Day 06 Routing , DV

7/28/2019 Day 06 Routing , DV

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2004 Cisco Systems, Inc. All rights reserved. ICND v2.23-2

Determining IP Routes

Introducing Routing


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To route, a router needs to do the following:

Discover the connected networks . Select the best paths (routes) to these networks.

Maintain and verify routing information using arouting table.

What Is Routing ?


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Routing table contains the best paths

discovered by a routing protocol

Routing table


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Routing protocolsare used between

routers to determine

paths and maintain

routing tables.ex. RIP , OSPF , EIGRP

Once the path is

determined, a router

can route arouted protocol which

support the method of

data delivery.

ex. IP , IPX , APPLETALK

Routing Protocol Vs. Routed Protocol


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Static Route

a route (path)that a network

administrator

enters into the

router manually

Dynamic Route

a route (path) thata network routing

protocol discovers

automatically and

adjusted when

topology changes

Routing Protocols


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Routing Protocols

Static Dynamic

Direct

connected

Static

route

Default

route

IGP EGP

(EGP ,

BGP)

Distancevector

(RIPv1 ,

IGRP)

Linkstate

(OSPF ,

ISIS)

Hybrid

(EIGRP ,

RIPv2)


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An autonomous system is a collection of networksunder a common administrative domain.

IGPs operate within an autonomous system.

EGPs connect different autonomous systems.

Autonomous Systems: Interior or

Exterior Routing Protocols


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Administrative Distance

it is a value between ( 0 255 ) that reflects the

truthfulness of routing protocol (the best protocolhas the least admin. distance)


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Selecting the

Best Route with Metrics

- The best path

has the least

metric.

- each routing

protocol use a

metric type

(hop count , BW ,

delay , load ,reliability , MTU)


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Static routing protocol

1- Direct connected networks :

- Direct connected networks are automatically detectedby the router without configuration

- symbol in routing table is C

- admin. Distance = 0

10.0.0.0 11.0.0.0 12.0.0.0 13.0.0.0

10.0.0.0

11.0.0.0

11.0.0.0

12.0.0.0

12.0.0.0

13.0.0.0

C

C C

C

C

C


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2- Static route :

-manually you can define a path to reach a certain network- symbol in routing table is S

- admin. Distance = 1192.168.1.0/24

192.168.1.0 S0

12.0.0.1/8

12.0.0.2/8

OR 192.168.1.0 12.0.0.2

Internet


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3- Default route :

- This route allows the stub network to reach all knownnetworks beyond router A (gateway of last resort)

- symbol in routing table is S*

192.168.1.0/24

12.0.0.1/812.0.0.2/8

12.0.0.1

InternetS0


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Displaying the routing table

router# show ip routeCodes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter areaE1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP

i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default

U - per-user static route

Gateway of last resort is 0.0.0.0 to network 0.0.0.0

C 12.0.0.0 is directly connected, Serial0

S* 0.0.0.0/0 is directly connected, Serial0


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Dynamic routing protocols

Distance Vector Routing Protocols :- each router detects its direct connected networks and

form its initial routing table

- routers pass periodic copies of routing table to neighbor

routers and learn the best paths to all networks

( the paths with the least metric ) and form the final

routing table (convergence)

- after convergence periodic updates (full routing table) are

sent to indicate any change in the topology .


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Distance Vector Routing

Protocols

10.0.0.0 11.0.0.0 12.0.0.0 13.0.0.0

10.0.0.0 11.0.0.0 12.0.0.011.0.0.0

12.0.0.0

13.0.0.0

12.0.0.0

10.0.0.0

13.0.0.0

13.0.0.0

11.0.0.0

10.0.0.0


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Routing loops

10.0.0.0

10.0.0.0 E0 16

down

10.0.0.0 S0 16

10.0.0.0 S1 3

10.0.0.0 S0 2

- when network 10.0.0.0 fails , router A will mark its metric

by 16 (a max. hop count value to avoid counting to infinity)

and send its routing table to B after the periodic interval.

- before B sends its periodic update to C , router C sent its

routing table to B containing a path to 10.0.0.0 with a better

metric so B think that 10.0.0.0 can be reached by C while C

depends on B for that so loop occurs .


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Routing loops solutions

1- Split Horizon :

route learned from an interface can not be

sent back on the same interface

10.0.0.0

10.0.0.0 E0 16

down

10.0.0.0 S0 16 10.0.0.0 S0 2


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2- Route Poisoning , Poison Reverse :

10.0.0.0

10.0.0.0 E0 16

down

10.0.0.0 S0 down 10.0.0.0 S0 2

Poison

route

Poison

reverse


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3- Hold-down Timers :- router that informed with a failed route dont accept any

update about it for a time equal to the hold down timer so

by the end of the timer all routers would know that route

failed ( it is useful in flapping networks ).

- hold finish if :

The hold-down timer expires.

Another update is received with a better metric.

10.0.0.0

10.0.0.0


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4- Triggered Updates :instead of sending updates after a time interval , router

sends the update as soon as a route fails or any change

occurs so other routers immediately modify their routingtables ( this is the most used solution ).


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Properties of Distance Vector

Routing Protocols

- simple configuration

- low processing / memory usage

- bandwidth waste due to the periodic updates

- unreliable (no ack. for the protocol messages)- updates are sent broadcast on all active interfacesso it may affect the hosts PCs

- classful :

do not include the subnet mask with the routeadvertisement and often sends a summary routes

- These are examples of distance vector protocols:

RIP version 1 (RIPv1)

IGRP


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1RIP v

- distance vector routing protocol

- symbol in routing table is R - admin. Distance = 120

- metric is hop count , metric 16 means unreachable

- full routing tables are flooded in the network till

convergence occurs (use Bellman Ford algorithm)

- after convergence , periodic updates are sent every 30

seconds , hold down timer = 180 sec

- at change , triggered update is sent- support load balancing if many paths to the same network

exist with an equal metric

(maximum is six paths , default = 4)

- classful


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- Starts the RIP routing process

Router(config)#router rip

Router(config-router)#network direct connected network

- Advertise about the connected networks

RIP Configuration


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RIP Configuration Example


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Verifying the RIP Configuration

Displaying the


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Displaying the

IP Routing Table


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debug ip rip Command


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IGRP- distance vector routing protocol

- Cisco proprietary

- symbol in routing table is I

- admin. Distance is 100

- metric is composite (bandwidth , delay by default) while (load ,

reliability , MTU) can be used , maximum hop count is 255

(more scalable)

- full routing tables are flooded in the network till convergence occurs

(use Bellman Ford algorithm)

- after convergence , periodic updates are sent every 90 seconds , hold

down timer = 270 sec- at change , triggered update is sent

- support load balancing if many paths to the same network exist even

if they have unequal metric (maximum is six paths , default = 4)

- classful


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Configuring IGRP

Router(config-router)#network network-number

Advertise the attached networks

Router(config)# router igrp autonomous-system Defines IGRP as the IP routing protocol


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IGRP Configuration Example


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Unequal load balancing

Router(config-router)# traffic-share {balanced | min}

Controls how load-balanced traffic is distributed

Router(config-router)# variancemultiplier Controls IGRP load balancing

Router(config)# router igrp autonomous-system


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Verifying the IGRP Configuration


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Displaying the IP Routing Table


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debug ip igrp transaction Command


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debug ip igrp events Command

RouterA#debug ip igrp eventsIGRP event debugging is on

RouterA#

00:23:44: IGRP: sending update to 255.255.255.255 via Ethernet0 (172.16.1.1)

00:23:44: IGRP: Update contains 0 interior, 2 system, and 0 exterior routes.

00:23:44: IGRP: Total routes in update: 2

00:23:44: IGRP: sending update to 255.255.255.255 via Serial2 (10.1.1.1)



00:23:48: IGRP: received update from 10.1.1.2 on Serial2



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