Department of Computer and IT EngineeringDepartment of Computer and IT EngineeringUniversity of KurdistanUniversity of Kurdistan

Computer Networks IIBorder Gateway protocol (BGP)

By: Dr. Alireza AbdollahpouriBy: Dr. Alireza Abdollahpouri

Internet structure: network of networks

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

Tier-2 ISPTier-2 ISP

Tier-2 ISP Tier-2 ISP

Tier-2 ISP

localISPlocal

ISPlocalISP

localISP

localISP Tier 3

ISP

localISP

localISP

localISP

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Hierarchical Routing aggregate routers into

regions, “autonomous systems” (AS)

routers in same AS run same routing protocol “intra-AS” routing

protocol routers in different

AS can run different intra-AS routing protocol

special routers in AS run intra-AS routing

protocol with all other routers in AS

also responsible for routing to destinations outside AS run inter-AS routing

protocol with other gateway routers

gateway routers

3

Internet’s Area Hierarchy

What is an Autonomous System (AS)? A set of routers under a single technical

administration, using Intra-AS routing protocols (e.g., RIP, OSPF) and common metrics to route packets within the AS and using an Inter-AS routing protocol to route packets to other AS’s

Each AS assigned unique ID

4

Intra-AS and Inter-AS routing

Host2

C

A

B

Intra-AS routingwithin AS A( RIP, OSPF, …)

Intra-AS routingwithin AS B( RIP, OSPF, …)Host

1

ab

ad

bc

ac

b

C.b

A.aB.a

Inter-AS routing

between A and

BA.c

BGP

5

AS Categories

Stub: an AS that has only a single connection to one other AS - carries only local traffic.

Multi-homed: an AS that has connections to more than one AS, but does not carry transit traffic

Transit: an AS that has connections to more than one AS, and carries both transit and local traffic (under certain policy restrictions)

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AS Categories

AS1

AS3AS2

AS1

AS2

AS3AS1

AS2

Stub

Multi-homed

Transit

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Internet inter-AS routing: BGP BGP (Border Gateway Protocol): the de facto

standard BGP is a Path Vector protocol:

similar to Distance Vector protocol each Border Gateway broadcast to neighbors

(peers) entire path (i.e., sequence of AS’s) to destination

BGP routes to networks (ASs), not individual hosts

E.g., Gateway X may send its path to dest. Z: Path (X,Z) = X,Y1,Y2,Y3,…,Z

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Internet inter-AS routing: BGP

Suppose: gateway X send its path to peer gateway W W may or may not select path offered by X

cost, policy (don’t route via competitors AS), loop prevention reasons . If W selects path advertised by X, then:

Path (W,Z) = W, Path (X,Z) Note: X can control incoming traffic by controlling its route advertisements to

peers: e.g., don’t want to route traffic to Z -> don’t advertise any routes to Z

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BGP: controlling who routes to you

A,B,C are provider networks X,W,Y are customer (of provider networks) X is dual-homed: attached to two networks

X does not want to route from B via X to C .. so X will not advertise to B a route to C

A

B

C

W X

Y

provider network

customer network

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BGP operation

Q: What does a BGP router do? Receiving and filtering route advertisements from

directly attached neighbor(s). Route selection.

To route to destination X, which path (of several advertised) will be taken?

Sending route advertisements to neighbors.

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Initial routing tables in path vector routing

12

Stabilized tables for four autonomous systems

13

14

BGP messages

BGP messages OPEN: opens TCP connection to peer and authenticates

sender

UPDATE: advertises new path (or withdraws old)

KEEPALIVE keeps connection alive in absence of UPDATES; also ACKs OPEN request (send periodically, every 30 seconds)

NOTIFICATION: reports errors in previous msg; also used to close connection

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Policy with BGP

BGP provides capability for enforcing various policies

Policies are not part of BGP: they are provided to BGP as configuration information

BGP enforces policies by choosing paths from multiple alternatives and controlling advertisement to other AS’s

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Examples of BGP Policies

A multi-homed AS refuses to act as transit Limit path advertisement

A multi-homed AS can become transit for some AS’s Only advertise paths to some AS’s

An AS can favor or disfavor certain AS’s for traffic transit from itself

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R3 R4R1

R2

E-BGP

I-BGP

AS1 AS2

External BGP (E-BGP): BGP runs between different ASs

Internal BGP (I-BGP):  BGP runs between two peers in the same AS

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I-BGP and E-BGP

AS-Path Sequence of AS’s a route traverses Used for loop detection and to apply policy

120.10.0.0/16130.10.0.0/16

110.10.0.0/16

AS-1

AS-2

AS-3 AS-4

AS-5

120.10.0.0/16 AS-2 AS-3 AS-4130.10.0.0/16 AS-2 AS-3110.10.0.0/16 AS-2 AS-5

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BGP Operations (Simplified)

Establish session on TCP port 179

Exchange all active routes

Exchange incremental updates

AS1

AS2

While connection is ALIVE exchange

route UPDATE messages

BGP session

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Example: Multiple AS Paths

AS701

AS73

AS7018

AS1239

AS9

128.2/16

128.2/169 701

128.2/169 7018 1239

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Shorter Doesn’t Always Mean Shorter

AS 4

AS 3

AS 2

AS 1

Path 4 1 is “better” than path 3 2 1

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23QuestionsQuestions