Download Introduction to networking

Introduction to networking
(Yarnfield)
Introduction to routing
Objectives
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Describe a routers primary aim
Differentiate between ports and interfaces
Describe the basic router configuration
Describe the routing table
Router overview
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Routers are the centre of a network
They can connect multiple networks
They are responsible for the efficient delivery of packets
Routers have interfaces that belong to different networks
Routers can forward packets to the network of the final
destination, or to another router that is used to reach the
final destination
Each network typically requires a separate physical
interface
Routes determine the best path
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The primary activity of the router is to forward packets
to local or remote networks
This is done by
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Determining the best path to send packets
Forwarding packets to their destination
This is achieved via the routing table
Received packets are referenced with the routing table
When a match is found the router will encapsulate the
packet into the corresponding data-link frame of the
outgoing interface and forward the packet toward its
destination
Ports and interfaces
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Ports usually refer to management ports, such as the
console port
Interfaces usually refer to the connectors that send and
receive user data
Interfaces come in two major categories
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LAN interfaces
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E.g. Fast Ethernet – must have an IP and subnet mask
WAN interfaces
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ISDN, Frame relay – must have an IP address
Forwarding packets
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Routers are considered layer 3 devices because their
forwarding decision is based upon the IP packet
Routing tables are searched for matching IP addresses
based upon a received packet, and forwarded to the next
router if no match has been found
Eventually a router will receive a packet that can forward
it to a directly connected interface, which will be the
packets final destination
Routers operate at layers 1, 2 and 3 of the OSI model
Basic router configuration
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Name the router
Set passwords
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Configure interfaces
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Console, AUX, telnet and secret
IP address, subnet mask, description (240 chars), clock rate, turn it
on!
Banner
Save changes
Verify configuration
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Show running-config
Show ip route
Show ip interface brief
Show interfaces
The routing table
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Routing tables store information about directly connected and
remote networks
They tell a router what the association between the ‘next hop’
and a network is
A directly connected network is one that is directly attached to
the routers interface
This happens when the interface is configured with an IP
address and subnet mask of a given network
A remote network is one where the router cannot connect to it
without the help of another router
This is possible via static and dynamic routing protocols
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Directly connected routes – visiting a neighbour
Static routes – used by trains
Dynamic routes – driving a car
Example
Routing table principles
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Every router make sits decision alone, based on the
information it has in its own routing table
The fact that one router has certain information in its
routing table does not mean that other routers have
the same information
Routing information about a path from one network to
another does not provide routing information about the
reverse, or return path
Asymmetric routing – packets can traverse a network in
one direction, using one path and return through
another path
Routing table principles
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PC1
2
R1
3
R2
PC2
5
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4.
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PC1 sends ping to PC2
RA has a route to PC2’s network
R2 is directly connected to PC2’s network
PC2 sends a reply ping to PC1
R2 does NOT have a route to PC1’s network, so it drops the packet
Routing factors
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Equal cost load balancing
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Unequal cost paths (IGRP
and EIGRP)
Packet forwarding involves
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Path determination via
metrics
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Directly connected network
– packet sent to host directly
Remote network – packet
sent to next router
No route determined –
packet discarded
Metrics
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Bandwidth – the data capacity of a link
Delay – the length of time required to move a packet
along each link from source to destination
Load – the amount of activity on a network resource such
as a router or a link
Reliability – an error rate of each network link
Hop count – the number of routers that a packet must
travel through before breaching its destination
Cost – an arbitrary value, usually base don bandwidth,
monetary expense, or other measurement, that is
assigned by an administrator
Routing factors
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Switching function
Accept packet on one interface and switch it out of another
1. Decapsulate layer 3 packet
2. Examine destination IP address to find best path
3. Encapsulate layer 3 packet into new layer 2 frame and
forward frame out of exit interface
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If TTL field reaches 0 then packet is discarded
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Layer 3 IP addresses do not change during routing, but
layer 2 data-link addresses change at every hop
Summary
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Routers have multiple network interfaces representing
multiple networks
The primary activity of a router is to forward packets via
the best path
Routers use a routing table to determine the best path
Ports and interfaces refer to two different aspects
Routers work at layers 1, 2 and 3 of the OSI model, with
layer 3 being the primary operating layer
Show ip route – shows the routing table
Each router interface must belong to a different network
Questions...
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...are there any?