What is the difference between a Layer 3 switch and a router? Cisco Forum FAQSome content part of this FAQ is taken out of the CiscoPress book Cisco LAN Switching, pp.451-453; authored by Kennedy Clark CCIE #2175 CCSI, and Kevin Hamilton CCSI Copyright© 1999 Cisco Press
In general, a Layer-3 switch (routing switch) is primarily a switch (a Layer 2 device) that has been enhanced or taught some routing (Layer 3) capabilities. A router is a Layer 3 device that simply do routing only. In the case of a switching router, it is primarily a router that uses switching technology (high-speed ASICs) for speed and performance (as well as also supporting Layer 2 bridging functions).
Several factors have created significant confusion surrounding the subject of Layer 3 switch and Layer 3 switching. Some of this bewilderment arises from the recent merging of several technologies. In the past, switches and routers have been separate and distinct devices. The term switch was reserved for hardware-based platforms that generally functioned at Layer 2. For example, ATM switches perform hardware-based forwarding of fixed-length cells whereas Ethernet switches use MAC addresses to make forwarding decisions. Conversely, the term router has been used to refer to a device that runs routing protocols to discover the Layer 3 topology and makes forwarding decisions based on hierarchical Layer 3 addresses. Because of the complexity of these tasks, routers have traditionally been software-based devices. Routers have also performed a wide variety of "high touch" and value added features such as tunneling, data-link switching (DLSw), protocol translation, access lists, and Dynamic Host Configuration Protocol (DHCP) relay.
As a broad category, routing switches use hardware to create shortcut paths through the middle of the network, by bypassing the traditional software-based router. However, unlike traditional routers that utilize general-purpose CPUs for both control-plane anddata-plane functions, Layer 3 switches use high-speed application specific integrated circuits (ASICs) in the data plane. By removing CPUs from the data-plane forwarding path, wire-speed performance can be obtained. This results in a much faster version of the traditional router.
Some network topologies as illustrations
1. Single Router
Internet
|
| 1.1.1.0/24
|
Router
|
LAN 1 with Unmanaged Switch (UM)
10.0.1.0/24
2. Single Router with multiple LAN subnets
Internet
|
| 1.1.1.0/24
|
Router --- LAN 2 with UM 10.0.2.0/24
|
LAN 1 with UM
10.0.1.0/24
3. Single Router with single connection to a switch and with multiple LAN subnets (also known as "Router on A Stick" design)
Internet
|
| 1.1.1.0/24
|
Router
*
* Single Connection to a Switch using feature called Trunking
*
Layer-2 Managed Switch
| | |
| LAN 2 |
| with UM |
| 10.0.2.0/24 |
| |
LAN 1 LAN 3
with UM with UM
10.0.1.0/24 10.0.3.0/24
4. Single Router with Layer-3 Switch and with multiple LAN subnets
Internet
|
| 1.1.1.0/24
|
Internet Router
|
| 10.0.0.0/24
|
Layer-3 Switch
| | |
| LAN 2 |
| with UM |
| 10.0.2.0/24 |
| |
LAN 1 LAN 3
with UM with UM
10.0.1.0/24 10.0.3.0/24
5. Multiple Routers with multiple unmanaged (dumb) switches and with multiple LAN subnets
Internet
|
| 1.1.1.0/24
|
Internet Router
|
| 10.0.0.0/24
|
Unmanaged Switch (UM)
| | |
| Router 2 |
| | |
| LAN 2 |
| with UM |
| 10.0.2.0/24 |
| |
Router 1 Router 3
| |
LAN 1 LAN 3
with UM with UM
10.0.1.0/24 10.0.3.0/24
Of the variety of other switching devices and terminology released by vendors, Layer 4 and Layer 7 switching have received considerable attention. In general, these approaches refer to the capability of a switch to act on Layer 4 (transport layer) information contained in packets. For example, Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) port numbers can be used to make decisions affecting issues such as security and Quality of Service (QoS). However, rather than being viewed as a third type of campus switching devices, these should be seen as a logical extension and enhancement to the two types of switches already discussed. In fact, both routing switches and switching routers can perform these upper-layer functions.
For further understanding, check out the following FAQ
»Cisco Forum FAQ »Should I use layer-3 switch or router?
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by rolande edited by aryoba
last modified: 2008-06-30 10:47:55