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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 Announcement

  • Local Policies
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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