A
router is a device that forwards
data packets between
computer networks, creating an overlay
internetwork.
A router is connected to two or more data lines from different
networks. When a data packet comes in one of the lines, the router reads
the address information in the packet to determine its ultimate
destination. Then, using information in its
routing table or
routing policy, it directs the packet to the next network on its journey. Routers perform the "traffic directing" functions on the
Internet.
A data packet is typically forwarded from one router to another through
the networks that constitute the internetwork until it gets to its
destination node.
[1]
The most familiar type of routers are
home and small office routers that simply pass data, such as web pages and email, between the home computers and the owner's
cable or
DSL modem, which connects to the Internet through an
ISP. More sophisticated routers, such as enterprise routers, connect large business or ISP networks up to the powerful
core routers that forward data at high speed along the
optical fiber lines of the
Internet backbone. Though routers are typically dedicated hardware devices, use of software-based routers has grown increasingly common.
When multiple routers are used in interconnected networks, the
routers exchange information about destination addresses, using a
dynamic routing protocol. Each router builds up a table listing the
preferred routes between any two systems on the interconnected networks.
A router has interfaces for different physical types of network
connections, (such as copper cables, fiber optic, or wireless
transmission). It also contains
firmware for different networking
protocol
standards. Each network interface uses this specialized computer
software to enable data packets to be forwarded from one protocol
transmission system to another.
 |
|
A typical home or small office router showing the ADSL telephone line and Ethernet network cable connections |
Routers may also be used to connect two or more logical groups of computer devices known as
subnets, each with a different sub-
network address. The subnets addresses recorded in the router do not necessarily map directly to the physical interface connections.
[2] A router has two stages of operation called planes:
[3]
- Routing
-Routing is the process of selecting paths in a network along
which to send network traffic. Routing is performed for many kinds of
networks, including the
telephone network (
circuit switching),
electronic data networks (such as the
Internet), and
transportation networks. This article is concerned primarily with routing in electronic data networks using
packet switching technology.
In packet switching networks, routing directs
packet forwarding, the transit of logically addressed packets from their source toward their ultimate destination through intermediate
nodes, typically hardware devices called
routers,
bridges,
gateways,
firewalls, or
switches. General-purpose
computers
can also forward packets and perform routing, though they are not
specialized hardware and may suffer from limited performance. The
routing process usually directs forwarding on the basis of
routing tables
which maintain a record of the routes to various network destinations.
Thus, constructing routing tables, which are held in the router's
memory, is very important for efficient routing. Most routing algorithms use only one network path at a time, but
multipath routing techniques enable the use of multiple alternative paths.
Routing, in a more narrow sense of the term, is often contrasted with
bridging in its assumption that
network addresses
are structured and that similar addresses imply proximity within the
network. Because structured addresses allow a single routing table entry
to represent the route to a group of devices, structured addressing
(routing, in the narrow sense) outperforms unstructured addressing
(bridging) in large networks, and has become the dominant form of
addressing on the Internet, though bridging is still widely used within
localized environments.
Delivery semantics
Routing schemes differ in their delivery semantics:
- anycast delivers a message to any one out of a group of nodes, typically the one nearest to the source
- broadcast delivers a message to all nodes in the network
- multicast delivers a message to a group of nodes that have expressed interest in receiving the message
- unicast delivers a message to a single specific node
- geocast delivers a message to a geographic area
Unicast is the dominant form of message delivery on the Internet, and this article focuses on unicast routing algorithms.
- Routing protocol
A
routing protocol specifies how
routers communicate with each other, disseminating information that enables them to select routes between any two
nodes on a
computer network, the choice of the route being done by
routing algorithms. Each router has
a priori
knowledge only of networks attached to it directly. A routing protocol
shares this information first among immediate neighbors, and then
throughout the network. This way, routers gain knowledge of the topology
of the network. For a discussion of the concepts behind routing
protocols, see:
Routing.
The term
routing protocol may refer specifically to one operating at layer three of the
OSI model, which similarly disseminates topology information between routers.
Although there are many types of routing protocols, three major classes are in widespread use on
IP networks:
Many routing protocols are defined in documents called
RFCs.
[1][2][3][4]
The specific characteristics of routing protocols include
- the manner in which they either prevent routing loops from forming or break them up if they do
- the manner in which they select preferred routes, using information about hop costs
- the time they take to converge
- how well they scale up
- many other factors
- Static routing
Static routing is a concept describing one way of configuring
path selection of
routers in
computer networks. It is the type of
routing characterized by the absence of communication between routers regarding the current
topology of the
network.
[1] This is achieved by manually adding
routes to the routing table. The opposite of static routing is
dynamic routing, sometimes also referred to as
adaptive routing.
In these systems, routes through a data network are described by
fixed paths (statically). These routes are usually entered into the
router by the system administrator. An entire network can be configured
using static routes, but this type of configuration is not fault
tolerant. When there is a change in the network or a failure occurs
between two statically defined nodes, traffic will not be rerouted. This
means that anything that wishes to take an affected path will either
have to wait for the failure to be repaired or the static route to be
updated by the administrator before restarting its journey. Most
requests will time out (ultimately failing) before these repairs can be
made. There are, however, times when static routes can improve the
performance of a network. Some of these include
stub networks and
default routes.
Example
To configure a static route to network 10.10.20.0/24, pointing to a
next-hop router with the IP address of 192.168.100.1, type: (Note that
this example is written in the
Cisco IOS command line syntax and will only work on certain Cisco routers
[2])
ip route 10.10.20.0 255.255.255.0 192.168.100.1
| Destination network |
10.10.20.0 |
| subnet |
255.255.255.0 |
| next-hop |
192.168.100.1 |
The other option is to define a static route with reference to the
outgoing interface which is connected to the next hop towards the
destination network.
ip route 10.10.20.0 255.255.255.0 Serial 0/0
| Destination network |
10.10.20.0 |
| subnet |
255.255.255.0 |
| next-hop |
Serial interface 0/0 (local exit) | | | | | | |
- Dynamic Routing
Dynamic Routing or
Adaptive routing describes the capability of a system, through
which routes are characterized by their destination, to alter the path
that the route takes through the system in response to a change in
conditions.
[1]
The adaptation is intended to allow as many routes as possible to
remain valid (that is, have destinations that can be reached) in
response to the change.
People using a transport system can display adaptive routing. For
example, if a local railway station is closed, people can alight from a
train at a different station and use another method, such as a bus, to
reach their destination. Another example of adaptive routing can be seen
within
financial markets. For example, ASOR or Adaptive Smart Order Router (developed by
Quod Financial), takes routing decisions dynamically and based on real-time market events.
The term is commonly used in
data networking
to describe the capability of a network to 'route around' damage, such
as loss of a node or a connection between nodes, so long as other path
choices are available. There are several
protocols used to achieve this:
 |
| Dynamic IP Routing multicast packets |
Systems that do not implement adaptive routing are described as using
static routing,
where routes through a network are described by fixed paths
(statically). A change, such as the loss of a node, or loss of a
connection between nodes, is not compensated for. This means that
anything that wishes to take an affected path will either have to wait
for the failure to be repaired before restarting its journey, or will
have to fail to reach its destination and give up the journey.
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