Routers connect multiple computer networks together and allow data to be sent between networks. Routers contain specialized operating systems, memory, and processors to perform routing and forwarding of information. Routers operate in both a control plane, where they learn the best paths between networks, and a forwarding plane, where they actually send packets along the paths. Common types of routers include edge routers, border routers, core routers, and access routers. Routing is the process of selecting the best path for sending data packets between source and destination devices across multiple networks using routers.

1. Concepts of Router and Routing
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2. Router
• A device that connects two or more computer networks
together is called as a Router
• It allows two or more different computer networks to
send data to each other
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3. Router Contd….
Fig.1
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4. Router Contd….
Fig.2
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5. Router Contd….
Fig.3
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6. Router Contd….
• A router is a computer whose software and hardware
are designed to perform the tasks of routing and
forwarding information (sending the information
to the destination).
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7. Router Contd….
Routers generally contain
• A specialized operating system
• RAM
• NVRAM
• Flash memory and
• one or more processors
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8. Router Contd….
• Routers operate in two different planes
• Control Plane: The router learns the outgoing interface
that is most appropriate for forwarding specific packets to
specific destinations
• Forwarding Plane: The router is responsible for the actual
process of sending a packet received on a logical interface
to an outbound logical interface
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9. Control Plane Activities
• Routers periodically disseminate topology information to all
the other routers with the help of control packets (e.g.
:HELLO, TC packets)
• Topology information contains the information about the one
hop and two hop neighbors to each of the router, link cost
etc
• Link cost can be defined in several terms: a time estimate, a
distance estimate, congestion estimate, bandwidth estimate
etc. for the communication link between router and its one
hop neighbor
• After learning the topology information, every router
computes shortest route from itself to every other router in
the Internet
• The computed shortest routes are stored in a route cache in
the form of routing table
• Routing table gives the nest hop router to which packet has
to be forwarded to send it in the shortest path for each
destination IP address ( network part of IP address) 9

10. Forward Plane Activities
• The Packet which arrive at Router’s interface is first
buffered (stored)
• The Router examines the packet’s header and the picks
the destination IP address to where the packet has to
reach
• It looks into the routing table to know the next hop router
to which the packet has to be forwarded in order to send it
in the shortest path
• Then Router forwards the packet to the corresponding
next Router address via one of its ports
• The same process is carried out at each Router along the
shortest path till the destination router is reached
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11. Types of Routers
Various types of routers
• Edge Router
• Border Router
• Core router
• Enterprise Router
• Access Router
• Distribution Router
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12. Symbol for Router
Fig.4
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13. 13
Router Connecting Various Subnets
TOKEN RI
NG
Fig.5
NETWORK
A BUS N
Y
Z
ROUTER
ETHERNE
X
T
NETWORK
C

14. Router Concepts
• Router must have three interfaces and three Network
Interface Cards (NICs) to interact with the networks A, B and
C at the junctions X and Y and Z as shown in figure 5
• In Internet, there are number of networks that communicate
with each other. Therefore, there would be as many routers
as networks
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15. Routing
• Routing is the process of selecting paths in a network to
send data or physical traffic
• Routing is performed for many kinds of networks,
including the telephone network , the Internet, and
transport networks
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16. Router Contd….
• Routing directs forwarding, the passing of logically
addressed packets from their source towards their
ultimate destination through intermediary nodes
• Nodes are hardware devices called routers, bridges,
gateways, firewalls, or switches
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17. Router Contd….
• Ordinary computers with multiple network cards 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
routers' memory, becomes very important for efficient
routing
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18. Router Contd….
• Therefore, Routing is the process of sending the data in
the form of packets called datagrams from the source to
the destination computers through many networks
• Routing algorithms take the decisions about routing
process and factors affecting them are as follows
• Least cost routing
• Distributed routing
• Packet life time
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19. ROUTING AND ROUTERS
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20. Summary
In this class, you have learnt about
• Router and routing concepts
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21. Quiz
1. A Router is connected to five networks.
Then number of NICs required by the
router is
a) 2
b) 3
c) 5
d) 7
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22. Frequently Asked Questions
1. Differentiate between Router and Routing?
2. List the various types of routers
3. Write short notes on routing
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