LAN switching

LAN switching

:"This article addresses packet switching in computer networks.

LAN switching is a form of packet switching used in local area networks. Switching technologies are crucial to network design, as they allow traffic to be sent only where it is needed in most cases, using fast, hardware-based methods.


Prices on OSI model layer 2 switching equipment dropped dramatically around the turn of the 21st century, making it easier to justify the cost of buying switches for an entire local area network. This doesn't mean that every business can afford switch ports for all users, but it does allow for a cost-effective upgrade method.

Layer 2 switching

Layer 2 switching is hardware based, which means it uses the media access control address (MAC address) from the host's network interface cards (NICs) to decide where to forward frames. Switches use application-specific integrated circuits(ASICs) to build and maintain filter tables (also known as MAC address tables). One way to think of a layer 2 switch is as a multiport bridge.

Layer 2 switching provides the following
* Hardware-based bridging (MAC)
* Wire speed
* High speed
* Low latency
* Low cost

Layer 2 switching is highly efficient because there is no modification to the data packet, only to the frame encapsulation of the packet, and only when the data packet is passing through dissimilar media (such as from Ethernet to FDDI). Layer 2 switching is used for workgroup connectivity and network segmentation (breaking up collision domains). This allows a flatter network design with more network segments than traditional 10BaseT shared networks.Layer 2 switching has helped develop new components in the network infrastructure
* Server farms — Servers are no longer distributed to physical locations because virtual LANs can be created to create broadcast domains in a switched internetwork. This means that all servers can be placed in a central location, yet a certain server can still be part of a workgroup in a remote branch, for example.
* Intranets — Allows organization-wide client/server communications based on a Web technology.

These new technologies are allowing more data to flow off of local subnets and onto a routed network, where a router's performance can become the bottleneck.


Layer 2 switches have the same limitations as bridge networks. Remember that bridges are good if a network is designed by the 80/20 rule: users spend 80 percent of their time on their local segment.

Bridged networks break up collision domains, but the network remains one large broadcast domain. Similarly, layer 2 switches (bridges) cannot break up broadcast domains, which can cause performance issues and limits the size of your network. Broadcast and multicasts, along with the slow convergence of spanning tree, can cause major problems as the network grows. Because of these problems, layer 2 switches cannot completely replace routers in the internetwork.

Layer 3 switching

The only difference between a layer 3 switch and a router is the way the administrator creates the physical implementation. Also, traditional routers use microprocessors to make forwarding decisions, and the switch performs only hardware-based packet switching. However, some traditional routers can have other hardware functions as well in some of the higher-end models. Layer 3 switches can be placed anywhere in the network because they handle high-performance LAN traffic and can cost-effectively replace routers. Layer 3 switching is all hardware-based packet forwarding, and all packet forwarding is handled by hardware ASICs. Layer 3 switches really are no different functionally than a traditional router and perform the same functions, which are listed here
* Determine paths based on logical addressing
* Run layer 3 checksums (on header only)
* Use Time to Live (TTL)
* Process and responds to any option information
* Can update Simple Network Management Protocol (SNMP) managers with Management Information Base (MIB) information
* Provide Security

The benefits of layer 3 switching include the following
* Hardware-based packet forwarding
* High-performance packet switching
* High-speed scalability
* Low latency
* Lower per-port cost
* Flow accounting
* Security
* Quality of service (QoS)

Layer 4 switching

Layer 4 switching is considered a hardware-based layer 3 switching technology that can also consider the application used (for example, Telnet or FTP).

Layer 4 switching provides additional routing above layer 3 by using the port numbers found in the Transport layer header to make routing decisions.

These port numbers are found in Request for Comments (RFC) 1700 and reference the upper-layer protocol, program, or application.

Layer 4 information has been used to help make routing decisions for quite a while. For example, extended access lists can filter packets based on layer 4 port numbers. Another example is accounting information gathered by NetFlow switching in Cisco's higher-end routers.

The largest benefit of layer 4 switching is that the network administrator can configure a layer 4 switch to prioritize data traffic by application, which means a QoS can be defined for each user.

For example, a number of users can be defined as a Video group and be assigned more priority, or band-width, based on the need for video conferencing.

However, because users can be part of many groups and run many applications, the layer 4 switches must be able to provide a huge filter table or response time would suffer. This filter table must be much larger than any layer 2 or 3 switch. A layer 2 switch might have a filter table only as large as the number of users connected to the network and may be even less if some hubs are used within the switched fabric. However, a layer 4 switch might have five or six entries for each and every device connected to the network. If the layer 4 switch does not have a filter table that includes all the information, the switch will not be able to produce wire-speed results.

Multi-layer switching (MLS)

Multi-layer switching combines layer 2, 3, and 4 switching technologies and provides high-speed scalability with low latency. It accomplishes this high combination of high-speed scalability with low latency by using huge filter tables based on the criteria designed by the network administrator.

Multi-layer switching can move traffic at wire speed and also provide layer 3 routing, which can remove the bottleneck from the network routers. This technology is based on the idea of route once, switch many.

Multi-layer switching can make routing/switching decisions based on the following
* MAC source/destination address in a Data Link frame
* IP source/destination address in the Network layer header
* Protocol field in the Network layer header
* Port source/destination numbers in the Transport layer header

There is no performance difference between a layer 3 and a layer 4 switch because the routing/switching is all hardware based.

ee also

* Network switch
* Content switch
* Residential gateway

External links

* [ Cisco IT Case Studies] about routing and switching

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