A network switch or switching hub is a
computer networking device that connects multiple computers together within one
local area network (LAN). Technically, network switches operate at layer two
(Data Link Layer) of the OSI model.
The network switch commonly refers to a
multi-port network bridge that processes and routes data at the data link layer
(layer 2) of the OSI model. Switches that additionally process data at the
network layer (layer 3) and above are often referred to as layer-3 switches or
multilayer switches.
Function
of Network Switch
A network switch is a telecommunication
device which receives a message from any device connected to it and then
transmits the message only to that device for which the message was meant. This
makes the switch a more intelligent device than hub (which receives a message
and then transmits it all the other devices on its network). The network switch
plays an integral part in most modern Ethernet local area networks (LANs).
Mid-to-large sized LANs contain a number of linked managed switches. Small
office/home office (SOHO) applications typically use a single switch, or an
all-purpose converged device such as a residential gateway to access small
office/home broadband services such as DSL or cable internet. In most of these
cases, the end-user device contains a router and components that interface to
the particular physical broadband technology. User devices may also include a
telephone interface for VoIP.
An Ethernet
switch operates at the data link layer of the OSI model to create a
separate collision domain for each switch port. With 4 computers (e.g., A, B,
C, and D) on 4 switch ports, A and B can transfer data back and forth, while C
and D also do so simultaneously, and the two conversations will not interfere
with one another. In the case of a hub, they would all share the bandwidth and
run in half duplex, resulting in collisions, which would then necessitate
retransmissions. Using a switch is called microsegmentation. This allows
computers to have dedicated bandwidth on a point-to-point connection to the
network and to therefore run in full duplex without collisions.
Role
of Switches in Networks
Switches may operate at one or more layers
of the OSI model, including data link and network. A device that operates
simultaneously at more than one of these layers is known as a multilayer
switch.
In switches intended for commercial use,
built-in or modular interfaces make it possible to connect different types of
networks, including Ethernet, Fibre Channel, ATM, ITU-T G.hn and 802.11. This
connectivity can be at any of the layers mentioned. While layer-2 functionality
is adequate for bandwidth-shifting within one technology, interconnecting
technologies such as Ethernet and token ring is easier at layer 3.
Devices that interconnect at layer 3 are
traditionally called routers, so layer-3 switches can also be regarded as
(relatively primitive) routers.
In some service provider and other
environments where there is a need for a great deal of analysis of network
performance and security, switches may be connected between WAN routers as
places for analytic modules. Some vendors provide firewallnetwork intrusion
detection,[4] and performance analysis modules that can plug into switch ports.
Some of these functions may be on combined modules.
In other cases, the switch is used to
create a mirror image of data that can go to an external device. Since most
switch port mirroring provides only one mirrored stream, network hubs can be
useful for fanning out data to several read-only analyzers, such as intrusion
detection systems and packet sniffers.
Layer-specific
Functionality
While switches may learn about topologies
at many layers, and forward at one or more layers, they do tend to have common
features. Other than for high-performance applications, modern commercial
switches use primarily Ethernet interfaces.
At any layer, a modern switch may implement
power over Ethernet (PoE), which avoids the need for attached devices, such as
a VoIP phone or wireless access point, to have a separate power supply. Since
switches can have redundant power circuits connected to uninterruptible power
supplies, the connected device can continue operating even when regular office
power fails.
Layer
1 Hubs vs. higher-layer switches
A network hub, or repeater, is a simple
network device. Hubs do not manage any of the traffic that comes through them.
Any packet entering a port is broadcast out or "repeated" on every
other port, except for the port of entry. Since every packet is repeated on
every other port, packet collisions affect the entire network, limiting its
capacity.
There are specialized applications where a
hub can be useful, such as copying traffic to multiple network sensors. High
end switches have a feature which does the same thing called port mirroring.
By the early 2000s, there was little price
difference between a hub and a low-end switch.
Layer
2
A network bridge, operating at the data
link layer, may interconnect a small number of devices in a home or the office.
This is a trivial case of bridging, in which the bridge learns the MAC address
of each connected device.
Single bridges also can provide extremely
high performance in specialized applications such as storage area networks.
Classic bridges may also interconnect using
a spanning tree protocol that disables links so that the resulting local area
network is a tree without loops. In contrast to routers, spanning tree bridges
must have topologies with only one active path between two points. The older
IEEE 802.1D spanning tree protocol could be quite slow, with forwarding
stopping for 30 seconds while the spanning tree would reconverge. A Rapid
Spanning Tree Protocol was introduced as IEEE 802.1w, but the newest edition of
IEEE 802.1D adopts the 802.1w extensions as the base standard.
The IETF is specifying the TRILL protocol,
which is the application of link-state routing technology to the layer-2
bridging problem. Devices which implement TRILL, called RBridges, combine the
best features of both routers and bridges.
While
layer 2 switch remains more of a marketing term than a technical
term,[citation needed] the products that were introduced as
"switches" tended to use microsegmentation and Full duplex to prevent
collisions among devices connected to Ethernet. By using an internal forwarding
plane much faster than any interface, they give the impression of simultaneous
paths among multiple devices.
Once a bridge learns the topology through a
spanning tree protocol, it forwards data link layer frames using a layer 2
forwarding method. There are four forwarding methods a bridge can use, of which
the second through fourth method were performance-increasing methods when used
on "switch" products with the same input and output port bandwidths:
Store
and forward: The switch buffers and verifies each
frame before forwarding it.
Cut
through: The switch reads only up to the frame's
hardware address before starting to forward it. Cut-through switches have to
fall back to store and forward if the outgoing port is busy at the time the
packet arrives. There is no error checking with this method.
Fragment
free: A method that attempts to retain the benefits
of both store and forward and cut through. Fragment free checks the first 64
bytes of the frame, where addressing information is stored. According to
Ethernet specifications, collisions should be detected during the first 64
bytes of the frame, so frames that are in error because of a collision will not
be forwarded. This way the frame will always reach its intended destination.
Error checking of the actual data in the packet is left for the end device.
Adaptive
switching: A method of automatically selecting
between the other three modes.
While there are specialized applications,
such as storage area networks, where the input and output interfaces are the
same bandwidth, this is not always the case in general LAN applications. In
LANs, a switch used for end user access typically concentrates lower bandwidth
and uplinks into a higher bandwidth.
Layer
3
Within the confines of the Ethernet
physical layer, a layer-3 switch can perform some or all of the functions
normally performed by a router. The most common layer-3 capability is awareness
of IP multicast through IGMP snooping. With this awareness, a layer-3 switch
can increase efficiency by delivering the traffic of a multicast group only to
ports where the attached device has signaled that it wants to listen to that
group.
Layer
4
While the exact meaning of the term layer-4
switch is vendor-dependent, it almost always starts with a capability for
network address translation, but then adds some type of load distribution based
on TCP sessions.
The device may include a stateful firewall,
a VPN concentrator, or be an IPSec security gateway.
Layer
7
Layer-7 switches may distribute loads based
on Uniform Resource Locator URL or by some installation-specific technique to
recognize application-level transactions. A layer-7 switch may include a web
cache and participate in a content delivery network.
Types
of switches
Form factor
- Desktop, not mounted in an enclosure, typically intended to be
used in a home or office environment outside of a wiring closet
- Rack mounted - A switch that mounts in an equipment rack
- Chassis - with swappable module cards
- DIN rail mounted - normally seen in industrial
environments or panels
Configuration
options
Unmanaged switches — these switches have no
configuration interface or options. They are plugandplay. They are typically
the least expensive switches, found in home, SOHO, or small businesses. They
can be desktop or rack mounted.
Managed
switches — these switches have one or more methods
to modify the operation of the switch. Common management methods include: a
command-line interface (CLI) accessed via serial console, telnet or Secure
Shell, an embedded Simple Network Management Protocol (SNMP) agent allowing
management from a remote console or management station, or a web interface for
management from a web browser. Examples of configuration changes that one can
do from a managed switch include: enable features such as Spanning Tree
Protocol, set port bandwidth, create or modify Virtual LANs (VLANs), etc. Two
sub-classes of managed switches are marketed today:
Smart
(or intelligent) switches — these are managed
switches with a limited set of management features. Likewise
"web-managed" switches are switches which fall in a market niche
between unmanaged and managed. For a price much lower than a fully managed
switch they provide a web interface (and usually no CLI access) and allow
configuration of basic settings, such as VLANs, port-bandwidth and duplex.
Enterprise
Managed (or fully managed) switches — these have a
full set of management features, including CLI, SNMP agent, and web interface.
They may have additional features to manipulate configurations, such as the
ability to display, modify, backup and restore configurations. Compared with
smart switches, enterprise switches have more features that can be customized
or optimized, and are generally more expensive than smart switches. Enterprise
switches are typically found in networks with larger number of switches and
connections, where centralized management is a significant savings in administrative
time and effort. A stackable switch is a version of enterprise-managed switch.
Traffic
Monitoring on a Switched Network
Unless port mirroring or other methods such
as RMON, SMON or sFlow are implemented in a switch,[10] it is difficult to
monitor traffic that is bridged using a switch because only the sending and
receiving ports can see the traffic. These monitoring features are rarely
present on consumer-grade switches.
Two popular methods that are specifically
designed to allow a network analyst to monitor traffic are:
Port
mirroring — the switch sends a copy of network
packets to a monitoring network connection.
SMON
— "Switch Monitoring" is described by RFC
2613 and is a protocol for controlling facilities such as port mirroring.
Another method to monitor may be to connect
a layer-1 hub between the monitored device and its switch port. This will
induce minor delay, but will provide multiple interfaces that can be used to
monitor the individual switch port.
Typical
Switch Management Features
HP Procurve rack-mounted switches mounted
in a standard Telco Rack 19-inch rack with network cables
Turn particular port
range on or off
Link bandwidth and
duplex settings
Priority settings for
ports
IP Management by IP
Clustering.
MAC filtering and
other types of "port security" features which prevent MAC flooding
Use of Spanning Tree
Protocol
SNMP monitoring of
device and link health
Port mirroring (also
known as: port monitoring, spanning port, SPAN port, roving analysis port or
link mode port)
Link aggregation
(also known as bonding, trunking or teaming)
VLAN settings
802.1X network access
control
IGMP snooping
Link aggregation allows the use of multiple
ports for the same connection achieving higher data transfer rates. Creating
VLANs can serve security and performance goals by reducing the size of the
broadcast domain.
More Reading at http://en.wikipedia.org/wiki/Network_switch
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