3.1 Routing Technology_revised

7/28/2019 3.1 Routing Technology_revised

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IP Routing Principles

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Router

Router

Application Application

Data

Transport Transport

TCP

Network Network

IP

Physical Physical

Data Link Data Link

HD TR



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IP Routing..

172.16.2.1 120.1.3.9

FDDITOKEN

RING

EthernetSerial

DATADATA



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IP Routing… Data link layer provide communications

across a physical pathCertain information about data link identifiers and encapsulation must be

aquired and stored in a database such as ARP cache

Network layer provide communications

across logical or virtual pathInformation about network layer is stored inrouting table or forwarding table



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IP Routing…. When a framed packet arrives at one of a

router’s interfaces: At layer-2 (Data Link Layer)

The frame will be accepted by a router only if

Data link identifier in destination addresscontains Identification of the router’s interfaceor a broadcast identifier

FCS field is checked for error detection, if any

If FCS is bad, frame is discarded

If FCS is good, frame header and trailer are strippedoff and IP Datagram is handed over to layer-3



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IP Routing….. At Layer-3 (Network layer) the destination

IP address is examinedIf the destination IP address is:

IP address of a router’s interface or an all host

broadcast addressThe protocol field of the packet is examined and theenclosed data is sent to the appropriate internalprocess

IP address of a host on a directly connectednetwork

The packet will be delivered after encapsulation



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IP Routing…… IP address of a host on a network not directly

connected to the routerThe packet will be routed after doing a route tablelookup

Each route entry must contain two items

The address of the network the router can reach

A pointer to the destination

the pointer will indicate the address of another router

on a directly connected network or the address of another router which is one hop closer to thedestination, known as next hop router



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IP Routing……. Direct Routing

If the datagram is routed locally i.e. if thedestination is on the same subnet as theoriginator.

Indirect RoutingIf the use of a forwarding device such as routeris invoked i.e. if the destination is remote



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IP Routing……..

E0 E1

172.16.2.1

172.16.2.2

172.16.2.160

172.16.3.5

172.16.3.100

172.16.3.150

172.16.2.3

172.16.3.1

Direct Routing Indirect Routing



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IP Routing

08.00.39.00.2F.C4 08.00.39.00.2F.C1 172.16.2.1 172.16.3.4 DATA CRC

08.00.39.00.2F.C8 08.00.39.00.2F.C5 172.16.2.1 172.16.3.4 DATA CRC

E0 E1

172.16.2.1

172.16.2.2

172.16.2.3

172.16.3.2

172.16.3.3

172.16.3.4

172.16.2.4

172.16.3.1

08.00.39.00.2F.C1

08.00.39.00.2F.C2

08.00.39.00.2F.C3

08.00.39.00.2F.C4

08.00.39.00.2F.C5

08.00.39.00.2F.C6

08.00.39.00.2F.C7

08.00.39.00.2F.C8

Routing Table

172.16.2.0/24 172.16.2.4172.16.3.0/24 172.16.3.1



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Route Table UpdationRoute table aquires information in two ways:

ManuallyStatic route entries

Automatically

Dynamic routing protocols



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Static RoutingRoutes to destinations are set up manually

Network reachability is not dependent onthe existence and state of the network

Route may be up or down but static routes

will remain in the routing tables and trafficwould still be sent towards the route

Not suitable for large networks

Also known as Non-adaptive routing



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Dynamic RoutingRoutes are learnt via an internal or external

routing protocolsRouting Protocol is a language a routerspeaks with other routers to shareinformation about the reachability and status

of the network Routing Protocol messages do not carry enduser traffic from network to network

Routing Protocol uses the routed protocol topass information between routers

RIP & OSPF are routing protocols



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Dynamic RoutingRouting decisions change to reflect the

changes in topologyNetwork reachability is dependent on theexistence and state of the network

If a router is down, its entry will be deletedfrom the routing table and traffic to that willnot be forwarded

Also known as Adaptive routing



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Routed ProtocolContains sufficient network layer addressing

information for user traffic to be directedfrom one network to another network

Define the format and use of the fields

within a packetPackets that use a routed protocol areconveyed from one end system to another

end system through an inter network Internet Protocol is a routed protocol





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Default Route..When a router receives a packet and its

table does not contain the network addressindicated in the packet, it is forwarded todefault router

The default router, too, may have a defaultrouterEasiest form of routing for a domainconnected to a single exit point

Default router is indicated as 0.0.0.0 with nosubnet mask



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Default Route… If there is no route or default route at any

stage, the router will send a controlmessage (through ICMP) to the originatingstation

Refers to “last resort” outlet



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Static and Default Routes

Traffic to network 192.168.5.0 (Static Route).

All outgoing traffic from network 192.168.5.0(Default Route).

R2R1

WAN

192.168.5.0



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Dynamic Routing Protocols All dynamic routing protocols are built

around an algorithm, which must specify: A procedure for passing network reachabilityinformation about networks to other routers

A procedure for receiving reachabilityinformation from other routers

A procedure for determining optimal routesbased on the reachability information it has and

for recording this information in a route table A procedure for reacting to and advertisingtopology changes in an network

C



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Convergence

10.1.2.010.1.1.0 10.1.3.0 10.1.4.0

.2.1 .2.1 .1.2

A B C

10.1.4.0 10.1.2.2 2 10.1.1.0 10.1.3.1 2

10.1.3.0 10.1.2.2 1 10.1.1.0 10.1.2.1 1

10.1.4.0 10.1.3.2 1

10.1.2.0 10.1.3.1 1

NW VIA HOP10.1.1.0 ---------- 0

10.1.2.0 ---------- 0

NW VIA HOP10.1.2.0 ---------- 0

10.1.3.0 ---------- 0

NW VIA HOP10.1.3.0 ---------- 0

10.1.4.0 ---------- 0

Routing Table-A Routing Table-B Routing Table-C

• All the routing tables contains information about all.



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ConvergenceThe process of bringing all route tables to a

state of consistency is called convergence.The time it takes to share information acrossan internetwork and for all routers to

calculate best paths is convergence time.

P th D t i ti



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Path Determination

A B

C

192.168.1.0 192.168.7.0

192.168.6.0

192.168.5.0

192.168.2.0

192.168.3.0

192.168.4.0

Routing Table of Router-A

Network Next Hop Router192.168.1.0 Direct192.168.2.0 Direct192.168.3.0 Direct

192.168.4.0 B,C192.168.5.0 B,C192.168.6.0 B,C192.168.7.0 B,C

•Networks192.168.4.0 to 192.168.7.0 can be reached viaeither router B or C, which path is preferable?•Metrics are needed to rank the alternatives.



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Metrics A metric is a variable assigned to routes as a

means of ranking them.Different routing protocols use different, andsometimes multiple metrics.

RIP defines the “best” route as one withminimum number of hops.

IGRP defines the “best” route on a combinationof lowest bandwidth along the route and the

total delay of the route.

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Metrics

Hop Count

Bandwidth

Load

Delay

Reliability

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Metrics

Hop Count

A hop count metric simply count router hops.

From router-A it is 1 hop to network 192.168.5.0 if packets are sent out interface

192.168.3.0 and 2 hops if sent out 192.168.1.0

A B

C

192.168.1.0 192.168.7.0

192.168.6.0

192.168.5.0

192.168.2.0

192.168.3.0

192.168.4.0

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Metrics

Bandwidth

A bandwidth metric would choose a higherbandwidth over a lower bandwidth.

A packet from router A to C will follow path A-B-C.

A B

C

192.168.1.0192.168.7.0

192.168.6.0

192.168.5.0

192.168.2.0

192.168.3.0

192.168.4.0

A-C: 64kbps A-B: 2mbpsB-C: 2mbps

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Metrics

Speed Cost

>= 100Mbps 1

Ethernet/802.3 10

E1(2.048Mbps) 48

64Kbps 1562

Metric=108 /Interface Speed in bits per sec.e.g. 100000000/2048000=48.828125

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MetricsLoad

The metric reflects the amount of trafficutilising the links along the path.

The best path is the one with the lowest load.

Unlike hop count and bandwidth, the load ona route changes and therefore the metric willchange and is called route flapping.

Route flaps have adverse effects on router’s

CPU and the overall stability of the network.

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MetricsDelay

Delay is the measure of time a packet takesto traverse a route

A protocol using delay as a metric wouldchoose the path with the least delay as the

best pathReliability

Reliability measures the likelihood that thelink will fail in some way

The path with highest reliability would beselected as the best

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MetricsCost

The term cost is often used as a generic termwhen speaking of router choices

RIP chooses the lowest-cost path based on the hopcount.

Another generic term is ShortestRIP chooses the shortest path based on the hopcount.

D i R ti P t l



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Dynamic Routing ProtocolsDistance Vector Routing Protocols

Distance vector algorithms are based on thework done of R.E.Bellman, L.R.Ford andD.R.Fulkerson

Often known as Bellman-ford or Ford-fulkerson

algorithms

Link State Advertisement Protocols

Built around a well known algorithm from graph

theory, E.W.Dijkstra’s shortest path algorithmCalled as Shortest path first or Distributivedatabase protocols

Di t V t R ti P t l



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Distance Vector Routing Protocols

Di t V t R ti P t l



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Distance Vector Routing ProtocolsRoutes are advertised as vectors of

<Distance, Direction>Distance is defined in terms of a metric

Direction is defined in terms of next hop router

Each router learns routes from itsneighboring router’s perspective and thenadvertises the routes from its ownperspective

Sometimes referred to as “Routing ByRumor”

Di t V t R ti P t l



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Distance Vector Routing ProtocolsRouting Information Protocol (RIP) for IP.

Xerox Networking System’s XNS RIP.Novell’s IPX RIP.

Cisco’s Internet Gateway Routing Protocol

(IGRP).

DEC’s DNA Phase IV.

Apple Talk’s Routing Maintenance Protocol

(RTMP).

Ro ting Loops



Routing Loops

Routing loops are

A condition in

which a packetis continuouslytransmitted within a series of

routers withoutever reaching itsdestination.

Routing Loops



Routing Loops

Routing loops may be caused by:Incorrectly configured static routes

Incorrectly configured route redistribution

Slow convergence

Incorrectly configured discard routes

Routing loops can create the followingissues:

Excess use of bandwidthCPU resources may be strained

Network convergence is degraded

Routing updates may be lost or not

processed in a timely manner

Routing Loops



Routing LoopsCount to Infinity

This is a routing loop whereby packetsbounce infinitely around a network

Routing Loops



Routing Loops

Setting a maximum

Distance Vector routing protocols seta specified metric value to indicate infinity

Once a router “counts to infinity” it marks theroute as unreachable



Routing LoopsPreventing loops with holddown timers

Holddown timers allow a router to notaccept any changes to a route for aspecified period of time

Point of using holddown timers• Allows routing updates to propagate throughnetwork with the most current information



Routing LoopsThe Split Horizon Rule is used toprevent routing loops

Split Horizon rule:

A router should not advertise a network through the interface from which the updatecame

Routing Loops



Routing Loops

Split horizon

with poisonreverse

The rule states

that once a routerlearns of anunreachable routethrough an

interface, advertiseit as unreachableback through thesame interface

Summary



SummaryD.V. routing protocols maintains routing tables by

RIP sending out periodic updates

RIP using 4 different timers to ensure informationis accurate and convergence is achieved in a timelymanner

EIGRP sending out triggered updatesD.V. routing protocols may be prone to routing loops

routing loops are a condition in which packetscontinuously traverse a network

Mechanisms used to minimize routing loops includedefining maximum hop count, holddown timers, splithorizon, route poisoning and triggered updates

Summary



Summary

Routing Information Protocol (RIP) A distance vector protocol that has 2versions

RIPv1 - a classful routing protocolRIPv2 - a classless routing protocol

Enhanced Interior Gateway RoutingProtocol (EIGRP)

A distance vector routing protocols that hassome features of link state routing protocols

A Cisco proprietary routing protocol

Link state Routing Protocols



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Link state Routing ProtocolsEach router originates information about

itself, its directly connected links and stateof those links

The information is passed around from

router to router, each router making a copyof it, but never changing it

Every router has identical information about

the internetwork Each router will independently calculate itsown best paths




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Link State Routing protocols are built around

well known algorithm from graph theoryE.W.Dijkstra shortest path algorithm.

Examples of LSR are:

Open Shortest Path First (OSPF)

The ISO’s Intermediate System to IntermediateSystem (IS-IS) for IP

DEC’s DNA Phase VNovell’s Netware Link State Protocol (NLSP)



Link-State Routing Protocols

Routingprotocol

Builds Topologica

lmap

Router canindependently determinethe shortest

path toevery

network.

Convergence

A periodic/event driven

routingupdates

Useof

LSP

Distancevector

No No Slow Generally No No

Link State Yes Yes Fast Generally Yes Yes

Advantages of a Link-State RoutingProtocol

Link State Routing Protocols




Requirements for using a link state routingprotocolMemory requirements

Typically link state routing protocolsuse more memoryProcessing Requirements

More CPU processing is required of link state routing protocolsBandwidth Requirements

Initial startup of link state routing

protocols can consume lots of




Link-State Routing Protocols2 link state routing protocols used forrouting IP

-Open Shortest Path First (OSPF)-Intermediate System-Intermediate

System (IS-IS)

Summary



SummaryLink State Routing protocols are also knownas Shortest Path First protocols

Summarizing the link state process

-Routers 1ST learn of directly connectednetworks

-Routers then say “hello” to neighbors-Routers then build link state packets

-Routers then flood LSPs to all neighbors

-Routers use LSP database to build anetwork topology map & calculate the bestpath to each destination

Summary



SummaryLink

An interface on the routerLink State

Information about an interface such as-IP address

-Subnet mask -Type of network -Cost associated with link -Neighboring routers on the link

Summary



SummaryLink State Packets

After initial flooding, additional LSP aresent out when a change in topologyoccurs

Examples of link state routing protocols

-Open shortest path first

-IS-IS

Autonomous System



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Autonomous SystemOld definition

An autonomous system is a group of routersunder a common administrative domain runninga common routing protocol

Contemporary definition

An autonomous system is an internetwork undera common administration

Interior-Exterior Gateway Protocols



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Interior Exterior Gateway Protocols

Interior Gateway Protocols

Routing Protocols which run within an Autonomous System are IGPs

IGPs discover paths between networks

Exterior Gateway ProtocolsRouting Protocols that route between

Autonomous System are EGPs

EGPs discover paths between autonomoussystems

IGPs-EGPs



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IGPs EGPs

AS-1 AS-2

AS-0

IGP

IGP IGP

BGP

BGP

BGP

Administrative Distances



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Administrative DistancesMetrics are assigned to the routes to

determine the most preferred route.RIP’s metric is hop count

IGRP uses bandwidth and delay

OSPF uses costEIGRP uses composite metric

Diversity of metrics poses problems inrouters running more than one routingprotocol.




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Administrative DistancesRouter may learn a route to the same

destination from each of the protocolsWhich route should be selected?

The answer to the problem is administrative

distance Administrative distances are the routesources to determine most preferred source

Administrative distance is a measure of believability




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Administrative DistancesThe administrative distance of various

protocols is as below:Connected Interface - 0

Static Route - 1

EIGRP summary route - 5

External BGP - 20

EIGRP - 90

IGRP - 100




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OSPF - 110

IS-IS - 115RIP - 120

EGP - 140

External EIGRP - 170Internal BGP - 200

Unknown - 255

The lower the administrative distance, themore believable the protocol.

Flow Chart of a Packet



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Packet Received

Received ARPReply

Send ICMP

error message

Discard original

Packet

Header &Checksum Valid

Route Found

Route table lookup

on Dest. Add.

YES

NO

Decrement TTL;

TTL>=0 YES

NO

YES

NO

If route available,

search MAC in ARP

cache

Default routeavailable

NO

YES

Send ARP

request and wait

for a response

Build new packet

with MAC address

and route throughport found in

routing table

MAC Address

Found YES

NO

Received ARP

reply, insert

MAC and IPaddress into

ARP table

YESNO

Flow Chart of a Packet



3.1 Routing Technology_revised

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