fault isolation.
• Simplify security management.
• Reduce broadcast traffic volumes.
• Make the configuration process for VLANs easy as a separate router is not required between each VLAN.
• Separate the routing tables and as a result, better segregate traffic.
• Simplifying troubleshooting like troubleshooting the L2 layer is tedious and time-consuming.
• Supports flow accounting and high-speed scalability.
• The lowest network latency since a packet does not have to make extra hops to go through a router.

Now that you simply know why your organization needs a Layer 3 switch, let's also check out the potential issues that accompany it.

Layer 3 switch disadvantages

Like any product, a Layer 3 switch also has its drawbacks. We will briefly discuss each of these disadvantages to help you make the right choice.

Cost

Cost is one of the main disadvantages of a Layer 3 switch. It costs much more than a traditional switch and configuring and managing these switches also requires more effort. Therefore, an organization must be ready to expend additional resources to configure Layer 3 switches.

Limited application

Layer 3 switches are most applicable only for large intranet environments with many device and traffic subnets. Homes and small organizations don't need these switches.

Lack of WAN functionality

The lack of WAN functionality is another major disadvantage of Layer 3 switches. This means that you cannot eliminate routers and you will need both Layer 3 routers and switches to route traffic in and out of your organization.

Multiple tenants and virtualization

Compared to Layer 2 switching, Layer 3 routing is comparatively slower. This can be a problem when you want to spread VLANs across multiple switches to support multiple tenants and virtualization.

Lack of flexibility

Since you are routing at the access layer, the VLANs will be local to that specific switch. In other words, a VLAN will be associated with one switch and cannot be used on other switches. This limitation means that you must plan well to prevent a LAN from using multiple switches.

Do you need it?

Now comes the big question: Do you need a Layer 3 switch for your network? Well, it depends on your network configuration and its size.

Generally speaking, you would like a Layer 3 switch if you answer yes to any of the subsequent questions.

• Do you have VLAN on your network? Do you plan to include them shortly?
• Do individual departments need separate broadcast domains for security and performance?
• Do you have subnets connected through a router?
• Does your network have more than 250 devices connected to the same VLAN?

If you answered yes to one or more of the above questions, you surely need a Layer 3 switch to improve performance. In all other cases, it is optional.

Layer 3 switches act as switches and routers. They are ideal for VLAN only, as they do not have a WAN interface. But within VLANs, gives you multiple options to manage your bandwidth efficiently. This is often why Layer 3 switches are a strong and scalable technology for building high-performance Ethernets.

Previously published at http://techgenix.com/layer-3-switch/