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Introduction to BGP. Lixin Gao ECE Dept. UMASS, Amherst. Border Gateway Protocol (BGP). Dynamic Inter-AS Routing Protocol Goals: Exchange reachability information between ASes Apply local policies for selecting route receiving and announcing route. BGP Sessions. eBGP.

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Introduction to bgp

Introduction to BGP

Lixin Gao

ECE Dept.

UMASS, Amherst

ECE697F: Special Topics-Internet Routing


Border gateway protocol bgp
Border Gateway Protocol (BGP)

  • Dynamic Inter-AS Routing Protocol

  • Goals:

    • Exchange reachability information between ASes

    • Apply local policies for

      • selecting route

      • receiving and announcing route

ECE697F: Special Topics-Internet Routing


Bgp sessions
BGP Sessions

eBGP

  • Used for Exchange Reachability Information

  • iBGP : fully meshed, might not directly connected,

    • Alternative to fully meshed: confederation, route reflector

  • eBGP

iBGP

AS701

MCI

AS7018

AS1

AT&T

BBN

ECE697F: Special Topics-Internet Routing


Route information exchange

Route Information Exchange

1.2.0.0/16 (1)

1.2.0.0/16 (7018,1)

eBGP

iBGP

AS701

1.2.0.0/16 (1)

1.2.0.0/16 (1)

1.2.0.0/16 (7018,1)

1.2.0.0/16 (7018,1)

MCI

1.2.0.0/16 ( )

AS7018

AS1

1.2.0.0/16 (1)

1.2.0.0/16 (1)

1.2.0.0/16

AT&T

BBN

ECE697F: Special Topics-Internet Routing


Route announcement
Route Announcement

  • Network Prefix: 12.0.0.0/8

  • Attributes: for reaching network prefix

    • AS path

    • Next hop : next hop IP addr

    • Local Preference

    • Multiple Exit Discriminator (MED)

    • Community Set

    • Origin code

    • Atomic Aggregate

    • Aggregator

    • …..

ECE697F: Special Topics-Internet Routing


As path
AS Path

  • Sequence of ASes traversed

  • Loop avoidance

  • Path selection: Path Length

  • Policy: AS7018 in AS path is more preferred

ECE697F: Special Topics-Internet Routing


Next hop
Next Hop

  • Next hop IP address to reach network prefix

  • EBGP: next hop IP addr of announcer

  • iBGP

    • From same AS:IP address of announcer

    • From diff AS: next hop unchanged

AS7018

AS1

2.3.4.1

3.4.5.2

1.2.0.0/16 (1) 2.3.4.1

1.2.0.0/16 (1) 2.3.4.1

1.2.0.0/16

1.2.0.0/16 ( ) 3.4.5.2

ECE697F: Special Topics-Internet Routing


Next hop1
Next hop

  • Determine next hop in forwarding table

    • Combing Inter-AS and Intra-AS routing table

    • Recursive route lookup

    • BGP abstract topology in AS level

ECE697F: Special Topics-Internet Routing


Local preference
Local Preference

eBGP

  • Local to AS

    • carry in iBGP session but not eBGP session

  • Used to influence path selection

    • Select the highest Local Pref path

iBGP

AS701

1.2.0.0/16 (1) 100

1.2.0.0/16 (1) 100

1.2.0.0/16 (7018,1) 200

1.2.0.0/16 (7018,1) 200

MCI

1.2.0.0/16 ( ) 300

AS7018

AS1

1.2.0.0/16 (1) 90

1.2.0.0/16 (1) 90

1.2.0.0/16

AT&T

BBN

ECE697F: Special Topics-Internet Routing


Multiple exit discriminator med
Multiple Exit Discriminator (MED)

  • Used for path selection

    • select an entry point among several to an AS (smaller MED is preferred)

    • meaningful among paths from the same AS

    • may use Intra-AS routing cost metrics

  • Non-transitive

AS701

1.2.0.0/16 (1) 200

1.2.0.0/16 (1) 100

AS1

1.2.0.0/16

ECE697F: Special Topics-Internet Routing


Community set
Community Set

  • Used for grouping destination

  • Simplify policy setting

AS701

1.2.0.0/16 (1) 1000

1.2.0.0/16 (1) 1000

3.0.0.0/18 (7018) 1000

3.0.0.0/16 (7018) 1000

MCI

3.0.0.0/8

AS7018

AS1

1.2.0.0/16

AT&T

BBN

ECE697F: Special Topics-Internet Routing


Origin code
Origin Code

  • IGP

    • interior to the originating AS.

  • EGP

    • learned via EGP (Exterior Gateway Protocol)

  • Incomplete

    • unknown

ECE697F: Special Topics-Internet Routing


Atomic aggregate
Atomic Aggregate

  • Aggregation that loses information

ECE697F: Special Topics-Internet Routing


Aggregator
Aggregator

  • Optional

  • For route that is an aggregation

ECE697F: Special Topics-Internet Routing


Announcement processing
Announcement Processing

  • Input Policy Engine

    • Receive from neigboring Ases new announcements

    • Attribute Manipulation and Filter announcement

    • Store announcements (Incremental protocol)

  • Path Selection among announcements

  • Output Policy Engine

    • Attribute Manipulation

    • Send to neighboring Ases new announcements

ECE697F: Special Topics-Internet Routing


Input policy engine
Input Policy Engine

  • Set local preference

  • Filter routes with the same AS number (loop avoidance)

  • Filter bogus routes

ECE697F: Special Topics-Internet Routing


Path selection process
Path Selection Process

  • For each prefix,

  • Prefer largest Local Preference

  • Prefer route this router originates

  • Prefer shorter AS path

  • prefer the lowest origin code (IGP<EGP<INCOMPLETE).

  • Prefer path with lowest MED

  • Prefer route with lowest IP addr

ECE697F: Special Topics-Internet Routing


Output policy engine
Output Policy Engine

  • Send to neighbors only the best path

  • Send to neighbors only the best path changes

  • Set MED

  • Set Community set

  • Set Atomic Aggregate

  • Set next hop

  • Append AS number

ECE697F: Special Topics-Internet Routing


Routing information storage
Routing Information Storage

  • Adj-RIBs-In

    • routes received

    • Input to route selection

  • Loc-RIB

    • Routes selected by selection process

    • Output of route selection

  • Adj-RIBs-Out

    • announcements to neighbor

    • Output of Output Policy Engine

ECE697F: Special Topics-Internet Routing


Bgp session implementation
BGP Session Implementation

  • OPEN message

    • establish TCP session

  • KEEPALIVE message

    • keep session going

  • Update message

    • announcement or withdrawal

  • Notification message

    • error in announcement

ECE697F: Special Topics-Internet Routing


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