external routing bgp

8/3/2019 External Routing BGP

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COLE POLYTECHNIQUEFDRALE DE LAUSANNE

BGPJeanYves Le Boudec

Fall 2009

Self-Organization

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Contents

A. What InterDomain Routing does

.

2. PolicyRouting

3. RouteAggregation

B. How BGP works

1. Howitworks

2. InteractionBGPIGPPacketForwarding

3. OtherBells

and

Whistles

C. Examples

D. Illustrations and Statistics

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A. WhatBGPDoes

1.Inter

Domain

Routing

Whyinvented?

Hierarchical routing is used

theInternetissplitintoDomains,orAutonomousSystems

withOSPF:largedomainsaresplitintoAreas

Routing protocols are saidinterior:(InternalGatewayProtocols,IGPs):insideASs:

RIP,OSPF(standard),IGRP(Cisco)

exterior:betweenASs:

3

EGP(old)andBGP1toBGP4(today),IDRP(tomorrow,maybe)


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WhatisanARD?AnAS?

ARD = Autonomous Routing Domain= routing domain under one single administration

oneormoreborderrouters

allsubnetworksinsideanARDshouldbeconnected

shouldlearnaboutothersubnetworkprefixes theroutingtablesofinternalroutersshouldcontainentriesofalldestinationoftheInternet

AS = Autonomous System= ARD with a number (AS number)

ASnumberis16bits

public:1

64511

private:64512 65535

ARDs that are do not need a number are typically those with default route tothe rest of the world

4

AS1942 CICGGRENOBLE,AS2200 Renater

AS559 SWITCHTeleinformaticsServices

AS5511 OPENTRANSIT

EPFL:oneARD,nonumber


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thefigureshowsthreedomains,orARDs.

ARDscanbetransit BandD stub A ormultihomed C .Onl nonstub

domainsneedanASnumber.

ARD D

D1area 0

C2

C1

IGRPA2

A1

A4 BGP-4

BGP-4

OSPF BGP-4

BGP-4 D2

D3D4

D6OSPF

area 2

area 1

D5

5

ARD C

C3B2

B4

B3

A3

Autonomous Routing Domain A

ARD B


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WhatdoesBGPdo?

WhatdoesBGPdo?

.

paths from one router in one ARD to any network prefix

anywhere in the world

There are only two levels for BGP :

Interdomain

withindomain

6

The method of routing is

Pathvector

Withpolicy


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PathVectorRouting

What is the requirement ?Findbestroutes,inasensethatcanbedecidedbyeveryARDusingtheirowncriteria

How does it work ?

anIPprefixexample:BA:n1

everyASusesitsownrulesfordecidingwhichpathisbetterBGPtable keepsarecordofbestpathstoalldestinationsASannouncesonlythebestpathsitknows

Q. Explain how E can choose the best paths to n1 and n2 ?

Q. How can loo s be avoided ? solution

B

En1, n2

A:n1,n2

B A:n1,n2

B:n5 dest AS path

n1 B A

n2 B A

n3 D C

n4 D

a e nn5

n4

7

A

CA:n1,n2

C A:n1,n2

C:n3

D

D C A:n1,n2

D C: n3

D: n4

n5 B

n3


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BorderGateways,E andIBGP

A router that runs BGP is called a BGP speakerAt the boundary between 2 ARDs there are 2 BGP speakers, one in eachdomain

Q:comparetoOSPF

Inside one ARD there are usually several BGP speakers

Theyalltalktoeachother,toexchangewhattheyhavelearnt

UsingInternalBGP(IBGP)

Over TCPconnections,fullmeshcalledtheBGPmesh

mesharenotrepeatedinsidethemesh

D1 D2A B

C D

X:n1 X:n1

A->C: D1 X:n1

E-BGP

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D4 D5

D3

G H

E F

C->E: D1 X:n1

E->G: D3 D1,X:n1

E-BGP

I-BGP


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Q1: Which of the following announcements do you think router D willsend to router E ?1. D>E: D2X:n1

2. D>E: D1X:n1

A:1only;2cannotbesentbecauseoverIBGPwedonotrepeataroutelearntromanot er spea er nt esame

Q2: Assume that in the figure below the router R does notrun BGP. Is thispossible ?A: yes.

A: no; C and F are not directly connected (are not onlink)

D1 D2A B

X:n1 X:n1

E-BGPsolution

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D4 D5

D3

G H

E F

E-BGP

I-BGP R


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2.PolicyRouting

Whyinvented?

Interconnection of ASs is selforganized

pointtopointlinksbetweennetworks:ex: EPFLtoSwitch,Switchto

Telianet

interconnectionpoints:NAP(NetworkAccessPoint),MAE(Metropolitan

AreaEthernet),CIX(CommercialInterneteXchange),GIX(GlobalInternet

eXchange),

IXP,

SFINX,

LINXMainly 3 types of relations, depending on money flows

customer:EPFLiscustomerofSwitch.EPFLpaysSwitch

provider:SwitchisproviderforEPFL;SwitchispaidbyEPFL

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peer:EPFLandCERNarepeers:costsofinterconnectionisshared

thetypeofrelationisnegotiatedinbilateralagreements

thereisnoarchitecturerule,justbusiness


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WhatistheGoalofPolicyRouting?

Example:

,

ISP2andISP3

ISP3 ISP1isalocal,inexpensivelink

CiiscustomerofISPi,ISPsarepeers

It is advantageous for ISP3 to send traffic toISP 1

ISP 3 ISP 2

n v a

ISP1 does not agree to carry traffic from C3to C2

ISP1offersatransitservicetoC1andanon

transitservicetoISP2andISP3

C1

C2C3

n2

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The goal of policy routing is to support

this and other similar requirementsprovider

customerpeers


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HowdoesPolicyRoutingWork?

Policy routing is implemented by rules imposed to BGP

routers inside an AS, who may

modifytheattributesthatcontrolwhichpathispreferred

(seelater)

Example

ISP1announcestoISP3allnetworksofC1 sothatC1canISP 1

ISP 3 ISP 2

bereachedbyallsourcesintheworld

ISP1

announces

to

C1

all

routes

it

has

learnt

from

ISP3

and

ISP2 sothatC1cansendtraffictoalldestinationsinthe

world

ISP2announcesISP2n2toISP3andISP1;assumethat

C1

C2C3

n2n3

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.

ISP3hastworouteston2:ISP2n2andISP1ISP2n2;

assumethatISP3givespreferencetothelatter

packetsfromn3ton2areroutedviaISP1 undesired

solution:ISP1announcestoISP3onlyroutestoISP3s

customers


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TypicalPolicyRoutingRules

Provider (ISP1) to customer (C1)

announceallrouteslearntfromotherISs

importonlyroutesthatbelongtodomainC1

example:importfromEPFLonlyoneroute

128.178/15

Customer (C1) to Provider (ISP1)

announceallroutesthatbelongtodomainC1ISP 1

ISP 3 ISP 2

importallroutes

Peers (ISP1 to ISP3)

announceonlyroutestoallcustomersofISP1

importonlyroutestoISP3scustomerC1

C2C3

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agreement

The rules are defined by every AS (self

organization) and implemented in all BGP

speakers in one AS


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3.Aggregation

Domains that do not have a default route (i.e. all transit ISPs)

inIProutingtablesunlessdefaultroutesareused

inBGPannouncements

Aggregation is a way to reduce the number of routes

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AggregationExample1

193.212.0.0/24

AS2

AS3 AS4

193.212.1.0/24

193.212.0.0/23 193.212.0.0/23

Assume AS3 aggregates the routes received from AS1 and AS2AS1:193.212.0.0/24 AS_PATH:1

AS2:193.212.1.0/24 AS_PATH:2

AS3:193.212.0.0/23 AS_PATH:3{12}

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. . . _


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AggregationExample2

197.8.2.0/24

AS2

AS3 AS4

197.8.3.0/24

197.8.2.0/23

197.8.3.0/24

197.8.2.0/23

n2= 197.8.3.0/24

n4

AS4 receives197.8.2.0/23 AS_PATH: 3 {1 2}

197.8.3.0/24 AS_PATH: 2

and assume that AS4 exports only the first route (aggregation is done)

But assume both routes are injected into AS4s local routing tables; Q:

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what happens to packets from n4 to n2 ?

If AS4 injects only the aggregate route into its local routing tables, whatis the answer now ?solution


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ExampleWithoutAggregation

197.8.2.0/24 197.8.2.0/23 197.8.2.0/23

AS2

AS3 AS4

197.8.3.0/24

197.8.3.0/24

197.8.2.0/24

197.8.3.0/24

197.8.2.0/24

197.8.3.0/24

Q: If AS3 does not aggregate, what are the routesannounced by AS 4 ? Is there any benefit ?

solution

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Morale

Aggregation should be performed whenever possible

whenallaggregatedprefixeshavethesamepathbeforetheaggregation

point(example2)

An AS can decide to

Aggregateseveralrouteswhenexportingthem

Butstill

maintain

different

routing

entries

inside

its

domain

(example

2)

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B.BGP(BorderGatewayProtocol)

1.Howitworks,FundamentalExamples

BGP4, RFC 1771

rou ers a oeac o erover connec ons

Maintainadjacencytopeers(overTCPconnection):open,keepalive

Transmitonlymodifications

How a BGP router works (see also next slide)

Receives routesfromneighbours

Accepts/rejectsthem(importpolicy)

Processesattributes

Stores

in

Adj

RIB

in

(one

per

BGP

peer,

internal

or

external)Appliesdecisionprocess andstoresresultsinLocRIB (globaltoBGPspeaker)(=decideswhichroutestoacceptamongallpresentinthedifferentAdjRIBins)

Foreverysubnetworkprefix,atmost1route isselectedSendstoneighbours

Decideswhethertosendornot(exportpolicy)

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Aggregatemultipleroutesintoone,ifapplicable

StoreresultinAdjRIBout(oneperBGPpeer)andsendtoneighbour

OnlyrouteslearntfromEBGParesenttoanIBGPneighbour

SendsupdateswhenAdjRIBoutchanges

Writeforwardingentries initsroutingtable,eitherdirectlyorbyredistributionintoIGP;thisisexplainedindetailinSection2.


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InsideBGP

One BGP Router

up a es n up a es ou

Adj-RIB-in

Import Policy

Decision

Process:

Ex ort - -- -

Adj-RIB-outLoc-RIB

IGP

Write forwarding

entries

one es

route /

destination

Policy

Internal routes

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TableStatic Configuration


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Routes,RIBs,RoutingTable

The records sent in BGP messages are called Routes. Routes + theirattributes are stored in the AdjRIBin, LocRIB, AdjRIBout.

destination(subnetworkprefix)

pathtothedestination(ASPATH)

Attributes

WellknownMandatory

ORIGIN(routelearntfromIGP,BGPorstatic)

NEXTHOP

Wellknown

Discretionary

LOCALPREF(seelater)

ATOMICAGGREGATE(=routecannotbedisaggregated)

OptionalTransitive

MULTIEXITDISC(MED)(seelater)

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OptionalNontransitive

WEIGHT(seelater)

In addition, like any IP host or router, a BGP router also has a RoutingTable = IP forwarding table

Usedforpacketforwarding,inrealtime

ThisisnotthesameasaRIB,wewillseetheinteractionlater


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TheDecisionProcess

The decisionprocess decides which route is selected;its output is written into LocRIB

At most one best route to exactly the same prefix ischosen

Onlyonerouteto2.2/16canbechosen

Buttherecanbedifferentroutesto2.2.2/24and2.2/16

Routes are com ared a ainst each other usin thefollowing sequence of criteria, until only one route

remains:0. Highestweight(Ciscoproprietary)

1. HighestLOCALPREF

2. ShortestASPATH

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3. LowestMED,iftakenseriouslybythisnetwork

4. EBGP>IBGP

5. ShortestpathtoNEXTHOP,accordingtoIGP

6. LowestBGPidentifier


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FundamentalExamples

AS xI-BGP

R4

R1

10.1/16 AS=(y)

10.2/16 AS=(y)

10.1/16 AS=(y)

10.2/16 AS=(y)E-BGP E-BGP

R3

1.1.1.1 2.2.2.2

1.1.1.2 2.2.2.1

AS y

10.1/16 10.2 16

Skrivawarandaol

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In this simple example there are 4 BGP routers. They communicate directly orindirectly via EBGP or IBGP, as shown on the figure.

There are 2 ASs, x and y. We do not show the details of the internals of y. R3 and R4send the BGP messages shown.

In the next slide, we show only the following attributes in the RIBs: destination,path, NEXTHOP


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R1 has the following RIBs, initially empty:Adj-RIB-In from R3Adj-RIB-In from R2Loc-RIBAdj-RIB-Out to R3

Adj-RIB-Out to R2R1 receives from R3 two announcements, made ofthe routes 10.1/16 AS=(y) and 10.2/16AS=(y). R1 associates the NEXTHOP attribute

R1 prepares to send these new routes to all its IBGP peers by writing them into AdjRIBOut toR2:

Adj-RIB-In from R310.1/16 AS=y NEXT-HOP=1.1.1.210.2/16 AS=y NEXT-HOP=1.1.1.2

Adj-RIB-In from R2Loc-RIB

1.1.1.2 to these routes and writes them in its AdjRIBIn:Adj-RIB-In from R3

10.1/16 AS=y NEXT-HOP=1.1.1.210.2/16 AS=y NEXT-HOP=1.1.1.2

Adj-RIB-In from R2Loc-RIBAdj-RIB-Out to R3Adj-RIB-Out to R2

R1 a lies its decision rocess, and selects both

. - . . .10.2/16 AS=y NEXT-HOP=1.1.1.2

Adj-RIB-Out to R3Adj-RIB-Out to R2


Since AdjRIBOut to R2 changed, a BGP messageis sent to R2

Similarly, at some time R1 will receive from R2 a=

routes:Adj-RIB-In from R3

10.1/16 AS=y NEXT-HOP=1.1.1.210.2/16 AS=y NEXT-HOP=1.1.1.2Adj-RIB-In from R2Loc-RIB



.

NEXT-HOP=2.2.2.1 and 10.2/16 AS=y NEXT-HOP=2.2.2.1:



Loc-RIB

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R1 prepares to send these new routes to all its EBGP peers by writing them into AdjRIBOut.However, the message is not sent to R3 as itwould create a loop in the ASpath.





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R1 applies again its decision process. Now ithas several possible routes to each prefix.The first applicable rule in the decision

process (slide The Decision Process) saysthat if a route is learnt from EBGP it hasprecedence over a route learnt from IBGP.Since all routes in Adj-RIB-In from R2 arelearnt from IBGP and all routes in Ad -RIB-In from R3 are learnt from EBGP, thewinners are the latter, so there is no changein Loc-RIB:


Adj-RIB-In from R210.1/16 AS=y NEXT-HOP=2.2.2.1

. =y - = . . .Loc-RIB




Since there is no change in Loc-RIB there is

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no change in Adj-RIB-Out and therefore nomessage is sent by R1.


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AS x I-BGP R21 I-BGP

I-BGP

FundamentalExamples,continued

R4

R1

10.1/16 AS=(y)10.2/16 AS=(y)

E-BGP E-BGP

R3

1.1.1.1 2.2.2.2

1.1.1.2 2.2.2.1

3.3.3.1

3.3.3.2

10.2/16 AS=(y)

AS y

10.1/16 10.2 16

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.TCP connections with each other (same in AS y, but not shown on figure).

An IGP (for example OSPF) also runs on R1, R21 and R22. All link costs are equal to 1.

The announcements made by R3 and R4 are different, as shown on the figure.


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Here is a possible sequence of messages and the resultingstates of RIBs, at R1, after message is processed:

R1 receives message 10.1/16 AS=(y) from R3 over E_BGP:Adj-RIB-In from R3

10.1/16 AS=y NEXT-HOP=1.1.1.2

Adj-RIB-In from R22Adj-RIB-In from R21Loc-RIB

10.1/16 AS=y NEXT-HOP=1.1.1.2Adj-RIB-Out to R3Adj-RIB-Out to R22

- -

Assume R21 also received the EBGP messages shown onthe figure and did its job. R1 receives message 10.2/16AS=y, NEXT-HOP=3.3.3.1 from R21 over IBGP:Adj-RIB-In from R3

10.1/16 AS=y NEXT-HOP=1.1.1.2

Adj-RIB-In from R2210.2/16 AS=y, NEXT-HOP=2.2.2.1


Loc-RIB10.1/16 AS=y NEXT-HOP=1.1.1.2

- -

R1 applies its decision process, accepts the route andsends it to R22 and R21Adj-RIB-In from R3

10.1/16 AS=y NEXT-HOP=1.1.1.2Adj-RIB-In from R22Adj-RIB-In from R21Loc-RIB


10.1/16 AS=y NEXT-HOP=1.1.1.2Adj-RIB-Out to R21

10.1/16 AS= NEXT-HOP=1.1.1.2

. = - = . . .Adj-RIB-Out to R3Adj-RIB-Out to R22


10.1/16 AS=y NEXT-HOP=1.1.1.2

The decision process now has to choose between tworoutes with same destination prefix 10.2/16. Both werelearnt from IBGP, so we apply criterion 5 in slide TheDecision Process.The distance, computed by the IGP, to 2.2.2.1 is 3 and thedistance to 3.3.3.1 is 2. Thus the route that has NEXT-

Assume R22 also received the EBGP messages shown onthe figure and did its job. R1 receives message 10.2/16

AS=y, NEXT-HOP=2.2.2.1 from R22 over IBGP, and thedecision process accepts it:

Adj-RIB-In from R310.1/16 AS=y NEXT-HOP=1.1.1.2


Adj-RIB-In from R21Loc-RIB

10.1/16 AS= NEXT-HOP=1.1.1.2

. . .Adj-RIB-In from R3

10.1/16 AS=y NEXT-HOP=1.1.1.2Adj-RIB-In from R22

10.2/16 AS=y, NEXT-HOP=2.2.2.1Adj-RIB-In from R21

10.2/16 AS=y, NEXT-HOP=3.3.3.1Loc-RIB




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10.1/16 AS=y NEXT-HOP=1.1.1.2

Since the new message is learnt from IBGP it is not putinto any AdjRIBOut

10.1/16 AS=y NEXT-HOP=1.1.1.2

Since the new message is learnt from IBGP it is not putinto any AdjRIBOut.


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Questions

What is the difference between IBGP and IGP ?

What is the difference between IBGP and EBGP ?

What is the difference between a RIB and a Routing Table ?

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Howareroutesinjectedinto BGP

BGP propagates route information, but how is this

Two methods

Staticconfiguration:tellthisBGProuterwhicharetheprefixesownedby

thisdomain

ImportfromIGP:tellthisroutertocopyalltheprefixesthattheIGPhas

earn

AssumestheIGPeitherdoesnotpropagateexternalprefixes,orhasawayto

differentiatethem

SuchroutesaresenttoEBGPneighboursonly,withORIGIN=IGP

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2.HowForwardingEntrieslearntbyBGParewritten

intoRoutingTables

So far, we have seen how BGP routers learn about all the

.

routers in their AS about these prefixes ( write forwarding

entries in slide Inside BGP ). This is the topic of this

section.

There are two possible ways for this, usually mutually

exc us ve:

Redistribution: routes learnt by BGP are passed to IGP (ex:OSPF)

CalledredistributionofBGPintoOSPF

OSPF ro a atestheroutesusin t e4LSAstoallroutersinOSPFcloud

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Notfoundmuchinpractice

Injection: routes learnt by BGP are written into the forwarding

table of this router

Routesdonotpropagate;thishelpsonlythisrouter


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RedistributionExample

AS xE-BGPR5 R6

AS z

R5 advertises 18.1/16 to R6 via EBGP E-BGP

R1

R218.1/16 I-BGP

IGP(OSPF)

IGP(OSPF)

2.2.2.2

AS y

R6 transmits it to R2 via IBGPTCPconnectionbetweenR6andR2

(redistributeBGPintoIGP) R6 injects 18.1/16 into IGP(OSPF)

OSPFpropagates18.1/16(type4LSA)andupdatesforwardingtables

AfterOSPFconver es,R1,R2nowhavearouteto18.1/6R2

R4

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advertisesroutetoR4viaEBGP

(synchronize with IGP) R2 must wait for the OSPF entryto 18.1/6 before advertising via EBGP

Packet to 18.1/16 from AS y finds forwarding tableentries in R2, R1 and R6


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ExamplewithReDistributionby

___,

F

learns

from

G

the

route

D2

D1

n1

CredistributestheexternalrouteD2:n2intoOSPF

y____, earns erou e :n ; y____ earns erou e :n ;

selectsD2:n2anddoesnotredistributeittoOSPF

by___,BlearnstherouteD2:n2

by___,AlearnstherouteDX:D2:n2

by___,Llearnstherouteton2viaC

D1 = EPFL D2 = UCB

n1

D1: n1

n2

A C

FD2 : n2

G

32

DX DYD2 : n2DY D2: n2

D1: n1B

D EL

physical link

TCP connectionsolution


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ReDistributionisConsideredHarmful

In practice, operators avoid redistribution of BGP into IGP

ConvergencetimeafterfailuresislargeifIGPhasmanyroutingtableentries

Therefore, injection is used instead

Injection assumes that routers in the AS employ recursivetable

lookup

WhenIPpacketissubmittedtorouter,theforwardingtablemayindicatea

NEXTHOP

which

is

not

on

link

with

router

Asecondtablelookupneedstobedonetoresolvethenexthopintoanon

linkneighbour

inpractice,secondlookupmaybedoneinadvance notinrealtime bypre

33

processingtheroutingtable


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Example:RecursiveTableLookup

At R1, data packet to 10.1.x.y is received

Q:whatarethenextevents?

Forwarding Table at R1

R1

To NEXT-HOP ayer- a r10.1/16 2.2.2.63 N/A

2.2.2.63 2.2.2.33 x09:F1:6A:33:76:21

2.2.2.33 2.2.2.6310.1/16R2

34

2.2.2.93

solution


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InjectionExample

AS xE-BGPR5 R6

AS z

R5 advertises 18.1/16, NEXT-HOP = 2.2.2.2 E-BGP

R1

R218.1/16 I-BGP

IGP

IGP(OSPF)

2.2.2.2

AS y

to R6 via EBGP

R6injects18.1/16, NEXT-HOP = 2.2.2.2 intoitslocalforwardingtable(doesnotredistributeinto

OSPF)

R2 learns route from R6 via IBGP

- =

R4

35

. , . . .

itslocalforwardingtable

Data packet to 18.1.2.3 is received by R2

RecursivetablelookupatR2canbeused

Q:thereisaproblematR1:howcanwesolveit?solution


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InjectioninPracticeRequiresallRouterstoRunBGP

AS xE-BGP

R2

R5 R6

18.1/16 I-BGPIGP

2.2.20.12.2.2.2

AS z

Practical solution often deployed

AllcoreroutersalsorunIBGP(inadditiontoIGP)

E-BGP

R4

R1 IGP

AS y

RouteslearntfromBGPareinjectedintolocal

forwarding

tableRecursivetablelookupisdoneatallrouters

Q:repeatthesequenceofpreviousslidewiththisnew

assumption

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IGP handles only internal networks very few

solution


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Questions

What is the difference between injection and redistribution ?

Can BGP routes be learnt by a non BGP router ?

What is recursive table lookup ? Where is it needed ?

37


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3.OtherBellsandWhistles

MULTIEXITDISC(MED)

packet to 10.1.2.3

AS x

R2R1

10.1/16 MED=1010.1/16 MED=50

10.2/16 MED=10E-BGP E-BGP

. . .

1.1.1.1 2.2.2.2

AS y

R4

.

10.1/16 10.2/16

R3

38

One AS connected to another over several links

ex:multinationalcompanyconnectedtoworldwideISP

ASyadvertisesitsprefixeswithdifferentMEDs(low=preferred)

IfASxacceptstouseMEDsputbyASy:trafficgoesonpreferredlink


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MEDExample

Q1: by which mechanisms will R1 and R2 make sure that

Q2: router R3 crashes; can 10.1/16 still be reached ? explain the

sequence of actions.

solution

39


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MEDQuestion

Q1: Assume now ASx and ASy are peers (ex: both are ISPs).

.

Q2: By which mechanisms can ASx pick the nearest route to ASy

?

solution

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LOCALPREF

AS x

E-BGP

R1 R2

R6

I-BGP

I-BGP E-BGP

E-BGP pref=10

pre =

Used inside an AS to select a bestAS

pathAssigned by border router when receiving route over E

BGP

PropagatedwithoutchangeoverIBGP

41

Example

R6associatespref=100,R2pref=10

R1choosesthelargestpreference

bgp default local-preference pref-value


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42

LOCALPREFExample

Q1: The link AS2AS4 is expensive. How should AS 4 set localprefs on routesreceived from AS 3 and AS 2 in order to route traffic preferably through AS 3 ?

: xp a n t e sequence o events or , an .

AS 1

AS1: 10.1/1610.1/16

AS1: 10.1/16

AS 3

AS 4R1R2

R3

42

AS 5

solution


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44

Weight

This is a route attribute given by Cisco or Zebra router

Neverpropagatedtootherrouters,eveninthesamecloud

Thereforethereisnoweightattributeinrouteannouncements

44


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45

RouteFlapDampening

Route modification propagates everywhere

successiveUPDATEandWITHDRAW

causedforexamplebyBGPspeakerthatoftencrashesandreboots

Solution:

decisionprocesseliminatesflappingroutes

How

withdrawnroutesarekeptinAdjRINin

ifcomesupagainsoon(ie:flap),routereceivesapenalty

ifpenalty suppresslimitrouteisnotselected

45

penaltyfadesoutexponentially,seenextslide


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46

RouteFlapDampening

suppress-limit

e

reuse-limit

time

t1 t2

46

Route suppressed att1, restored att2


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47

AvoidIBGPMesh:Confederations

E-BGP

I-BGP I-BGP I-BGP

E-BGP

AS decomposed into subAS

privateASnumber

z

E-BGPE-BGP E-BGP

47

similartoOSPFareas

IBGPinsidesubAS(fullinterconnection)

EBGPbetweensubAS


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48

AvoidIBGPMesh:Routereflectors

I-BGP

I-BGP

I-BGP I-BGP I-BGP

I-BGP

RR RR RR

cluster 1 cluster 2 cluster 3

Cluster of routers

oneIBGPsessionbetweenoneclientandRR

CLUSTER_ID

z

E-BGPE-BGP E-BGP

48

Route reflector

readvertisesaroutelearntviaIBGP

toavoidloops

ORIGINATOR_IDattributeassociatedwiththeadvertisement


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49

IBGPconfiguration

lo0lo0I-BGP

I-BGP

IBGP configured on loopback interface (lo0)

AS z

49

interfacealwaysup

IPaddressassociatedwiththeinterface

IGProutingguaranteespacketforwardingtotheinterface


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50

AvoidEBGPmesh:Routeserver

E-BGP

50

t nterconnect on po nt

Instead ofn(n1)/2 peer to peer EBGP connections

n connections to Route Server

To avoid loops ADVERTISER attribute indicates which router inthe AS generated the route


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51

Communities

Other attributes can be associated with routes in order to

.

Predefined:Example:NOEXPORT(awellknown,predefinedattribute)

seelaterforanexample

DefinedbyoneAS(alabeloftheformASN:xwhereAS=ASnumber,x=a2

bytenumber)

51


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52

NOEXPORT

Written on EBGP by one AS, transmitted on IBGP by accepting AS, notforwarded

Example: AS2 has different routes to AS1 but AS2 sends only one aggregateroute to AS3

simplifiestheaggregationrulesatAS2

Whatistheroutefollowedbyapacketsentto2.2.48receivedbyR4?

2.2.0/17 NO-EXPORT

2.2.0/17

.

2.2/16R3

R4

52

2.2.128/17

2.2.128/17 NO-EXPORT2.2/16

R2

solution


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53

C.Examples

rules for

Importpolicy(acceptrouteornot,ifrequiredmodify/add/deleteattributes)

Exportpolicy(exportornotaroute,decidewhichattributevalue)

We illustrate two scenarios:

Dual Homing

53


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54

Ex1: StubArea

Operator AS

R1

R2

BGP not needed between Client and Operator

No AS number for client

R2 learns all prefixes in Client by static configuration or RIP on link R1R2

Client AS

54

Example: EPFL and Switch

what if R1 fails ?


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55

Ex2:StubArea,DualHomingtoSingleProvider

AS x

R2R1

10.1/16 MED=50

Provider

R4

.

10.2/16 MED=5010.2/16 MED=10

10.1/16 10.2/16

- -

R3

Client

With numbered Client AS

UseMEDtosharetrafficfromISPtoClientontwolinks

UseClientIGPconfigurationtosharetrafficfromClienttotwolinks

y

55

Q1:isitpossibletoavoiddistributingBGProutesintoClientIGP?

Q2:isitpossibletoavoidassigninganASnumbertoClient?

Q3:isitpossibletoavoidBGPbetweenClientandProvider?

solution


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56

Ex3:StubArea,DualHomingtoSeveralProviders

AS x AS y

AS z

R4

E-BGP E-BGP

R3

Provi ers

Client has own address space and AS number

Q: how can routes be announced between AS 100 and AS x ? AS x and AS z ?

AS 100

2.0/17 .ent

56

Q: assume Client wants most traffic to favour AS x. How can that be done ?

solution


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57

Ex4:HotPotatoRouting

Customer 1

ISP1R11 R12

ISP2R21 R22

Customer 2

Packets from Customer 2 to Customer 1BothR21andR22havearoutetoCustomer1

ShortestpathroutingfavoursR21

Q1:bywhichmechanismisthatdone?

Q2: what is the path followed in the reverse direction ?

57

solution


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58

D.Illustrations: TheSwitchNetwork

www.switch.ch

58


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59

AnInterconnectionPoint

59


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60


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61

Type Next HOP MED Origin Community RRC ID

A128.178.0.0/15

2003-10-0205:05:49Z

129.250.0.232

129.250.0.232

9 Not defined2914 1299559

2914:4202914:20002914:3000

RIPE NCC

rom www.ris.ripe.net: all routes to 128.178.0.0/15 on RIPE Route Collectors

A128.178.0.0/15

2003-10-0206:16:00Z

193.10.252.5

193.10.252.5

0 IGP2603 33561299 559

2603:6663356:23356:863356:5073356:6663356:2076

Netnod

A128.178.0.0/15

2003-10-0206:16:17Z

194.68.48.1 194.68.48.1 0 IGP12381 16532603 20965559

12381:1653 Netnod

A128.178.0.0/15

2003-10-0206:16:37Z

194.68.48.1 194.68.48.1 0 IGP12381 16532603 3356

1299 559

12381:1653 Netnod

A128.178.0.0/15

2003-10-0206:21:08Z

193.10.252.5

193.10.252.5

0 IGP2603 20965559

2603:2222603:66620965:155

Netnod

A128.178.0.0/15

2003-10-0206:21:17Z

194.68.48.1 194.68.48.1 0 IGP12381 16532603 20965559

12381:1653 Netnod

61

A128.178.0.0/15

2003-10-0207:24:06Z

129.250.0.232

129.250.0.232

9 Not defined2914 3549559

2914:420

P r e f ixT i m eP e e rA S p a th


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TheWorldseenfromEPFL

62

http://www.ris.ripe.net/bgpviz/


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63

Somestatistics

Source: http://www.cidrreport.org

Address prefixes

ASs

63


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64

Numberofhosts

64


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65

65


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66

Exercise

What ASs does EPFL receive service from ?

What ASs does Switch receive service from ?

Find the names of the networks that have these AS numbers

66


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67

Exercise

Lookup http://rpsl.info.ucl.ac.be. to find out the relationships

How does the software on this site decide whether a

relationship is client, provider or peer ?

67


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68

FurtherReading

Slow convergence after route suppression BGP path

,

convergence. Is in the nature of path vector routing with

explicit suppression.

CraigLabovitz,AbhaAhuja,AbhijitBose,FarnamJahanian:DelayedInternet

routingconvergence.IEEE/ACMTrans.Netw.9(3):293306(2001)

oute ap ampen ng s ows own convergence

ZhuoqingMorley

Mao,

Ramesh

Govindan,

George

Varghese,

Randy

H.

Katz:

Routeflapdampingexacerbatesinternetroutingconvergence.SIGCOMM

2002:221233

Path vector + policy may suffer from incompatibilities (loops)

68

Thestablepathsproblemandinterdomainrouting

Griffin,T.G.;Shepherd,F.B.;Wilfong,G.

ACM/IEEEToNApril2002, Page(s):232243


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69

References

Timothy Griffins home page at Intel

C. Huitema, Le Routage dans lInternet

John W. Stewart III BGP 4

www.ris.ripe.net: AS paths

www.c rrepor .org aggrega on s a s cs

www.caida.org map of Internetrpsl.info.ucl.ac.be relations between ASs

69


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Solutions

70


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71

PathVectorRouting

Q. Explain how E can choose the best paths to n1 and n2A.EreceivestheroutesBAn1andDCAn1.EselectsasbestroutestheoneswithshorterAS ath.

Q. How can loops be avoided ?A:BGProutersrecognizeloopingannouncementsbytherepetitionofthesameASinthepath.Suchannouncementsarediscarded

back

B

En1, n2

A:n1,n2

B A:n1,n2

B:n5 dest AS path

n1 B A

n2 B A

n3 D C

n4 D

a e nn5

n4

71

A

CA:n1,n2

C A:n1,n2

C:n3

D

D C A:n1,n2

D C: n3

D: n4

n5 B

n3


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72

Q1: Which of the following announcements do you think router D willsend to router E ?1. D>E: D2X:n1

2. D>E: D1X:n1

A:1only;2cannotbesentbecauseoverIBGPwedonotrepeataroutelearntromanot er spea er nt esame

Q2: Assume that in the figure below the router R does notrun BGP. Is thispossible ?A: yes.

A: no; C and F are not directly connected (are not onlink)

D1 D2A B

X:n1 X:n1

E-BGPback

72

D4 D5

D3

G H

E F

E-BGP

I-BGP R


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73

AggregationExample2

197.8.2.0/24

AS2

AS3 AS4

197.8.3.0/24

197.8.2.0/23

197.8.3.0/24

197.8.2.0/23

n2

n4

Q:what

happens

to

packets

from

n4

to

n2

?

A:TherearetworoutingentriesinAS4routers:onefor197.8.2.0/23 andonefor197.8.3.0/24. Longestprefixmatchinthepacketforwardingalgorithmensuresthatpacketston2goonthedirectroute(senttoAS2).

73

IfAS4injectsonlytheaggregaterouteintoitslocalroutingtables,whatistheanswernow?

Thereisnowonlyoneentry(197.8.2.0/23 )andallpacketston2aresenttoAS3back


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74

AggregationExample3

197.8.2.0/24

AS2

AS3 AS4

197.8.3.0/24

197.8.2.0/23 197.8.2.0/23

197.8.3.0 24 AS5 AS6

197.8.3.0/24n2

n4

AS4 receives197.8.2.0/23 AS_PATH: 3 {1 2}

197.8.3.0/24 AS_PATH: 6 5 2

197.8.3.0/24

74

.A. they go the long wayAssume now that only shortest AS paths routes are injected into routing tablesQ: what happens to packets from n4 to n2?

A: they go via AS3

back


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75

ExampleWithoutAggregation

197.8.2.0/24 197.8.0.0/23 197.8.0.0/23

AS2

AS3 AS4

197.8.3.0/24

197.8.3.0/24

197.8.2.0/24

197.8.3.0/24

197.8.2.0/24

197.8.3.0/24

Q: If AS3 does not aggregate, what are the routes announced by AS 4 ?Is there any benefit ?

A:

197.8.0.0/23 AS_PATH: 4 3

75

. . . _

197.8.3.0/24 AS_PATH: 4 2

A: there is no benefit since all routes go via AS 4 anyhow. AS4 shouldaggregate.back


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76

NEXTHOP

AS xE-BGP R6

AS z

-

R1 R2

R5I-BGP

I-BGP

AS y10.1/16 10.2/16

R3 R4

76

R3 advertises 10.2/16 to R1, NEXTHOP = R4 IP address

R6 advertises 10.2/16 to R5, NEXTHOP = R6 IP address

Q. where is such a scenario likely to happen ?

A: in interconnection points with many providers interconnected on one

LAN back


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77

MEDExample

Q1: by which mechanisms will R1 and R2 make sure that

A:

R1andR2exchangetheirroutestoAXyviaIBGP

R1has2routesto10.1/16,onelearntbyEBGP(MED=10),onelearntbyI

BGP(MED=5=);preferstheformer

. ,

Q2: router R3 crashes; can 10.1/16 still be reached ? explain thesequence of actions.

A:

R1clearsroutestoASylearntfromR3(keepalivemechanism)

77

R2isinformedoftheroutesuppressionbyIBGP

R2hasnowonly1routeto10.1/16and1routeto10.2/16;

trafficto10.1/16nowgoestoR2 back


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78

MEDQuestion

Q1: Assume now ASx and ASy are peers (ex: both are ISPs). Explain why ASx

is not interested in taking MED into account.

A: ASx is interested in sending traffic to ASy to the nearest exit, avoiding

transit inside ASx as much as possible. Thus ASx will choose the nearest

route to ASy, and will ignore MEDs

Q2: By which mechanisms can ASx pick the nearest route to ASy ?

A: it depends on the IGP. With OSPF: all routes to ASy are injected into OSPF

by means type 5 LSAs. These LSAs say: send to router R3 or R4. Every OSPF

router inside ASx knows the cost (determined by OSPF weights) of the pathfrom self to R3 and R4. Packets to 10.1/16 and 10.2/16 are routed to the

nearest among R3 and R4 (nearest = lowest OSPF cost).

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79

LOCALPREFExample

Q1: The link AS2AS4 is expensive. How should AS 4 set localprefs on routesreceived from AS 3 and AS 2 in order to route traffic preferably through AS 3 ?

all routes received from AS 2

AS 1

AS1: 10.1/1610.1/16AS1: 10.1/16 Q2: Explain the sequence of events

for R1, R2 and R3

AS 3

AS 4R1R2

R3

:R1 receives the route AS2 AS1 10.1/16

over E-BGP; sets LOCAL-PREF to 50R2 receives the route AS3 AS1 10.1/16over E-BGP; sets LOCAL-PREF to 100

R3 receives AS2 AS1 10.1/16, LOCAL-PREF=50 from R1 over I-BGP and

AS3 AS1 10.1/16, LOCAL-PREF=100from R1 over I-BGPAS3 AS1: 10.1/16

79

AS 5

se ec s . , -PREF=100 and installs it into local-RIB

R3 announces only AS3 AS1 10.1/16 toAS 5

back


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80

LOCALPREFQuestion

Q: Compare MED to LOCALPREF

MEDisusedbetweenASs(i.e.overEBGP);LOCALPREFisusedinsideone

AS(overIBGP)

MEDisusedtotelloneproviderASwhichentrylinktoprefer;LOCALPREFisusedtotelltherestoftheworldwhichASpath wewanttouse,bynotannouncin theotherones.

back

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81

ExamplewithReDistributionby

IBGP,

F

learns

from

G

the

route

to

D2

D1

n1

CredistributestheexternalrouteD2:n2intoOSPF;

y , earns erou e :n ; y earns erou e :n ;

selectsD2:n2anddoesnotredistributeittoOSPF

byIBGP,BlearnstherouteD2:n2fromC

byEBGP,AlearnstherouteDX:D2:n2

byOSPF,Llearnstherouteton2viaC

I-BGP - internal BGP

E-BGP - external BGP

D1 = EPFL D2 = UCB

n1

D1: n1

n2

A C

FD2 : n2

G

81

DX DYD2 : n2DY D2: n2

D1: n1B

D EL

physical link

TCP connectionback


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82

Example:RecursiveTableLookup

At R1, data packet to 10.1.x.y is received

Q:whatarethenextevents?

A:first,thenexhop2.2.2.63isfound;asecondlookupfor2.2.2.63isdone;

thepacketissenttoMACaddressx09:F1:6A:33:76:21

R1

To NEXT-HOP ayer- a r10.1/16 2.2.2.63 N/A

2.2.2.63 2.2.2.33 x09:F1:6A:33:76:21

2.2.2.33 2.2.2.63

10.1/16R2

82

2.2.2.93

back


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83

AvoidRedistribution:CombineRecursiveLookup

andNEXTHOP

AS xE-BGPR5 R6

AS z

Data packet to 18.1.2.3 is received by R2 E-BGP

R1

R218.1/16 I-BGP

IGP

IGP(OSPF)

2.2.2.2

AS y

: ere sapro ema : owcanweso ve

A:theproblemis:PacketissentbyR2towardsR1,

whichis

the

neighbour

towards

2.2.2.2.

At

R1,

there

is

noforwardingentryto18.1/16 blackhole!Asolutionwouldbetouseloosesourcerouting:R2adds2.2.2.2asloosesourceroutinginfointopacket.Anothersolutionislabelswitching(seeMPLSmodule)alsocalledtagswitching.R2establishesalabelswitched

R4

83

pathto2.2.2.2andforwardsthepacketonthispath.

Inpracticehowever,anothersolution,asshownnext.

back


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84

AvoidRedistribution:PracticalSolution

AS xE-BGP

R2

R5 R6

18.1/16 I-BGPIGP

2.2.20.12.2.2.2

AS z

Q:repeatthesequenceofpreviousslidewiththisnewassumption

R5advertises18.1/16, NEXT-HOP = 2.2.2.2 toR6viaEBGP

R6transmits18.1/16, NEXT-HOP = 2.2.2.2 to R1andR2via

E-BGP

R4

R1 IGP

AS y

IBGP

R6injects18.1/16, NEXT-HOP = 2.2.2.2 intoitslocalforwardingtableR2injects18.1/16, NEXT-HOP = 2.2.2.2 intoitslocalforwardingtable

Independently,IGPfindsthat,atR2,packetsto2.2.10.1shouldbesenttoR1

Datapacketto18.1.2.3isreceivedbyR2

AtR2,recursivetablelookupdeterminesthatpacketshouldbeforwardedto

84

R1

AtR1,recursivetablelookupdeterminesthatpacketshouldbeforwardedtoR6

AtR6,recursivetablelookupdeterminesthatpacketshouldbeforwardedto2.2.2.2

back


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85

NOEXPORT

Q:Whatistheroutefollowedbyapacketsentto2.2.48receivedbyR4?

A:the acketissentviaR3andR1

2.2.0/17 NO-EXPORT

2.2.0/17

.

2.2/16R3

R4

85

2.2.128/17

2.2.128/17 NO-EXPORT2.2/16

R2

back


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86

Sol.Ex2:StubArea,DualHomingtoSingleProvider

AS x

R2R1

10.1/16 MED=50

Provider

R4

.

10.2/16 MED=5010.2/16 MED=10

10.1/16 10.2/16

- -

R3

Client

Q1:is

it

possible

to

avoid

distributing

BGP

routes

into

Client

IGP

?

A:yes,forexample:configureR3andR4asdefaultroutersinClientAS;trafficfromClientASisforwardedtonearestofR3andR4.IfR3orR4fails,totheremainingone

Q2:isitpossibletoavoidassigninganASnumbertoClient?

A:Yes itissufficienttoassi ntoClienta rivateASnumber:Providertranslatesthis

y

86

numbertoitsown.

Q3:isitpossibletoavoidBGPbetweenClientandProvider?

A:Yes,byrunningaprotocollikeRIPbetweenClientandProviderandredistributingClientroutesintoProviderIGP.ThusProviderpretendstotherestoftheworldthattheprefixesofClientareitsown. back


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87

Sol.Ex3:StubArea,DualHomingtoSeveralProviders

AS x AS y

AS z

R4

E-BGP E-BGP

R3

Provi ers

Client has own address space and AS number

Q: how can routes be announced between AS 100 and AS x ? AS x and AS z ?A: R3 announces 2.0 17 and 2.0 16 traffic from AS x to 2.0 17 will flow via AS x if

AS 100

2.0/17 .ent

87

R3 fails, it will use the longer prefix and flow via Asy.ASx announces 2.0/17 and 2.0/16 to AS z

Q: assume Client wants most traffic to prefer AS y. How can that be done ?A: R3 announces an artificially inflated path: 100 100 100 100 : 2.0/17. AS z willfavour the path via AS y which has a shorter AS path length back


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88

Sol.Ex4:HotPotatoRouting

Customer 1

ISP1R11 R12

ISP2R21 R22

Customer 2

Packets from Customer 2 to Customer 1BothR21andR22havearoutetoCustomer1

ShortestpathroutingfavoursR21

Q1:bywhichmechanismisthatdone?

88

: o ceo t e estroute cr ter a ,assum nga routers n run

Q2: what is the path followed in the reverse direction ?

A:seepicture.Notetheasymmetricroutingback


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89

Exercise

What ASs does EPFL receive service from ?

,

What ASs does Switch receive service from ?

fromthepreviousroutesweseethatthereareatleast:

AS1299

AS20965

AS3549

Find the names of the networks that have these AS numbersfromwhoisonwww.ripe.net:

AS1299:Telianet

AS20965:Geant

89

AS3549:GlobalCrossing


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90

Exercise

Lookup http://rpsl.info.ucl.ac.be. to find out the relationships

How does the software on this site decide whether a

relationship is client, provider or peer ?

ASXisclientofSwitchifASXacceptsANYpathandannouncesonlyself(AS

X)

ASXisproviderofSwitchifASXannouncesANYpathandacceptsonlyAS

SwitchASXisapeerifASXacceptsandannouncesonlyasmallsetofroutes

90


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external routing bgp

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