Definition:

Layer 3 switching is solely based on (destination) IP address stored in the header of IP datagram (see layer 4 switching later on this page for the difference). The difference between a layer 3 switch and a router is the way the device is making the routing decision. Traditionally, routers use microprocessors to make forwarding decisions in software, while the switch performs only hardware-based packet switching (by specialized ASIC with the help of content-addressable memory). However, some traditional routers can have advanced hardware functions as well in some of the higher-end models.

What Condition Is Required to Enable Layer 3 Switching Quizlet:

A network device that forwards traffic based on layer 3 information at very high speeds. Traditionally, routers, which inspect layer 3, were considerably slower than layer 2 switches. In order to increase routing speeds, many "cut-through" techniques were used, which perform an "inspect the first packet at layer 3 and send the rest at layer 2" type of processing. Ipsilon's IP Switch and Cabletron's SecureFast switches were pioneers in cut-through switching. 

Layer 3 switching is a relatively new term, which has been? Extended? by a numerous vendors to describe their products. For example, one school uses this term to describe fast IP routing via hardware, while another school uses it to describe Multi Protocol Over ATM (MPOA). For the purpose of this discussion, Layer 3 switches are superfast rout-ers that do Layer 3 forwarding in hardware. In this article, we will mainly discuss Layer 3 switching in the context of fast IP routing, with a brief discussion of the other areas of application. Intelligent packet forwarding (routing) based on Layer 3 information is traditionally the function of routers. It's here that IP addresses are found, for example, enabling a router to link different subnets together. Specialised routing protocols also use Layer 3, enabling routers to "learn" routes between networks.

In recent years, however, that same functionality has also been built into network switches. Routers are still used to forward packets across (relatively) slow WAN (Wide Area Network) connections but on local networks, high-performance Layer 3 switches - sometimes referred to as "switch routers" or "routing switches" - have largely replaced them.

For the sake of allowing viewers better to know and understand  layer 3 switching, apart from introducing the conditions of layer 3 switching, we will introduce it from three aspects as below:

How Layer 3 Switches Work

A traditional switch dynamically routes traffic between its individual physical ports according to the physical addresses (MAC addresses) of devices connected. Layer 3 switches use this capability when managing traffic within a LAN. They also expand on this capability by utilizing IP address information to make routing decisions when managing traffic between LANs.

Layer 3 Routing Versus Layer 3 Switching

It is important to understand the difference between Layer 3 routing and Layer 3 switching. Both terms are open to some interpretation; however, the distinction between both can perhaps be best explained by examining how an IP packet is routed. The process of routing an IP packet can be divided into two distinct processes:

Control plane—The control plane process is responsible for building and maintaining the IP routing table, which defines where an IP packet should be routed to based upon the destination address of the packet, which is defined in terms of a next hop IP address and the egress interface that the next hop is reachable from. Layer 3 routing generally refers to control plane operations.

Data plane—The data plane process is responsible for actually routing an IP packet, based upon information learned by the control plane. Whereas the control plane defines where an IP packet should be routed to, the data plane defines exactly how an IP packet should be routed. This information includes the underlying Layer 2 addressing required for the IP packet so that it reaches the next hop destination, as well as other operations required on for IP routing, such as decrementing the time-to-live (TTL) field and recomputing the IP header checksum. Layer 3 switching generally refers to data plane operations.

Benefits of Layer 3 switches

Performance versus Cost—Layer 3 switches are much more cost effective than routers for delivering high-speed inter-VLAN routing. High performance routers are typically much more expensive than Layer 3 switches. For example, a Catalyst 3550-24 EMI switch sets you back $4,990 U.S. list, which provides a packet forwarding rate of 6.6 million packets per second with 24 * 10/100BASE-T ports and 2 * 1000BASE-X ports. A Cisco 7300 router with an NSE-100 engine provides a packet forwarding rate of 3.5 million packets per second, but sets you back $22,000 U.S. list and has only 2 * 1000BASE-T ports in its base configuration. Of course, the Cisco 7300 router has many more features and can support a wide variety of WAN media options; however, many of these extra features are not required for inter-VLAN routing.

Port density—Layer 3 switches are enhanced Layer 2 switches and, hence, have the same high port densities that Layer 2 switches have. Routers on the other hand typically have a much lower port density.

Flexibility—Layer 3 switches allow you to mix and match Layer 2 and Layer 3 switching, meaning you can configure a Layer 3 switch to operate as a normal Layer 2 switch, or enable Layer 3 switching as required.

Layer 3 switching is cheap because Layer 3 switches are targeted specifically for inter-VLAN routing, where only Ethernet access technologies are used in high densities. This makes it easy for Layer 3 switch vendors such as Cisco to develop high performance Layer 3 switches, as vendors can develop hardware chips (known as application-specific integrated circuits or ASICs) that specifically route traffic between Ethernet networks, without having to worry about the complexities of also supporting WAN technologies such as Frame Relay or ATM. Routing over WAN networks can still be supported, simply by plugging a traditional router that connects to the WAN networks into the LAN network. Figure 6-1 illustrates the concept of Layer 3 switching.

Conclusion:

The Layer 3 switch must have an IP routing enabled to enable layer 3 switching.  A traditional switch dynamically routes traffic between its individual physical ports according to the physical addresses (MAC addresses) of devices connected. Layer 3 switches use this capability when managing traffic within a LAN. They also expand on this capability by utilizing IP address information to make routing decisions when managing traffic between LANs. (In contrast, Layer 4 switches also utilize TCP or UDP port numbers.