Network switch
From Wikipedia, the free encyclopedia
A
network switch (sometimes known as a
switching hub) is a
computer networking device that is used to connect devices together on a
computer network. A switch is considered more advanced than a
hub
because a switch will only send a message to the device that needs or
requests it, rather than broadcasting the same message out of each of
its ports.
[1]
A switch is a multi-port
network bridge that processes and forwards data at the
data link layer (layer 2) of the
OSI model. Some switches have additional features, including the ability to
route packets. These switches are commonly known as
layer-3 or
multilayer switches. Switches exist for various types of networks including
Fibre Channel,
Asynchronous Transfer Mode,
InfiniBand,
Ethernet and others. The first Ethernet switch was introduced by
Kalpana in 1990.
[2]
Overview
A switch is a device used on a
computer network
to physically connect devices together. Multiple cables can be
connected to a switch to enable networked devices to communicate with
each other. Switches manage the flow of data across a network by only
transmitting a received message to the device for which the message was
intended. Each networked device connected to a switch can be identified
using a
MAC address,
allowing the switch to regulate the flow of traffic. This maximises
security and efficiency of the network. Because of these features, a
switch is often considered more "intelligent" than a
network hub.
Hubs neither provide security, or identification of connected devices.
This means that messages have to be transmitted out of every port of the
hub, greatly degrading the efficiency of the network.
Network design
An Ethernet switch operates at the data link layer of the
OSI model to create a separate
collision domain
for each switch port. With four computers (e.g., A, B, C and D) on four
switch ports, any pair (e.g. A and B) can transfer data back and forth
while the other pair (e.g. C and D) also do so simultaneously, and the
two conversations will not interfere with one another. In
full duplex mode, these pairs can also overlap (e.g. A transmits to B, simultaneously B to C, and so on). In the case of using a
repeater hub, they would all share the bandwidth and run in
half duplex, resulting in collisions which would require retransmissions.
Applications
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.
Microsegmentation
Segmentation is the use of a bridge or a switch (or a router) to
split a larger collision domain into smaller ones in order to reduce
collision probability and improve overall throughput. In the extreme, i.
e. microsegmentation, each device is located on a dedicated switch
port. In contrast to an Ethernet hub, there is a separate collision
domain on each of the switch ports. This allows computers to have
dedicated bandwidth on point-to-point connections to the network and
also to run in full-duplex without collisions. Full-duplex mode has only
one transmitter and one receiver per 'collision domain', making
collisions impossible.
Role of switches in a network
Switches may operate at one or more layers of the
OSI model,
including the data link and network layers. 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.
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
firewall,
[3][4] network
intrusion detection,
[5] and performance analysis modules that can plug into switch ports. Some of these functions may be on combined modules.
[6]
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
A modular network switch with three network modules (a total of 24 Ethernet and 14 Fast Ethernet ports) and one power supply.
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 versus higher-layer switches)
A
network hub,
or repeater, is a simple network device. Repeater hubs do not manage
any of the traffic that comes through them. Any packet entering a
port
is flooded 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.
A switch creates the – originally mandatory – Layer 1 end-to-end
connection only virtually. Its bridge function selects which packets are
forwarded to which port(s) on the basis of information taken from layer
2 (or higher), removing the requirement that every node be presented
with all data. The connection lines are not "switched" literally, it
only appears like this on the packet level. "Bridging hub", "switching
hub", or "multiport bridge" would be more appropriate terms.
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.
[7]
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 reconverged. A
Rapid Spanning Tree Protocol was introduced as IEEE
802.1w. The newest standard
Shortest path bridging (
IEEE 802.1aq) is the next logical progression and incorporates all the older Spanning Tree Protocols (
IEEE 802.1D STP,
IEEE 802.1w RSTP,
IEEE 802.1s
MSTP) that blocked traffic on all but one alternative path. IEEE
802.1aq (Shortest Path Bridging SPB) allows all paths to be active with
multiple equal cost paths, provides much larger layer 2 topologies (up
to 16 million compared to the 4096 VLANs limit),
[8]
faster convergence, and improves the use of the mesh topologies through
increase bandwidth and redundancy between all devices by allowing
traffic to load share across all paths of a mesh network.
[9][10][11][12]
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. '
Non-blocking' devices use a forwarding plane or equivalent method fast enough to allow full duplex traffic for each port simultaneously.
Once a bridge learns the addresses of its connected nodes, 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.
[13]
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.
[14]
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.
Configuration options
- Unmanaged switches — These switches have no configuration interface or options. They are plug and play. They are typically the least expensive switches, and therefore often used in a small office/home office environment. Unmanaged switches 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 into 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.[15]
- 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.
Typical switch management features
Traffic monitoring on a switched network
Unless port mirroring or other methods such as
RMON, SMON or
sFlow are implemented in a switch,
[16]
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.
See also
References
- Jump up ^ "Hubs Versus Switches — Understand the Tradeoffs" (PDF). ccontrols.com. 2002. Retrieved 2013-12-10.
- Jump up ^ Robert J. Kohlhepp (2000-10-02). "The 10 Most Important Products of the Decade". Network Computing. Retrieved 2008-02-25.
- Jump up ^ Cisco Catalyst 6500 Series Firewall Services Module, Cisco Systems,2007
- Jump up ^ Switch 8800 Firewall Module, 3Com Corporation, 2006
- Jump up ^ Cisco Catalyst 6500 Series Intrusion Detection System (IDSM-2) Module, Cisco Systems,2007
- Jump up ^ Getting Started with Check Point Fire Wall-1, Checkpoint Software Technologies Ltd., n.d.
- Jump up ^ Matthew Glidden (October 2001). "Switches and Hubs". About This Particular Macintosh blog. Retrieved June 9, 2011.
- Jump up ^ Shuang Yu. "IEEE APPROVES NEW IEEE 802.1aq™ SHORTEST PATH BRIDGING STANDARD". IEEE Standards Association. Retrieved 19 June 2012.
"Using the IEEE’s next-generation VLAN, called a Service Interface
Identifier (I-SID), it is capable of supporting 16 million unique
services compared to the VLAN limit of four thousand."
- Jump up ^ Peter Ashwood-Smith (24 Feb 2011). "Shortest Path Bridging IEEE 802.1aq Overview". Huawei. Retrieved 11 May 2012.
- Jump up ^ Jim Duffy (11 May 2012). "Largest Illinois healthcare system uproots Cisco to build $40M private cloud". PC Advisor. Retrieved 11 May 2012. "Shortest Path Bridging will replace Spanning Tree in the Ethernet fabric."
- Jump up ^ "IEEE Approves New IEEE 802.1aq Shortest Path Bridging Standard". Tech Power Up. 7 May 2012. Retrieved 11 May 2012.
- Jump up ^ D. Fedyk, Ed.,; P. Ashwood-Smith, Ed.,; D. Allan, A. Bragg,; P. Unbehagen (April 2012). "IS-IS Extensions Supporting IEEE 802.1aq". IETF. Retrieved 12 May 2012.
- Jump up ^ S. Sathaye (January 1999), The Ins and Outs of Layer 4+ Switching,
NANOG 15, "It usually means one of two things: - 1. Layer 4 information
is used to prioritize and queue traffic (routers have done this for
years) - 2. Layer 4 information is used to direct application sessions
to different servers (next generation load balancing)."
- Jump up ^ How worried is too worried? Plus, a Global Crossing Story., NANOG mailing list archives, S. Gibbard,October 2001
- Jump up ^ Tech specs for a sample HP "web-managed" switch (archived page from web.archive.org)
- Jump up ^ Remote Network Monitoring Management Information Base, RFC 2819, S. Waldbusser,May 2000
External links
