Border Gateway Protocol (BGP) Presentation

BorderGatewayProtocol Rappelsroutage
Static Routing vsdynamic routing
•StaticRoutes
–nopathinformation
–veryversatile
–lowprotocoloverhead
–highmaintainance
–verybadconvergencetime
–requiresmanualconfiguration
•Dynamicprotocolsovercometheselimitations&
problems
–Collectionofnetworkswithsamepolicy
–Singleroutingprotocol
–Usuallyundersingleadministrativecontrol
–StandardRange:1to65535(16bits,extendedto32bitsbyRFC
48931)
–Private:64512to65534
Autonomous System(AS)

IGP
Interior GatewayProtocol
–Protocolederoutageutilisédansàl’intérieurd’un
systèmeautonome
–Rôle
•Établirlesroutesoptimalesentreunpointduréseauet
touteslesdestinationsdisponibles,
•éviterlesboucles,
•Assurerlaconvergenceduréseaudanslesplusbrefsdélais
encasdemodification/évènement.
09/11/2022 5
EGP
•Exterior GatewayProtocol
–UsedtoconveyroutinginformationbetweenAutonomous
Systems
–DecoupledfromtheIGP
–CurrentonlyEGPisBGP
Why:
–Scalingtolargenetwork
•Hierarchy
•Limitscopeoffailure
–Policy
•Controlreachabilitytoprefixes
09/11/2022 6
Interiorvs.ExteriorRoutingProtocols
–Interior
•Automaticdiscovery
•GenerallytrustyourIGProuters
•RoutesgotoallIGProuters
–Exterior
•Specificallyconfiguredpeers
•Connectingwithoutsidenetworks
•Setadministrativeboundaries
09/11/2022 7
Administrativedistance
•Distanceadministrativepour
différencierlaprioritédes
protocoles
–Définitunordredeprioritépourle
routeurpourchoisirunemême
routeprésentedans2protocoles
deroutagesdifférents.
–Définitlafiabilitéd'unprotocolede
routage,duplusfiable(distance
faible)aumoinsfiable(distance
forte).
–Valeurentre0et255,plusfaible
l’emporte
–Possibilitédemodifierladistance
administrativeviaconfiguration
09/11/2022 8

09/11/2022 9
BGP
•BorderGatewayProtocol
–RFC1771;CurrentlyVersionBGP4
–PathVectorProtocol,RunsoverTCP(port179)
–IncrementalUpdates
–Runswithprefix(X.X.X.X/Xroutes)
–PropagatesONLYthebestpath
–Manyoptionsforpolicyenforcement
–WidelyusedforInternetbackbone
09/11/2022 10
eBGP
•ExternalBGPPeering:between2AS
–BetweenBGPspeakersindifferentAS
–Shouldbedirectlyconnected
–DonotrunanIGPbetweeneBGP peers
09/11/2022 11
iBGP
•InternalBGPPeering
–LearnsmultiplepathsviainternalandexternalBGPspeakers
–PicksthebestpathandinstallsintheIPforwardingtable
–Policiesappliedbyinfluencingthebestpathselection
–Notrequiredtobedirectlyconnected
–iBGP speakersneedtobefullymeshed(theydonotpassonprefixeslearnedfromotheriBGP speakerstoavoid
loops)
Informationtransfer

09/11/2022 13
ASPATH
•BGPstoresthelist ofcrossed AS
OrderedsequenceofASaroutehastraversed
09/11/2022 14
ASPATHuse
–Loopdetection
–Applypolicy(filters,priority…)
Présentationetdéfinitiondesattributs
•BGPAttributes
–Isacharacteristicofaprefixinadditiontothe
network/mask
–IstransferredbetweenBGPpeers
–Attributescanbe:
•ASpath
•Nexthop
•Localpreference
•Multi‐ExitDiscriminator(MED)
•BGPcommunity
•Others
09/11/2022 15
L’attributWeight
–Ciscospecific(sortofrouter’sinternallocalpreference)
–Localtotherouter
•Notpropagated
–Value:0‐ 65535
–Default:
•originatedlocally=32768
•other=0
–Routeswithahigherweightarepreferredwhenthere
aremultipleroutestothesamedestination
09/11/2022 16

L’attribut‘LocalPreference’
–GlobaltothelocalAS
–UsedtoinfluenceBGPpathselection
–Pathwithhighestlocalpreferencewins
09/11/2022 17
L’attribut‘LocalPreference’
–LocaltoanAS– non‐transitive
•localpreferencesetto100whenheardfromneighbouring AS
–UsedtoinfluenceBGPpathselection
•determinesbestpathforoutboundtraffic
–PathswithhighestLOCAL‐PREFaremostdesirable(default=100)
•bgp defaultlocal‐preference<value>
09/11/2022 18
AS‐PathPrepending
–AS‐PATHisupdatedbythesendingrouterwithits
ownASnumbertoincreasePATHLength
09/11/2022 19
L’attribut‘ORIGIN'
–Conveystheoriginoftheprefix
–Values
•IGP(i)=vianetworkcommand
•EGP(e)=learnedfromEGP
•incomplete(?)=redistribution
–Influencesbestpathselection
–route‐mapSETORIGINpermit10
–setoriginigp
09/11/2022 20