Network-to-Network Communications : ARP operation within a subnet
Network Layer (Part V)
Network-to-Network Communications :
ARP operation within a subnet
• If a host wants to send data to another
host, it must know the destination IP
address.
• If it is unable to locate a MAC address
for the destination in its own ARP
table, the host initiates a process
called an ARP request.
• An ARP request enables it to discover
the destination MAC address..
Network-to-Network Communications :
ARP operation within a subnet
Network-to-Network Communications :
ARP operation within a subnet
Network-to-Network Communications :
ARP operation within a subnet
• A host builds an ARP request packet
and sends it to all devices on the
network.
• To ensure that all devices see the
ARP request, the source uses a
broadcast MAC address.
• The broadcast address in a MAC
addressing scheme has all places set
to hexadecimal F.
• Thus, a MAC broadcast address
Network-to-Network Communications :
ARP operation within a subnet
Network-to-Network Communications :
ARP operation within a subnet
• Because ARP request packets travel in a
broadcast mode, all devices on the local
network receive the packets and pass
them up to the network layer for further
examination.
• If the IP address of a device matches the
destination IP address in the ARP request,
that device responds by sending the
source its MAC address.
• This is known as the ARP reply
Network-to-Network Communications :
ARP operation within a subnet
Network-to-Network Communications :
ARP operation within a subnet
• Example:
Source device 197.15.22.33 is asking
for the MAC address of the
destination with IP address
197.15.22.126, Destination device
197.15.22.126 picks up the ARP
request and responds with an ARP
reply containing its MAC address.
Network-to-Network Communications :
ARP operation within a subnet
• Once the originating device receives the
ARP reply, it extracts the MAC address from
the MAC header, and updates its ARP table.
• The originating device can then properly
address its data with both, a destination
MAC address, and a destination IP address.
• It uses this new information to perform
Layer 2 and Layer 3 encapsulations of the
data, before it sends them out over the
network.
Network-to-Network Communications :
ARP operation within a subnet
• When the data arrives at the destination, the data
link layer makes a match, strips of the MAC header,
and transfers the data up to the network layer.
• The network layer examines the data and fnds that
the IP address matches the destination IP address
carried in the IP header.
• The network layer strips of the IP header, and
transfers the encapsulated data to the next highest
layer in the OSI model, the transport layer (Layer 4).
• This process is repeated until the rest of the packet's
partially decapsulated data reaches the application,
where the user data may be read.
Advanced ARP Concepts :
Default gateway
• In order for a device to communicate with
another device on another network, you
must supply it with a default gateway.
• A default gateway is the IP address of the
interface on the router that connects to
the network segment which the source
host is located on.
• The default gateway’s IP address must be
in the same network segment as the
source host.
Advanced ARP Concepts :
Default gateway
• If
no
default
gateway
is
defned,
communication is possible only on the device’s
own logical network segment.
• The computer that sends the data does a
comparison between the IP address of the
destination and its own ARP table.
• If it fnds no match, it must have a default IP
address to use.
• Without a default gateway, the source
computer has no destination MAC address, and
the message is undeliverable.
Advanced ARP Concepts : Problems with
sending data to nodes on diferent subnets
• One of the major problems in networking
is how to communicate with devices that
are not on the same physical network
segment.
• There are two parts to the problem.
• The frst is obtaining the MAC address of
the destination host, and the second is
transferring the data packets from one
network segment to another, to get to the
destination host.
Advanced ARP Concepts : How ARP
sends data to remote networks
• ARP uses broadcast packets to accomplish its function.
• Routers, however, do not forward broadcast packets.
• In order for a device to send data to the address of a
device that is on another network segment, the source
device sends the data to a default gateway.
• The default gateway is the IP address of the router
interface that is connected to the same physical network
segment as the source host.
• The source host compares the destination IP address and
its own IP address to determine if the two IP addresses are
located on the same segment.
• If the receiving host is not on the same segment, the
source host sends the data to the default gateway.
Advanced ARP Concepts : Proxy
ARP
• Proxy ARP is a variation of the ARP protocol.
• In this case an intermediate device (e.g.
router) sends an ARP response, on behalf of
an end node, to the requesting host.
• Routers running proxy ARP capture ARP
packets.
• They respond with their MAC addresses for
those requests in which the IP address is not
in the range of addresses of the local subnet.
Advanced ARP Concepts : Proxy
ARP
• In the previous description of how data is sent
to a host on a diferent subnet, the default
gateway is confgured.
• If the source host does not have a default
gateway confgured, it sends an ARP request.
• All hosts on the segment, including the router,
receive the ARP request.
• The router compares the IP destination address
with the IP subnet address to determine if the
destination IP address is on the same subnet as
the source host.
Advanced ARP Concepts : Proxy
ARP
• If the subnet address is the same, the router
discards the packet.
• The reason that the packet is discarded is that
the destination IP address is on the same
segment as the source's IP address.
• This means another device on the segment
should respond to the ARP request.
• The exception to this is that the destination IP
address is not currently assigned, which will
generate an error response on the source host.
Advanced ARP Concepts : Proxy
ARP
• If the subnet address is diferent, the router will
respond with its own MAC address for the
interface that is directly connected to the
segment on which the source host is located.
• This is the proxy ARP. Since the MAC address is
unavailable for the destination host, the router
supplies its MAC address in order to get the
packet.
• Then the router can forward the ARP request
(based on the destination IP address) to the
proper subnet for delivery.
Advanced ARP Concepts:Four
Layer 3 fowcharts
Advanced ARP Concepts:Four
Layer 3 fowcharts
• Create fowcharts for the following
processes:
– ARP
– RARP
– BOOTP
– DHCP
Routable Protocols : Routed
protocols
• IP is a network layer protocol, and
because of that, it can be routed
over an internetwork, which is a
network of networks.
• Protocols that provide support for
the network layer are called routed
or routable protocols.
Routable Protocols:Other routed
protocols
• The focus of this course is on the
most
commonly
used
routable
protocol, which is IP.
• Even though you will concentrate on
IP, it is important to know that there
are other routable protocols.
• Two of them are IPX/SPX and
AppleTalk.
Routable Protocols : Routable
and non-routable protocols
• Protocols such as IP, IPX/SPX and AppleTalk
provide Layer 3 support and are, therefore,
routable.
• However, there are protocols that do not
support Layer 3; these are classed as nonroutable protocols.
• The most common of these non-routable
protocols is NetBEUI.
• NetBEUI is a small, fast, and efcient protocol
that is limited to running on one segment.
Routable Protocols :
Characteristics of a routable
protocol
• In order for a protocol to be routable, it must
provide the ability to assign a network number, as
well as a host number, to each individual device.
• Some protocols, such as IPX, only require that you
assign a network number; they use a host's MAC
address for the physical number.
• Other protocols, such as IP, require that you
provide a complete address, as well as a subnet
mask.
• The network address is obtained by ANDing the
address with the subnet mask.
Routing Protocols:Examples of
routing protocols
• Routing protocols (Note: Do not
confuse with routed protocols.)
determine the paths that routed
protocols follow to their destinations.
• Examples of routing protocols include
the Routing Information Protocol
(RIP), the Interior Gateway Routing
Protocol
(IGRP),
the
Enhanced
Interior Gateway Routing Protocol
(EIGRP), and Open Shortest Path First
Routing Protocols:Examples of
routing protocols
• Routing protocols enable routers that
are connected, to create a map,
internally, of other routers in the
network or on the Internet.
• This allows routing (i.e. selecting the
best path, and switching) to occur.
Such maps become part of each
router's routing table.
Routing Protocols :Defnition of
routing protocol
• Routers use routing protocols to
exchange routing tables and to share
routing information.
• Within a network, the most common
protocol used to transfer routing
information between routers, located
on the same network, is Routing
Information Protocol (RIP).
Routing Protocols :Defnition of
routing protocol
• This Interior Gateway Protocol (IGP) calculates
distances to a destination host in terms of how
many hops (i.e. how many routers) a packet
must pass through.
• RIP enables routers to update their routing
tables at programmable intervals, usually every
30 seconds.
• One disadvantage of routers that use RIP is that
they are constantly connecting to neighboring
routers to update their routing tables, thus
creating large amounts of network trafc.
Routing Protocols :Defnition of
routing protocol
• RIP allows routers to determine
which path to use to send data. It
does so by using a concept known as
distance-vector.
• Whenever data goes through a
router, and thus, through a new
network number, this is considered
to be equal to one hop.
Routing Protocols :Defnition of
routing protocol
• A path which has a hop count of four
indicates that data traveling along
that path would have to pass through
four routers before reaching the fnal
destination on the network.
• If there are multiple paths to a
destination, the path with the least
number of hops would be the path
chosen by the router.
Routing Protocols :Defnition of
routing protocol
• Because hop count is the only routing metric used
by RIP, it doesn’t necessarily select the fastest path
to a destination.
• A metric is a measurement for making decisions. You
will soon learn that other routing protocols use many
other metrics besides hop count to fnd the best
path for data to travel.
• Nevertheless, RIP remains very popular, and is still
widely implemented.
• This may be due primarily to the fact that it was one
of the earliest routing protocols to be developed
Routing Protocols :Defnition of
routing protocol
• One other problem posed by the use of
RIP is that sometimes a destination may
be located too far away to be reachable.
• When using RIP, the maximum number
of hops that data can be forwarded
through is ffteen.
• The destination network is considered
unreachable if it is more than ffteen
router hops away.
•
•
•
•
•
Routing Protocols : Routing
encapsulation sequence
At the data link layer, an IP datagram is
encapsulated into a frame.
The datagram, including the IP header, is
treated as data.
A router receives the frame, strips of the
frame header, then checks the destination
IP address in the IP header.
The router then looks for that destination
IP address in its routing table,
encapsulates the data in a data link layer
frame, and sends it out to the appropriate
interface.
If it does not fnd the destination IP
Routing Protocols : Multiprotocol routing
• Routers are capable of supporting
multiple
independent
routing
protocols, and of maintaining routing
tables for several routed protocols,
concurrently.
• This capability allows a router to
deliver packets from several routed
protocols over the same data links.
Other Network Layer
Services : Connectionless
network services
• Most network services use a connectionless
delivery system.
• They treat each packet separately, and
send it on its way through the network.
• The packets may take diferent paths to get
through the network, but are reassembled
when they arrive at the destination.
• In a connectionless system the destination
is not contacted before a packet is sent
Other Network Layer Services :
Connectionless network services
• A good analogy for a connectionless
system is a postal system.
• The recipient is not contacted before
a letter is sent from one destination
to another.
• The letter is sent on its way, and the
recipient learns of the letter when it
arrives.
Other Network Layer Services :
Connection-oriented network services
• In connection-oriented systems, a
connection is established between
the sender and the recipient before
any data is transferred.
• An example of a connection-oriented
network is the telephone system.
• You place a call, a connection is
established, and then communication
occurs.
Other Network Layer Services :
Comparing connectionless and
connection-oriented network processes
• Connectionless network processes are often
referred to as packet switched.
• In these processes, as the packets pass from
source to destination, they can switch to
diferent paths, as well as (possibly) arrive
out of order.
• Devices make the path determination for
each packet based on a variety of criteria.
• Some of the criteria (e.g. available
bandwidth) may difer from packet to packet.
Other Network Layer Services :
Comparing connectionless and
connection-oriented network processes
• Connection-oriented
network
processes are often referred to as
circuit switched.
• These
processes
establish
a
connection with the recipient, frst,
and then begin the data transfer.
• All packets travel sequentially across
the same physical circuit, or more
commonly, across the same virtual
circuit
Other Network Layer Services :
Comparing connectionless and
connection-oriented network processes
• The Internet is one huge connectionless
network in which all packet deliveries
are handled by IP.
• TCP (Layer 4) adds connection-oriented
services on top of IP (Layer 3).
• TCP segments are encapsulated into IP
packets for transport across the Internet.
• TCP
provides
connection-oriented
session services to reliably deliver data.
Other Network Layer Services:IP
and the transport layer
• IP is a connectionless system; it treats each packet
independently.
• For example, if you use an FTP program to download a
fle, IP does not send the fle in one long stream of data.
• It treats each packet independently. Each packet can
travel diferent paths.
• Some may even get lost.
• IP relies on the transport layer protocol to determine
whether packets have been lost, and to request
retransmission.
• The transport layer is also responsible for reordering the
packets.
ARP Tables:Internetworking
devices that have ARP tables
• You have learned that the port, or
interface, where a router connects to a
network, is considered part of that
network; therefore, the router interface
connected to the network has an IP
address for that network.
• Routers, just like every other device on
the network, send and receive data on the
network, and build ARP tables that map IP
addresses to MAC addresses.
ARP Tables :Comparing router ARP tables with
ARP tables kept by other networking devices
• Routers can be connected to multiple
networks, or subnetworks.
• Generally speaking, network devices map
the IP addresses and MAC addresses that
they see on a regular and repeated basis.
• This means that a typical device contains
mapping information pertaining only to
devices on its own network.
• It knows very little about devices beyond its
LAN.
ARP Tables :Comparing router ARP tables with
ARP tables kept by other networking devices
• Routers build tables that describe all
networks connected to them.
• ARP tables kept by routers can
contain IP addresses and MAC
addresses of devices located on
more than one network
ARP Tables :Comparing router ARP tables with
ARP tables kept by other networking devices
• In addition to mapping IP addresses
to MAC addresses, router tables also
map ports.
• Can you think of a reason why
routers would need to do this? (Note:
Examine the router's ARP table
below.)
ARP Tables : Other router table
addresses
• What happens if a data packet reaches a router that is
destined for a network to which it is not connected?
• In addition to IP addresses and MAC addresses of
devices located on networks to which it connects, a
router also possesses IP addresses and MAC addresses
of other routers.
• It uses these addresses to direct data toward its fnal
destination.
• If a router receives a packet whose destination address
is not in its routing table, it forwards it to the address of
another router that most likely does contain information
about the destination host in its routing table.
ARP Tables : ARP requests and
ARP replies
• ARP is used only on a local network.
• What would happen if a local router
wanted to ask a non-local router to
provide indirect routing (next-hop)
services, but did not know the MAC
address of the non-local router?
ARP Tables : ARP requests and
ARP replies
• When a router does not know the MAC address
of the next-hop router, the source router (router
that has the data to be sent on) issues an ARP
request.
• A router that is connected to the same segment
as the source router receives the ARP request.
• This router issues an ARP reply to the router
that originated the ARP request.
• The reply contains the MAC address of the nonlocal router.
ARP Tables:Proxy ARP
• A device on one network cannot send
an ARP request to a device on
another network.
• Can you think of a reason why this is
so?
ARP Tables:Proxy ARP
• What happens in the case of
subnetworks?
• Can a device on one subnetwork fnd
the MAC address of a device on
another subnetwork?
• The answer is yes, provided the
source directs its question to the
router.
• Working through a third party is
called proxy ARP, and it allows the
ARP Tables : Indirect
routing
• Sometimes a source resides on a network
that has a diferent network number than
the desired destination.
• If the source doesn't know the MAC address
of the destination it must use the services
of a router.
• With the router's aid, the source's data can
reach its destination.
• A router that is used for this purpose is
called a default gateway.
ARP Tables : Indirect
routing
• To obtain the services of a default
gateway, a source encapsulates the
data so that it contains the
destination MAC address of the router.
• A source uses the destination IP
address of the host device, and not
that of a router, in the IP header,
because it wants the data delivered to
the host device and not to a router.
ARP Tables : Indirect
routing
• When a router picks up data, it strips
of the data link layer information
that is used in the encapsulation.
• It then passes the data up to the
network layer where the router
examines the destination IP address.
• It compares the destination IP
address with information contained
in its routing tables.
ARP Tables : Indirect
routing
• If the router locates the mapped destination IP
address and the MAC address, and learns that the
location of the destination network is attached to
one of its ports, it encapsulates the data with the
new MAC address information, and forwards it to the
correct destination.
• If the router cannot locate the mapped destination
address and MAC address of the device of the fnal
target device, it locates the MAC address of another
router that can perform this function, and forwards
the data to that router.
• This type of routing is referred to as indirect routing.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :
Routed protocols and routing protocols
• You have learned that protocols are like
languages.
• One protocol that you have been learning
about is IP, or the Internet Protocol.
• You know that IP is a network layer protocol.
• Because IP is routed over an internetwork, it
is called a routed protocol.
• Examples of other types of routed protocols
include Novell's IPX, and Appletalk.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :
Routed protocols and routing protocols
• Routers use routing protocols to exchange
routing tables and share routing information.
In other words, routing protocols determine
how routed protocols are routed. Examples
of routing protocols include the following:
– RIP - Routing Information Protocol
– IGRP - Interior Gateway Routing Protocol
– EIGRP - Enhanced Interior Gateway Routing
Protocol
– OSPF - Open Shortest Path First
Interior Gateway Protocol (IGP) and Exterior
Gateway Protocol (EGP) : IGPs and EGPs
• Two types of routing protocols are the
Exterior Gateway Protocols (EGPs)
and the Interior Gateway Protocols
(IGPs).
• Exterior Gateway Protocols route
data between autonomous systems.
• An example of an EGP is BGP (Border
Gateway Protocol), the primary
exterior routing protocol of the
Internet.
Interior Gateway Protocol (IGP) and Exterior
Gateway Protocol (EGP) : IGPs and EGPs
• Can you think of an example where an
Exterior Gateway Protocol would be used?
• Interior Gateway Protocols route data in an
autonomous system. Examples of IGPs are:
–
–
–
–
RIP
IGRP
EIGRP
OSPF
• Can you think of an example where an
Interior Gateway Protocol would be used?
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) : RIP
• The most common method to transfer routing
information between routers that are located
on the same network is RIP.
• This Interior Gateway Protocol calculates
distances to a destination.
• RIP allows routers that use this protocol to
update their routing tables at programmable
intervals, typically every thirty seconds.
• However, because it is constantly connecting
neighboring routers, this can cause network
trafc to build.
Interior Gateway Protocol
(IGP) and Exterior Gateway
Protocol (EGP) : RIP
• RIP allows routers to determine which path it
will use to send data, based on a concept
known as distance-vector.
• Whenever data travels on a router, and thus
through a new network number, it is
considered to have traveled one hop.
• A path that has a hop count of four indicates
that data traveling along that path must
have passed through four routers before
reaching its fnal destination on the network.
Interior Gateway Protocol
(IGP) and Exterior Gateway
Protocol (EGP) : RIP
• If there are multiple paths to a destination, the
router, using RIP, selects the path with the
least number of hops.
• However, because hop count is the only
routing metric used by RIP in determining best
path, it is not necessarily the fastest path.
• Nevertheless, RIP remains very popular, and is
widely implemented.
• This is primarily because it was one of the
earliest routing protocols to be developed.
Interior Gateway Protocol
(IGP) and Exterior Gateway
Protocol (EGP) : RIP
• Another problem with using RIP is
that a destination may be located too
far away for the data to reach it.
• With RIP, the maximum number of
hops that data can travel is ffteen.
• Because of this, if the destination
network is more than ffteen routers
away, it is considered unreachable.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) : RIP
• Dynamic routing protocols like RIP, or IGRP,
difer in the metrics they use when calculating
the best path.
• RIP uses a metric measured by the number of
routers or hops a packet has to go through to
reach a destination.
• If multiple path exist to a destination, the path
with the least number of hops is the path
chosen.
• RIP is not concerned with speed, only the hop
count.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) : RIP
• You could compare this to deciding how to
drive to work, based only the number of
trafc lights and stop signs.
• This might not be the fastest route, since
the route with the least number of trafc
lights could be the most congested route,
have a speed limit of only twenty-fve
miles per hour, or go through areas under
construction.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) : RIP
• In other words, even though the hop
count is low, the route may be slower
than another option.
• This poses the question : if RIP
doesn’t necessarily take the quickest
route, why is it so popular ?
Interior Gateway Protocol (IGP) and Exterior
Gateway Protocol (EGP) : IGRP and EIGRP
• GRP and EIGRP are routing protocols
that were developed by Cisco
Systems, Inc., therefore, they are
considered
proprietary
routing
protocols.
• IGRP was developed specifcally to
address problems associated with
routing,
in
large
multi-vendor
networks, that were beyond the
scope of protocols such as RIP.
Interior Gateway Protocol (IGP) and Exterior
Gateway Protocol (EGP) : IGRP and EIGRP
• Like RIP, IGRP is a distance-vector
protocol; however, when determining
the best path, it also takes into
consideration
such
things
as
bandwidth,
load,
delay,
and
reliability.
•
Network
administrators
can
determine the importance given to
any one of these metrics.
• Or, allow IGRP to automatically
Interior Gateway Protocol (IGP) and Exterior
Gateway Protocol (EGP) : IGRP and EIGRP
• EIGRP is an advanced version of
IGRP.
• Specifcally, EIGRP provides superior
operating efciency and combines
the
advantages
of
link-state
protocols with those of distancevector protocols.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) : OSPF
• OSPF means "open shortest path frst".
• A better description, however, might be
"determination
of
optimum
path",
because this Interior Gateway Protocol
actually
uses
several
criteria
to
determine the best route to a destination.
• These criteria include cost metrics, which
factor in such things as route speed,
trafc, reliability, and security.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) : How
routers recognize networks
• So how does route information get into a
routing table in the frst place?
• The network administrator can manually
enter the information in the router.
• Or, routers can learn the information, on the
fy, from each other.
• Manual entries in routing tables are called
"static routes".
• Routes learned automatically are called
"dynamic routes".
Interior Gateway Protocol (IGP) and Exterior
Gateway Protocol (EGP) : Examples of static
routing
• If
routers
can
learn
routing
information automatically, it might
seem pointless to manually enter
information into a router's routing
tables.
• However, such manual entries can be
useful
whenever
a
network
administrator wants to control which
path a router will select.
Interior Gateway Protocol (IGP) and Exterior
Gateway Protocol (EGP) : Examples of static
routing
• For example, routing tables that are
based on static information could be
used to test a particular link in the
network, or to conserve wide area
bandwidth.
• Static routing is also the preferred
method for maintaining routing
tables whenever there is only one
path to a destination network
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :
Examples of static routing
• This type of network is referred to as
a stub network.
• There is only one way to get to this
network, so it is important to indicate
this situation to prevent routers from
trying to fnd another way to this
stub network if it's connection fails.
Interior Gateway Protocol (IGP) and Exterior
Gateway Protocol (EGP) : Example of dynamic
routing
• Adaptive, or dynamic, routing occurs
when routers send periodic routing update
messages to each other.
• Each time a router receives a message
containing new information, it recalculates
the new best route, and sends the new
updated information to other routers.
• By using dynamic routing, routers can
adjust to changing network conditions.
Interior Gateway Protocol (IGP) and Exterior
Gateway Protocol (EGP) : Example of dynamic
routing
• Before the advent of dynamic
updating of routing tables, most
vendors had to maintain router
tables for their clients.
• This meant that vendors had to
manually enter network numbers,
their associated distances, and port
numbers into the router tables of all
the equipment they sold or leased.
Interior Gateway Protocol (IGP) and Exterior
Gateway Protocol (EGP) : Example of dynamic
routing
• As networks grew larger, this became
an increasingly cumbersome, timeconsuming,
and
ultimately,
expensive, task.
• Dynamic routing eliminates the need
for
network
administrators
or
vendors
to
manually
enter
information into routing tables.
Interior Gateway Protocol (IGP) and Exterior
Gateway Protocol (EGP) : Example of dynamic
routing
• It works best when bandwidth and
large amounts of network trafc are
not issues.
• RIP, IGRP, EIGRP, and OSPF are all
examples
of
dynamic
routing
protocols because they allow this
process to occur.
• Without dynamic routing protocols,
the Internet would be impossible
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :How
routers use RIP to route data through a
network
• You have a Class B network that is
divided into eight subnetworks that
are connected by three routers.
• Host A has data it wants to send to
host Z.
• It passes the data down through the
OSI model, from the application layer
to the data link layer, where host A
encapsulates
the
data
with
information provided by each layer.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :How
routers use RIP to route data through a
network
• When the data reaches the network
layer, source A uses its own IP
address and the destination IP
address of host Z, because that is
where it wants to send the data.
• Then, host A passes the data to the
data link layer.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :How
routers use RIP to route data through a
network
• At the data link layer, source A places the
destination MAC address of the router, to which
it is connected, and its own MAC address in the
MAC header.
• Source A does this because it sees subnetwork
8 as a separate network.
• It knows that it cannot send data directly to a
diferent network, but must pass such data
through a default gateway.
• In this example, the default gateway for source
A is router 1.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :How
routers use RIP to route data through a
network
• The data packet travels along subnetwork 1. All
hosts that it passes by, examine it, but do not
copy it, when they see that the destination MAC
address carried by the MAC header does not
match their own.
• The data packet continues along subnetwork 1
until it reaches router 1.
• Like the other devices on subnetwork 1, router
1 sees the data packet, and picks it up,
because it recognizes that its own MAC address
is the same as the destination MAC address.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :How
routers use RIP to route data through a
network
• Router 1 strips of the MAC header of
the data and passes the data up to
the network layer where it looks at
the destination IP address in the IP
header.
• The router then searches its routing
tables in order to map a route, for
the
network
address
of
the
destination, to the MAC address of
the router that is connected to
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :How
routers use RIP to route data through a
network
• The router is using RIP as its routing protocol,
therefore, it determines that the best path for the
data is one that places the destination only three
hops away.
• Next, the router determines that it must send the
data packet through whichever one of its ports is
attached to subnetwork 4, in order for the data
packet to reach its destination via the selected
path.
• The router passes the data down to the data link
layer, where it places a new MAC header on the
data packet.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :How
routers use RIP to route data through a
network
• The new MAC header contains the
destination MAC address of router 2,
and the MAC address of the frst
router that became the new source.
• The IP header remains unchanged.
• The frst router passes the data
packet through the port that it
selects, and on to subnetwork 4.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :How
routers use RIP to route data through a
network
• The data passes along subnetwork 4.
• All hosts that it passes by, examine
it, but do not copy it, when they see
that the destination MAC address
carried by the MAC header does not
match their own.
• The data packet continues along
subnetwork 4 until it reaches router
2.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :How
routers use RIP to route data through a
network
• Like the other devices on subnetwork
4, the router 2 sees the data packet.
• This time it picks it up because it
recognizes that its own MAC address
is the same as the destination MAC
address.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :How
routers use RIP to route data through a
network
• At the data link layer, the router strips of
the MAC header, and passes the data up
to the network layer.
•
There, it examines the destination
network IP address, and looks in its routing
table.
• The router, using RIP as its routing
protocol, determines that the best path for
the data is one that places the destination
only two hops away.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :How
routers use RIP to route data through a
network
• Next, the router determines that it
must send the data packet through
whichever one of its ports is attached
to subnetwork 5, in order for the data
packet to reach its destination via
the selected path.
• The router passes the data down to
the data link layer where it places a
new MAC header on the data packet
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :How
routers use RIP to route data through a
network
• The new MAC header contains the
destination MAC address of router 2,
and the MAC address of the frst
router becomes the new source MAC.
• The IP header remains unchanged.
The frst router passes the data
packet through the port that it
selects and on to subnetwork 5.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :How
routers use RIP to route data through a
network
• The data passes along subnetwork 5.
The data packet continues along
subnetwork 5 until it reaches router
3.
• Like the other devices on
subnetwork 5, router 3 sees the data
packet.
• This time it picks it up because it
recognizes that its own MAC address
is the same as the destination MAC
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :How
routers use RIP to route data through a
network
• At the data link layer, the router strips of the
MAC header, and passes it up to the network
layer.
• There, it sees that the destination IP address in
the IP header matches that of a host that is
located on one of the subnetworks to which it is
attached.
• Next, the router determines that it must send
the data packet through whichever one of its
ports is attached to subnetwork 8, in order for
the data packet to reach its destination address.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :How
routers use RIP to route data through a
network
• It places a new MAC header on the
data. This time, the new MAC header
contains the destination MAC address
of host Z, and the source MAC
address of router 3.
• As before, the IP header remains
unchanged. Router 3 sends the data
through the port that is attached to
subnetwork 8.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :How
routers use RIP to route data through a
network
• The data packet travels along subnetwork
8. All hosts that it passes by, examine it,
but do not copy it, when they see that the
destination MAC address carried by the
MAC header does not match their own.
• Finally, it reaches host Z, which picks it up
because it sees that its MAC address
matches the destination MAC address
carried in the MAC header of the data
packet.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :How
routers use RIP to route data through a
network
• Host Z strips of the MAC header and passes the data
to the network layer.
• At the network layer, host Z sees that its IP address,
and the destination IP address carried in the IP
header, match.
• Host Z strips of the IP header and passes the data up
to the transport layer of the OSI model.
• Host Z continues to strip of the layers that
encapsulate the data packet, and to pass the data to
the next layer of the OSI model.
• This continues until the data fnally arrives at the top
layer, the application layer, of the OSI model.
Network-to-Network Communications :
ARP operation within a subnet
• If a host wants to send data to another
host, it must know the destination IP
address.
• If it is unable to locate a MAC address
for the destination in its own ARP
table, the host initiates a process
called an ARP request.
• An ARP request enables it to discover
the destination MAC address..
Network-to-Network Communications :
ARP operation within a subnet
Network-to-Network Communications :
ARP operation within a subnet
Network-to-Network Communications :
ARP operation within a subnet
• A host builds an ARP request packet
and sends it to all devices on the
network.
• To ensure that all devices see the
ARP request, the source uses a
broadcast MAC address.
• The broadcast address in a MAC
addressing scheme has all places set
to hexadecimal F.
• Thus, a MAC broadcast address
Network-to-Network Communications :
ARP operation within a subnet
Network-to-Network Communications :
ARP operation within a subnet
• Because ARP request packets travel in a
broadcast mode, all devices on the local
network receive the packets and pass
them up to the network layer for further
examination.
• If the IP address of a device matches the
destination IP address in the ARP request,
that device responds by sending the
source its MAC address.
• This is known as the ARP reply
Network-to-Network Communications :
ARP operation within a subnet
Network-to-Network Communications :
ARP operation within a subnet
• Example:
Source device 197.15.22.33 is asking
for the MAC address of the
destination with IP address
197.15.22.126, Destination device
197.15.22.126 picks up the ARP
request and responds with an ARP
reply containing its MAC address.
Network-to-Network Communications :
ARP operation within a subnet
• Once the originating device receives the
ARP reply, it extracts the MAC address from
the MAC header, and updates its ARP table.
• The originating device can then properly
address its data with both, a destination
MAC address, and a destination IP address.
• It uses this new information to perform
Layer 2 and Layer 3 encapsulations of the
data, before it sends them out over the
network.
Network-to-Network Communications :
ARP operation within a subnet
• When the data arrives at the destination, the data
link layer makes a match, strips of the MAC header,
and transfers the data up to the network layer.
• The network layer examines the data and fnds that
the IP address matches the destination IP address
carried in the IP header.
• The network layer strips of the IP header, and
transfers the encapsulated data to the next highest
layer in the OSI model, the transport layer (Layer 4).
• This process is repeated until the rest of the packet's
partially decapsulated data reaches the application,
where the user data may be read.
Advanced ARP Concepts :
Default gateway
• In order for a device to communicate with
another device on another network, you
must supply it with a default gateway.
• A default gateway is the IP address of the
interface on the router that connects to
the network segment which the source
host is located on.
• The default gateway’s IP address must be
in the same network segment as the
source host.
Advanced ARP Concepts :
Default gateway
• If
no
default
gateway
is
defned,
communication is possible only on the device’s
own logical network segment.
• The computer that sends the data does a
comparison between the IP address of the
destination and its own ARP table.
• If it fnds no match, it must have a default IP
address to use.
• Without a default gateway, the source
computer has no destination MAC address, and
the message is undeliverable.
Advanced ARP Concepts : Problems with
sending data to nodes on diferent subnets
• One of the major problems in networking
is how to communicate with devices that
are not on the same physical network
segment.
• There are two parts to the problem.
• The frst is obtaining the MAC address of
the destination host, and the second is
transferring the data packets from one
network segment to another, to get to the
destination host.
Advanced ARP Concepts : How ARP
sends data to remote networks
• ARP uses broadcast packets to accomplish its function.
• Routers, however, do not forward broadcast packets.
• In order for a device to send data to the address of a
device that is on another network segment, the source
device sends the data to a default gateway.
• The default gateway is the IP address of the router
interface that is connected to the same physical network
segment as the source host.
• The source host compares the destination IP address and
its own IP address to determine if the two IP addresses are
located on the same segment.
• If the receiving host is not on the same segment, the
source host sends the data to the default gateway.
Advanced ARP Concepts : Proxy
ARP
• Proxy ARP is a variation of the ARP protocol.
• In this case an intermediate device (e.g.
router) sends an ARP response, on behalf of
an end node, to the requesting host.
• Routers running proxy ARP capture ARP
packets.
• They respond with their MAC addresses for
those requests in which the IP address is not
in the range of addresses of the local subnet.
Advanced ARP Concepts : Proxy
ARP
• In the previous description of how data is sent
to a host on a diferent subnet, the default
gateway is confgured.
• If the source host does not have a default
gateway confgured, it sends an ARP request.
• All hosts on the segment, including the router,
receive the ARP request.
• The router compares the IP destination address
with the IP subnet address to determine if the
destination IP address is on the same subnet as
the source host.
Advanced ARP Concepts : Proxy
ARP
• If the subnet address is the same, the router
discards the packet.
• The reason that the packet is discarded is that
the destination IP address is on the same
segment as the source's IP address.
• This means another device on the segment
should respond to the ARP request.
• The exception to this is that the destination IP
address is not currently assigned, which will
generate an error response on the source host.
Advanced ARP Concepts : Proxy
ARP
• If the subnet address is diferent, the router will
respond with its own MAC address for the
interface that is directly connected to the
segment on which the source host is located.
• This is the proxy ARP. Since the MAC address is
unavailable for the destination host, the router
supplies its MAC address in order to get the
packet.
• Then the router can forward the ARP request
(based on the destination IP address) to the
proper subnet for delivery.
Advanced ARP Concepts:Four
Layer 3 fowcharts
Advanced ARP Concepts:Four
Layer 3 fowcharts
• Create fowcharts for the following
processes:
– ARP
– RARP
– BOOTP
– DHCP
Routable Protocols : Routed
protocols
• IP is a network layer protocol, and
because of that, it can be routed
over an internetwork, which is a
network of networks.
• Protocols that provide support for
the network layer are called routed
or routable protocols.
Routable Protocols:Other routed
protocols
• The focus of this course is on the
most
commonly
used
routable
protocol, which is IP.
• Even though you will concentrate on
IP, it is important to know that there
are other routable protocols.
• Two of them are IPX/SPX and
AppleTalk.
Routable Protocols : Routable
and non-routable protocols
• Protocols such as IP, IPX/SPX and AppleTalk
provide Layer 3 support and are, therefore,
routable.
• However, there are protocols that do not
support Layer 3; these are classed as nonroutable protocols.
• The most common of these non-routable
protocols is NetBEUI.
• NetBEUI is a small, fast, and efcient protocol
that is limited to running on one segment.
Routable Protocols :
Characteristics of a routable
protocol
• In order for a protocol to be routable, it must
provide the ability to assign a network number, as
well as a host number, to each individual device.
• Some protocols, such as IPX, only require that you
assign a network number; they use a host's MAC
address for the physical number.
• Other protocols, such as IP, require that you
provide a complete address, as well as a subnet
mask.
• The network address is obtained by ANDing the
address with the subnet mask.
Routing Protocols:Examples of
routing protocols
• Routing protocols (Note: Do not
confuse with routed protocols.)
determine the paths that routed
protocols follow to their destinations.
• Examples of routing protocols include
the Routing Information Protocol
(RIP), the Interior Gateway Routing
Protocol
(IGRP),
the
Enhanced
Interior Gateway Routing Protocol
(EIGRP), and Open Shortest Path First
Routing Protocols:Examples of
routing protocols
• Routing protocols enable routers that
are connected, to create a map,
internally, of other routers in the
network or on the Internet.
• This allows routing (i.e. selecting the
best path, and switching) to occur.
Such maps become part of each
router's routing table.
Routing Protocols :Defnition of
routing protocol
• Routers use routing protocols to
exchange routing tables and to share
routing information.
• Within a network, the most common
protocol used to transfer routing
information between routers, located
on the same network, is Routing
Information Protocol (RIP).
Routing Protocols :Defnition of
routing protocol
• This Interior Gateway Protocol (IGP) calculates
distances to a destination host in terms of how
many hops (i.e. how many routers) a packet
must pass through.
• RIP enables routers to update their routing
tables at programmable intervals, usually every
30 seconds.
• One disadvantage of routers that use RIP is that
they are constantly connecting to neighboring
routers to update their routing tables, thus
creating large amounts of network trafc.
Routing Protocols :Defnition of
routing protocol
• RIP allows routers to determine
which path to use to send data. It
does so by using a concept known as
distance-vector.
• Whenever data goes through a
router, and thus, through a new
network number, this is considered
to be equal to one hop.
Routing Protocols :Defnition of
routing protocol
• A path which has a hop count of four
indicates that data traveling along
that path would have to pass through
four routers before reaching the fnal
destination on the network.
• If there are multiple paths to a
destination, the path with the least
number of hops would be the path
chosen by the router.
Routing Protocols :Defnition of
routing protocol
• Because hop count is the only routing metric used
by RIP, it doesn’t necessarily select the fastest path
to a destination.
• A metric is a measurement for making decisions. You
will soon learn that other routing protocols use many
other metrics besides hop count to fnd the best
path for data to travel.
• Nevertheless, RIP remains very popular, and is still
widely implemented.
• This may be due primarily to the fact that it was one
of the earliest routing protocols to be developed
Routing Protocols :Defnition of
routing protocol
• One other problem posed by the use of
RIP is that sometimes a destination may
be located too far away to be reachable.
• When using RIP, the maximum number
of hops that data can be forwarded
through is ffteen.
• The destination network is considered
unreachable if it is more than ffteen
router hops away.
•
•
•
•
•
Routing Protocols : Routing
encapsulation sequence
At the data link layer, an IP datagram is
encapsulated into a frame.
The datagram, including the IP header, is
treated as data.
A router receives the frame, strips of the
frame header, then checks the destination
IP address in the IP header.
The router then looks for that destination
IP address in its routing table,
encapsulates the data in a data link layer
frame, and sends it out to the appropriate
interface.
If it does not fnd the destination IP
Routing Protocols : Multiprotocol routing
• Routers are capable of supporting
multiple
independent
routing
protocols, and of maintaining routing
tables for several routed protocols,
concurrently.
• This capability allows a router to
deliver packets from several routed
protocols over the same data links.
Other Network Layer
Services : Connectionless
network services
• Most network services use a connectionless
delivery system.
• They treat each packet separately, and
send it on its way through the network.
• The packets may take diferent paths to get
through the network, but are reassembled
when they arrive at the destination.
• In a connectionless system the destination
is not contacted before a packet is sent
Other Network Layer Services :
Connectionless network services
• A good analogy for a connectionless
system is a postal system.
• The recipient is not contacted before
a letter is sent from one destination
to another.
• The letter is sent on its way, and the
recipient learns of the letter when it
arrives.
Other Network Layer Services :
Connection-oriented network services
• In connection-oriented systems, a
connection is established between
the sender and the recipient before
any data is transferred.
• An example of a connection-oriented
network is the telephone system.
• You place a call, a connection is
established, and then communication
occurs.
Other Network Layer Services :
Comparing connectionless and
connection-oriented network processes
• Connectionless network processes are often
referred to as packet switched.
• In these processes, as the packets pass from
source to destination, they can switch to
diferent paths, as well as (possibly) arrive
out of order.
• Devices make the path determination for
each packet based on a variety of criteria.
• Some of the criteria (e.g. available
bandwidth) may difer from packet to packet.
Other Network Layer Services :
Comparing connectionless and
connection-oriented network processes
• Connection-oriented
network
processes are often referred to as
circuit switched.
• These
processes
establish
a
connection with the recipient, frst,
and then begin the data transfer.
• All packets travel sequentially across
the same physical circuit, or more
commonly, across the same virtual
circuit
Other Network Layer Services :
Comparing connectionless and
connection-oriented network processes
• The Internet is one huge connectionless
network in which all packet deliveries
are handled by IP.
• TCP (Layer 4) adds connection-oriented
services on top of IP (Layer 3).
• TCP segments are encapsulated into IP
packets for transport across the Internet.
• TCP
provides
connection-oriented
session services to reliably deliver data.
Other Network Layer Services:IP
and the transport layer
• IP is a connectionless system; it treats each packet
independently.
• For example, if you use an FTP program to download a
fle, IP does not send the fle in one long stream of data.
• It treats each packet independently. Each packet can
travel diferent paths.
• Some may even get lost.
• IP relies on the transport layer protocol to determine
whether packets have been lost, and to request
retransmission.
• The transport layer is also responsible for reordering the
packets.
ARP Tables:Internetworking
devices that have ARP tables
• You have learned that the port, or
interface, where a router connects to a
network, is considered part of that
network; therefore, the router interface
connected to the network has an IP
address for that network.
• Routers, just like every other device on
the network, send and receive data on the
network, and build ARP tables that map IP
addresses to MAC addresses.
ARP Tables :Comparing router ARP tables with
ARP tables kept by other networking devices
• Routers can be connected to multiple
networks, or subnetworks.
• Generally speaking, network devices map
the IP addresses and MAC addresses that
they see on a regular and repeated basis.
• This means that a typical device contains
mapping information pertaining only to
devices on its own network.
• It knows very little about devices beyond its
LAN.
ARP Tables :Comparing router ARP tables with
ARP tables kept by other networking devices
• Routers build tables that describe all
networks connected to them.
• ARP tables kept by routers can
contain IP addresses and MAC
addresses of devices located on
more than one network
ARP Tables :Comparing router ARP tables with
ARP tables kept by other networking devices
• In addition to mapping IP addresses
to MAC addresses, router tables also
map ports.
• Can you think of a reason why
routers would need to do this? (Note:
Examine the router's ARP table
below.)
ARP Tables : Other router table
addresses
• What happens if a data packet reaches a router that is
destined for a network to which it is not connected?
• In addition to IP addresses and MAC addresses of
devices located on networks to which it connects, a
router also possesses IP addresses and MAC addresses
of other routers.
• It uses these addresses to direct data toward its fnal
destination.
• If a router receives a packet whose destination address
is not in its routing table, it forwards it to the address of
another router that most likely does contain information
about the destination host in its routing table.
ARP Tables : ARP requests and
ARP replies
• ARP is used only on a local network.
• What would happen if a local router
wanted to ask a non-local router to
provide indirect routing (next-hop)
services, but did not know the MAC
address of the non-local router?
ARP Tables : ARP requests and
ARP replies
• When a router does not know the MAC address
of the next-hop router, the source router (router
that has the data to be sent on) issues an ARP
request.
• A router that is connected to the same segment
as the source router receives the ARP request.
• This router issues an ARP reply to the router
that originated the ARP request.
• The reply contains the MAC address of the nonlocal router.
ARP Tables:Proxy ARP
• A device on one network cannot send
an ARP request to a device on
another network.
• Can you think of a reason why this is
so?
ARP Tables:Proxy ARP
• What happens in the case of
subnetworks?
• Can a device on one subnetwork fnd
the MAC address of a device on
another subnetwork?
• The answer is yes, provided the
source directs its question to the
router.
• Working through a third party is
called proxy ARP, and it allows the
ARP Tables : Indirect
routing
• Sometimes a source resides on a network
that has a diferent network number than
the desired destination.
• If the source doesn't know the MAC address
of the destination it must use the services
of a router.
• With the router's aid, the source's data can
reach its destination.
• A router that is used for this purpose is
called a default gateway.
ARP Tables : Indirect
routing
• To obtain the services of a default
gateway, a source encapsulates the
data so that it contains the
destination MAC address of the router.
• A source uses the destination IP
address of the host device, and not
that of a router, in the IP header,
because it wants the data delivered to
the host device and not to a router.
ARP Tables : Indirect
routing
• When a router picks up data, it strips
of the data link layer information
that is used in the encapsulation.
• It then passes the data up to the
network layer where the router
examines the destination IP address.
• It compares the destination IP
address with information contained
in its routing tables.
ARP Tables : Indirect
routing
• If the router locates the mapped destination IP
address and the MAC address, and learns that the
location of the destination network is attached to
one of its ports, it encapsulates the data with the
new MAC address information, and forwards it to the
correct destination.
• If the router cannot locate the mapped destination
address and MAC address of the device of the fnal
target device, it locates the MAC address of another
router that can perform this function, and forwards
the data to that router.
• This type of routing is referred to as indirect routing.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :
Routed protocols and routing protocols
• You have learned that protocols are like
languages.
• One protocol that you have been learning
about is IP, or the Internet Protocol.
• You know that IP is a network layer protocol.
• Because IP is routed over an internetwork, it
is called a routed protocol.
• Examples of other types of routed protocols
include Novell's IPX, and Appletalk.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :
Routed protocols and routing protocols
• Routers use routing protocols to exchange
routing tables and share routing information.
In other words, routing protocols determine
how routed protocols are routed. Examples
of routing protocols include the following:
– RIP - Routing Information Protocol
– IGRP - Interior Gateway Routing Protocol
– EIGRP - Enhanced Interior Gateway Routing
Protocol
– OSPF - Open Shortest Path First
Interior Gateway Protocol (IGP) and Exterior
Gateway Protocol (EGP) : IGPs and EGPs
• Two types of routing protocols are the
Exterior Gateway Protocols (EGPs)
and the Interior Gateway Protocols
(IGPs).
• Exterior Gateway Protocols route
data between autonomous systems.
• An example of an EGP is BGP (Border
Gateway Protocol), the primary
exterior routing protocol of the
Internet.
Interior Gateway Protocol (IGP) and Exterior
Gateway Protocol (EGP) : IGPs and EGPs
• Can you think of an example where an
Exterior Gateway Protocol would be used?
• Interior Gateway Protocols route data in an
autonomous system. Examples of IGPs are:
–
–
–
–
RIP
IGRP
EIGRP
OSPF
• Can you think of an example where an
Interior Gateway Protocol would be used?
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) : RIP
• The most common method to transfer routing
information between routers that are located
on the same network is RIP.
• This Interior Gateway Protocol calculates
distances to a destination.
• RIP allows routers that use this protocol to
update their routing tables at programmable
intervals, typically every thirty seconds.
• However, because it is constantly connecting
neighboring routers, this can cause network
trafc to build.
Interior Gateway Protocol
(IGP) and Exterior Gateway
Protocol (EGP) : RIP
• RIP allows routers to determine which path it
will use to send data, based on a concept
known as distance-vector.
• Whenever data travels on a router, and thus
through a new network number, it is
considered to have traveled one hop.
• A path that has a hop count of four indicates
that data traveling along that path must
have passed through four routers before
reaching its fnal destination on the network.
Interior Gateway Protocol
(IGP) and Exterior Gateway
Protocol (EGP) : RIP
• If there are multiple paths to a destination, the
router, using RIP, selects the path with the
least number of hops.
• However, because hop count is the only
routing metric used by RIP in determining best
path, it is not necessarily the fastest path.
• Nevertheless, RIP remains very popular, and is
widely implemented.
• This is primarily because it was one of the
earliest routing protocols to be developed.
Interior Gateway Protocol
(IGP) and Exterior Gateway
Protocol (EGP) : RIP
• Another problem with using RIP is
that a destination may be located too
far away for the data to reach it.
• With RIP, the maximum number of
hops that data can travel is ffteen.
• Because of this, if the destination
network is more than ffteen routers
away, it is considered unreachable.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) : RIP
• Dynamic routing protocols like RIP, or IGRP,
difer in the metrics they use when calculating
the best path.
• RIP uses a metric measured by the number of
routers or hops a packet has to go through to
reach a destination.
• If multiple path exist to a destination, the path
with the least number of hops is the path
chosen.
• RIP is not concerned with speed, only the hop
count.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) : RIP
• You could compare this to deciding how to
drive to work, based only the number of
trafc lights and stop signs.
• This might not be the fastest route, since
the route with the least number of trafc
lights could be the most congested route,
have a speed limit of only twenty-fve
miles per hour, or go through areas under
construction.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) : RIP
• In other words, even though the hop
count is low, the route may be slower
than another option.
• This poses the question : if RIP
doesn’t necessarily take the quickest
route, why is it so popular ?
Interior Gateway Protocol (IGP) and Exterior
Gateway Protocol (EGP) : IGRP and EIGRP
• GRP and EIGRP are routing protocols
that were developed by Cisco
Systems, Inc., therefore, they are
considered
proprietary
routing
protocols.
• IGRP was developed specifcally to
address problems associated with
routing,
in
large
multi-vendor
networks, that were beyond the
scope of protocols such as RIP.
Interior Gateway Protocol (IGP) and Exterior
Gateway Protocol (EGP) : IGRP and EIGRP
• Like RIP, IGRP is a distance-vector
protocol; however, when determining
the best path, it also takes into
consideration
such
things
as
bandwidth,
load,
delay,
and
reliability.
•
Network
administrators
can
determine the importance given to
any one of these metrics.
• Or, allow IGRP to automatically
Interior Gateway Protocol (IGP) and Exterior
Gateway Protocol (EGP) : IGRP and EIGRP
• EIGRP is an advanced version of
IGRP.
• Specifcally, EIGRP provides superior
operating efciency and combines
the
advantages
of
link-state
protocols with those of distancevector protocols.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) : OSPF
• OSPF means "open shortest path frst".
• A better description, however, might be
"determination
of
optimum
path",
because this Interior Gateway Protocol
actually
uses
several
criteria
to
determine the best route to a destination.
• These criteria include cost metrics, which
factor in such things as route speed,
trafc, reliability, and security.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) : How
routers recognize networks
• So how does route information get into a
routing table in the frst place?
• The network administrator can manually
enter the information in the router.
• Or, routers can learn the information, on the
fy, from each other.
• Manual entries in routing tables are called
"static routes".
• Routes learned automatically are called
"dynamic routes".
Interior Gateway Protocol (IGP) and Exterior
Gateway Protocol (EGP) : Examples of static
routing
• If
routers
can
learn
routing
information automatically, it might
seem pointless to manually enter
information into a router's routing
tables.
• However, such manual entries can be
useful
whenever
a
network
administrator wants to control which
path a router will select.
Interior Gateway Protocol (IGP) and Exterior
Gateway Protocol (EGP) : Examples of static
routing
• For example, routing tables that are
based on static information could be
used to test a particular link in the
network, or to conserve wide area
bandwidth.
• Static routing is also the preferred
method for maintaining routing
tables whenever there is only one
path to a destination network
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :
Examples of static routing
• This type of network is referred to as
a stub network.
• There is only one way to get to this
network, so it is important to indicate
this situation to prevent routers from
trying to fnd another way to this
stub network if it's connection fails.
Interior Gateway Protocol (IGP) and Exterior
Gateway Protocol (EGP) : Example of dynamic
routing
• Adaptive, or dynamic, routing occurs
when routers send periodic routing update
messages to each other.
• Each time a router receives a message
containing new information, it recalculates
the new best route, and sends the new
updated information to other routers.
• By using dynamic routing, routers can
adjust to changing network conditions.
Interior Gateway Protocol (IGP) and Exterior
Gateway Protocol (EGP) : Example of dynamic
routing
• Before the advent of dynamic
updating of routing tables, most
vendors had to maintain router
tables for their clients.
• This meant that vendors had to
manually enter network numbers,
their associated distances, and port
numbers into the router tables of all
the equipment they sold or leased.
Interior Gateway Protocol (IGP) and Exterior
Gateway Protocol (EGP) : Example of dynamic
routing
• As networks grew larger, this became
an increasingly cumbersome, timeconsuming,
and
ultimately,
expensive, task.
• Dynamic routing eliminates the need
for
network
administrators
or
vendors
to
manually
enter
information into routing tables.
Interior Gateway Protocol (IGP) and Exterior
Gateway Protocol (EGP) : Example of dynamic
routing
• It works best when bandwidth and
large amounts of network trafc are
not issues.
• RIP, IGRP, EIGRP, and OSPF are all
examples
of
dynamic
routing
protocols because they allow this
process to occur.
• Without dynamic routing protocols,
the Internet would be impossible
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :How
routers use RIP to route data through a
network
• You have a Class B network that is
divided into eight subnetworks that
are connected by three routers.
• Host A has data it wants to send to
host Z.
• It passes the data down through the
OSI model, from the application layer
to the data link layer, where host A
encapsulates
the
data
with
information provided by each layer.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :How
routers use RIP to route data through a
network
• When the data reaches the network
layer, source A uses its own IP
address and the destination IP
address of host Z, because that is
where it wants to send the data.
• Then, host A passes the data to the
data link layer.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :How
routers use RIP to route data through a
network
• At the data link layer, source A places the
destination MAC address of the router, to which
it is connected, and its own MAC address in the
MAC header.
• Source A does this because it sees subnetwork
8 as a separate network.
• It knows that it cannot send data directly to a
diferent network, but must pass such data
through a default gateway.
• In this example, the default gateway for source
A is router 1.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :How
routers use RIP to route data through a
network
• The data packet travels along subnetwork 1. All
hosts that it passes by, examine it, but do not
copy it, when they see that the destination MAC
address carried by the MAC header does not
match their own.
• The data packet continues along subnetwork 1
until it reaches router 1.
• Like the other devices on subnetwork 1, router
1 sees the data packet, and picks it up,
because it recognizes that its own MAC address
is the same as the destination MAC address.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :How
routers use RIP to route data through a
network
• Router 1 strips of the MAC header of
the data and passes the data up to
the network layer where it looks at
the destination IP address in the IP
header.
• The router then searches its routing
tables in order to map a route, for
the
network
address
of
the
destination, to the MAC address of
the router that is connected to
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :How
routers use RIP to route data through a
network
• The router is using RIP as its routing protocol,
therefore, it determines that the best path for the
data is one that places the destination only three
hops away.
• Next, the router determines that it must send the
data packet through whichever one of its ports is
attached to subnetwork 4, in order for the data
packet to reach its destination via the selected
path.
• The router passes the data down to the data link
layer, where it places a new MAC header on the
data packet.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :How
routers use RIP to route data through a
network
• The new MAC header contains the
destination MAC address of router 2,
and the MAC address of the frst
router that became the new source.
• The IP header remains unchanged.
• The frst router passes the data
packet through the port that it
selects, and on to subnetwork 4.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :How
routers use RIP to route data through a
network
• The data passes along subnetwork 4.
• All hosts that it passes by, examine
it, but do not copy it, when they see
that the destination MAC address
carried by the MAC header does not
match their own.
• The data packet continues along
subnetwork 4 until it reaches router
2.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :How
routers use RIP to route data through a
network
• Like the other devices on subnetwork
4, the router 2 sees the data packet.
• This time it picks it up because it
recognizes that its own MAC address
is the same as the destination MAC
address.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :How
routers use RIP to route data through a
network
• At the data link layer, the router strips of
the MAC header, and passes the data up
to the network layer.
•
There, it examines the destination
network IP address, and looks in its routing
table.
• The router, using RIP as its routing
protocol, determines that the best path for
the data is one that places the destination
only two hops away.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :How
routers use RIP to route data through a
network
• Next, the router determines that it
must send the data packet through
whichever one of its ports is attached
to subnetwork 5, in order for the data
packet to reach its destination via
the selected path.
• The router passes the data down to
the data link layer where it places a
new MAC header on the data packet
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :How
routers use RIP to route data through a
network
• The new MAC header contains the
destination MAC address of router 2,
and the MAC address of the frst
router becomes the new source MAC.
• The IP header remains unchanged.
The frst router passes the data
packet through the port that it
selects and on to subnetwork 5.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :How
routers use RIP to route data through a
network
• The data passes along subnetwork 5.
The data packet continues along
subnetwork 5 until it reaches router
3.
• Like the other devices on
subnetwork 5, router 3 sees the data
packet.
• This time it picks it up because it
recognizes that its own MAC address
is the same as the destination MAC
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :How
routers use RIP to route data through a
network
• At the data link layer, the router strips of the
MAC header, and passes it up to the network
layer.
• There, it sees that the destination IP address in
the IP header matches that of a host that is
located on one of the subnetworks to which it is
attached.
• Next, the router determines that it must send
the data packet through whichever one of its
ports is attached to subnetwork 8, in order for
the data packet to reach its destination address.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :How
routers use RIP to route data through a
network
• It places a new MAC header on the
data. This time, the new MAC header
contains the destination MAC address
of host Z, and the source MAC
address of router 3.
• As before, the IP header remains
unchanged. Router 3 sends the data
through the port that is attached to
subnetwork 8.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :How
routers use RIP to route data through a
network
• The data packet travels along subnetwork
8. All hosts that it passes by, examine it,
but do not copy it, when they see that the
destination MAC address carried by the
MAC header does not match their own.
• Finally, it reaches host Z, which picks it up
because it sees that its MAC address
matches the destination MAC address
carried in the MAC header of the data
packet.
Interior Gateway Protocol (IGP) and
Exterior Gateway Protocol (EGP) :How
routers use RIP to route data through a
network
• Host Z strips of the MAC header and passes the data
to the network layer.
• At the network layer, host Z sees that its IP address,
and the destination IP address carried in the IP
header, match.
• Host Z strips of the IP header and passes the data up
to the transport layer of the OSI model.
• Host Z continues to strip of the layers that
encapsulate the data packet, and to pass the data to
the next layer of the OSI model.
• This continues until the data fnally arrives at the top
layer, the application layer, of the OSI model.