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 does routing only. In the case of a switching router, it is primarily a
router that may use switching technology (high-speed ASICs) for speed and performance
(as well as also supporting Layer-2 bridging functions).
As illustration, here are some examples
Layer-2 switches
Cisco: Catalyst 2950, 2960 series
Layer-3 switches or routing switches
Cisco: Catalyst 3550, 3560, 3750, 4500, 6500 series
Juniper: EX series
Routers (with some bridging and/or security features) or switching routers
Cisco: 1800, 1900, 2600, 2800, 2900, 3700, 3800, 3900, 7200, 7600, ASR 1000
series
Juniper: MX series, J series, M series
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.
To understand better of switching router and routing switch differences,
following is an illustration. In early Cisco switches (i.e. Catalyst 3500
switches), there are only basic Layer-2 capabilities such as bridging and
switching. With newer models (i.e. Catalyst 3550 or 3560 switches), there are
also some routing capabilities such as terminating multiple Layer-3 interfaces
and running dynamic routing protocol. In router world, early Cisco routers
(i.e. 1600 or 2500 model), there are only basic Layer-3 capabilities such as
running dynamic routing protocol, terminating Serial ports, and running non-IP
protocols such as IPX and SNA. With newer models (i.e. 1700, 1800, 2600 or 2800
models), there are also some Layer-2 capabilities such as bridging and
switching. In addition there are some WIC (WAN Interface Cards) and NM (Network
Modules) with Ethernet ports supporting bridging and switching in those newer
router models even further such as WIC-4ESW Ethernet Switching card for 1700
series, HWIC-4ESW High-Density Ethernet Switching card for 1800 and 2800
series, and NM-16ESW Ethernet Switching module for 2600 and 2800 series.
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 and data-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. In Cisco world, this routing switch ASIC technology implementation as
example applies to Catalyst 6500 switch series. These kind of switches are
typically blade or module based switch which you have to specify which
"switch brain" (called Supervisor Engine in Cisco world) and which
port modules you like the switch to have.
In the case of a switching router as primarily a router that uses switching
technology (high-speed ASICs) for speed and performance (as well as also
supporting Layer-2 bridging functions), there are Cisco 7600 series and Juniper
MX series routers as examples. These kind of routers are typically blade or
module-based router which you have to specify which "router brain"
(also called Supervisor Engine in Cisco world) and which port modules you like
the router to have.
Further, the Cisco 7600 series router Supervisor Engine modules are compatible
with the Cisco Catalyst 6500 series switch due to identical architecture
between the router and the switch. In other words, you could use the same
Supervisor Engine model on either Cisco 7600 series router or Catalyst 6500
series switch.
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.
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