Inter-Autonomous-System Routing: Border Gateway Protocol · Routing Goal: each router u must be able to compute, for each other router v, the next-hop neighbor x that is on the least-cost

Inter-Autonomous-System Routing:

Border Gateway Protocol

Antonio Carzaniga

Faculty of InformaticsUniversity of Lugano

June 14, 2005

c© 2005 Antonio Carzaniga

Outline

Hierarchical routing

BGP


Routing


Routing

Goal: each router u must be able to compute, for each otherrouter v , the next-hop neighbor x that is on the least-cost pathfrom u to v


Routing

Goal: each router u must be able to compute, for each otherrouter v , the next-hop neighbor x that is on the least-cost pathfrom u to v

u

v

x1x2

x3

x4


Network Model


Network Model

So far we have studied routing over a “flat” network model


Network Model


a b c

d e f

g h j

Also, our objective has been to find the lowest-cost pathsbetween sources and destinations


Network Model


a b c

d e f

g h j

3 4

3 9

211 1 1

1 1 24

14

Also, our objective has been to find the lowest-cost pathsbetween sources and destinations


More Realistic Topologies


More Realistic Topologies


Even More Realistic


Even More Realistic

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ast


An Internet Map

c©1999 Lucent Technologies


Higher-Level Objectives



ScalabilityI hundreds of millions of hosts in today’s Internet



ScalabilityI hundreds of millions of hosts in today’s InternetI transmitting routing information (e.g., LSAs) would be too

expensive




expensiveI forwarding would also be too expensive





Administrative autonomy





Administrative autonomyI one organization might want to run a distance-vector routing

protocol, while another might want to run a link-state protocol





Administrative autonomyI one organization might want to run a distance-vector routing

protocol, while another might want to run a link-state protocolI an organization might not want to expose its internal network

structure


Hierarchical Structure

Today’s Internet is organized in autonomous systems (ASs)

I independent administrative domains





Gateway routers connect an autonomous system with otherautonomous systems





Gateway routers connect an autonomous system with otherautonomous systems

An intra-autonomous system routing protocol runs within anautonomous system

I this protocol determines internal routesI internal router ↔ internal routerI internal router ↔ gateway routerI gateway router ↔ gateway router


Inter-AS Routing

An inter-autonomous system routing protocol determines routingat the autonomous-system level


Inter-AS Routing

An inter-autonomous system routing protocol determines routingat the autonomous-system level

Once computed by the gateway routers, inter-AS routinginformation is propagated within an autonomous system

I all routers use both inter-AS and intra-AS routes to compile theirforwarding tables


Hierarchical Routing

Destinations within the same autonomous system are reachedas usual




What about a destination x outside the autonomous system?





I routers use inter-AS information to figure out that destination x isreachable through gateway Gx






I routers use intra-AS information to figure how to send the packetto Gx






I routers use intra-AS information to figure how to send the packetto Gx

I in cases in which x is reachable through multiple gateway routersGx , G′

X , . . ., routers use the closest gatewayI “hot-potato” routing


Benefits of Hierarchical Routing

Administrative autonomy



Administrative autonomyI each autonomous system decides what intra-AS routing to use



Administrative autonomyI each autonomous system decides what intra-AS routing to useI an autonomous system needs to expose only minimal information

about the internal structure of its networkI essentially only (sub)net addresses





Scalability





ScalabilityI routers within an autonomous system need to know very little

about the internal structure of other autonomous systems






about the internal structure of other autonomous systemsI essentially only (sub)net addresses






about the internal structure of other autonomous systemsI essentially only (sub)net addresses

External subnet addresses are likely to “aggregate” in groupsthat admit compact representations

I this process is called supernettingc© 2005 Antonio Carzaniga

Inter-AS Routing in the Internet



The Border Gateway Protocol (BGP) is the inter-AS routingprotocol in today’s Internet




I provides reachability information from neighbor ASs




I provides reachability information from neighbor ASsI transmits reachability information to all internal routers within an

AS





ASI determines good routes to all outside subnets






I based on reachability information






I based on reachability informationI based on policies






I based on reachability informationI based on policies

I BGP is a distance-vector protocol (or better path-vector)


BGP Architecture and Terminology

BGP session: a semi-permanent connection between tworouters




BGP peers: two routers engaged in a BGP sessionI BGP sessions are established over TCP





BGP external session (eBGP): a session across twoautonomous systems





BGP external session (eBGP): a session across twoautonomous systems

BGP internal session (iBGP): a session within an autonomoussystem

I note that internal sessions carry inter-AS informationI intra-AS routing is a separate protocol (e.g., OSPF)


BGP Architecture and Terminology (2)

BGP advertisement: a router advertises a routes to networks,much like an entry in a distance-vector

I destinations are denoted by address prefixes




I destinations are denoted by address prefixesI an AS may or may not forward an advertisement for a foreign

network. Doing so means being willing to carry traffic for thatnetwork






I this is where a router may aggregate prefixes (a.k.a.,“supernetting”)E.g.,

128.138.242.0/24128.138.243.0/24

}

→ 128.138.242.0/23







128.138.242.0/24128.138.243.0/24

}

→ 128.138.242.0/23

191.224.128.0/22191.224.136.0/21191.224.132.0/22

→







128.138.242.0/24128.138.243.0/24

}

→ 128.138.242.0/23

191.224.128.0/22191.224.136.0/21191.224.132.0/22

→ 191.224.128.0/20



Autonomous system number (ASN): a unique identifier for eachAS (with more than one gateway)




BGP attributes: a route advertisement includes a number ofattributes

I AS-PATH: sequence of ASNs through which the advertisementhas been sent




BGP attributes: a route advertisement includes a number ofattributes

I AS-PATH: sequence of ASNs through which the advertisementhas been sent

I NEXT-HOP: specifies the interface to use to forward packetstowards the advertised destination. This is used to resolveambiguous cases where an AS can be reached through multiplegateways (interfaces)


Route Selection

1. Router preference: routes are ranked according to a preferencevalue

I configured at the routerI or learned from another router within the same ASI essentially a configuration parameter for the AS


Route Selection



2. Shortest AS-PATH


Route Selection



2. Shortest AS-PATH

3. Closest NEXT-HOP router


Route Selection



2. Shortest AS-PATH

3. Closest NEXT-HOP router

4. . . .


Documents

Inter-Autonomous-System Routing: Border Gateway Protocol · Routing Goal: each router u must be able to compute, for each other router v, the next-hop neighbor x that is on the least-cost