Network-to-Network Communications : ARP operation within a subnet

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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}

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• _{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}

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• _{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.

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• _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

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• _{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}

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• _{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}

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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.

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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}

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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.}

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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.

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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.

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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}

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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}

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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}

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Advanced ARP Concepts:Four

Layer 3 fowcharts

• _{Create fowcharts for the following}

processes:

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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

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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

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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 non-routable protocols.

• The most common of these non-routable protocols is NetBEUI.

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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}

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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

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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}

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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

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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}

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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}

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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}

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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.

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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.

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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.

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Routing Protocols :

Multi-protocol 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}

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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}

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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}

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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}

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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}

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• _{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}

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• _{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}

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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}

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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.

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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.

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• _{Routers build tables that describe all}

networks connected to them.

• _{ARP tables kept by routers can}

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• _{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

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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}

• _{If a router receives a packet whose destination address}

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ARP Tables

: ARP requests and

ARP replies

• _{ARP is used only on a local network.}

• _{What would happen if a local router}

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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}

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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}

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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}

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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}

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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}

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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}

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ARP Tables : Indirect

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.

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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.

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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

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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}

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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}

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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}

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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}

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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}

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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}

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• _{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}

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• _{You could compare this to deciding how to} drive to work, based only the number of trafc lights and stop signs.

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• _{In other words, even though the hop}

count is low, the route may be slower than another option.

• _{This poses the question : if RIP}

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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

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• _{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}

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• _{EIGRP is an advanced version of}

IGRP.

• _{Specifcally, EIGRP provides superior}

operating efciency and combines

the advantages of link-state

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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}

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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".

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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

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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}

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Examples of static routing

• _{This type of network is referred to as}

a stub network.

• _{There is only one way to get to this}

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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.

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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}

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routing

• _{As networks grew larger, this became}

an increasingly cumbersome,

time-consuming, and ultimately,

expensive, task.

• _{Dynamic routing eliminates the need}

for network administrators or

vendors to manually enter

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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,}

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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

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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}

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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}

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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}

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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}

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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}

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network

• _{Like the other devices on subnetwork}

4, the router 2 sees the data packet.

• _{This time it picks it up because it}

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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.

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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}

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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.}

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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}

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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.

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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}

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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.

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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.

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