Department of Computer and IT Engineering

University of Kurdistan

Computer Networks II

Border Gateway protocol (BGP)

By: Dr. Alireza Abdollahpouri

Internet structure: network of networks

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

Tier-2 ISP Tier-2 ISP

Tier-2 ISP Tier-2 ISP

Tier-2 ISP

local ISP local

ISP local ISP

local ISP

local ISP Tier 3

ISP

local ISP

local ISP

local ISP

2

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 routing within AS A ( RIP, OSPF, …)

Intra-AS routing within AS B ( RIP, OSPF, …) Host1

a b

a d

b c

a c

b

C.b

A.a

B.a

A.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)

6

AS Categories

AS1

AS3 AS2

AS1

AS2

AS3 AS1

AS2

Stub

Multi-homed

Transit

7

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

8

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

9

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

10

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.

11

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

15

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

16

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

17

R3 R4

R1

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

18

I-BGP and E-BGP

AS-Path

� Sequence of AS’s a route traverses

� Used for loop detection and to apply policy

120.10.0.0/16 130.10.0.0/16

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-4 130.10.0.0/16 AS-2 AS-3

110.10.0.0/16 AS-2 AS-5

19

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

20

Example: Multiple AS Paths

AS

701

AS

73

AS

7018

AS

1239

AS

9

128.2/16

128.2/16

9 701 128.2/16

9 7018 1239

21

Shorter Doesn’t Always Mean Shorter

AS 4

AS 3

AS 2

AS 1

Path 4 1 is “better”

than path 3 2 1

22

23

Questions Questions