Introduction to the Border Gateway Protocol (BGP) Part-1

BGP

  • What
  • Why
  • Who
  • When
  • Where

BGP version 4

  • RFC 1771
  • Intelligent route selection based  on most specific prefix and shortest Autonomous System (AS) path
  • Classless Inter Domain Routing (CIDR) support
  • Current Internet standard for inter-domain (AS) exterior routing

Border Gateway Protocol (BGP)

  • BGP version 4 is the current  de facto exterior routing protocol in the Internet
  • BGP- 4 is required for CIDR

Introduction

  • Used for carrying routing information between AS
  • Path vector protocol
  • Incremental
  • Runs over TCP (port 179)
  • Conveys information about AS path topology

BGP Between AS’s

PPT Slide

General Operation

  • Learns multiple paths via internal
    and external BGP speakers
  • Picks the best path and installs in
    the IP forwarding table
  • Policies applied by influencing the
    best path selection

When should I use BGP?

  • Dual- or multi-homed
  • Providing partial or full Internet routing to a downstream customer
  • Anytime the AS path information is required

Multi-Homed AS

  • AS 100
  • AS 200
  • AS 300
  • A
  • B
  • C
  • D

BGP is not needed…

  • if singularly homed
  • if not providing downstream routing
  • Use a default route!

PPT Slide

  • Default Route
  • Static Route
  • 10.0/23
  • Upstream
  • Provider
  • AS100

BGP Attributes

  • What is an attribute?
  • AS path
  • Next hop
  • Local preference
  • Multi-exit discriminator (MED)
  • Others

What Is an Attribute?

  • Describes the characteristics of a particular prefix
  • Transitive or non-transitive
  • 13 attributes
  • Some are mandatory

BGP Attributes—AS Path

  • Sequence of AS’s a route has traversed
  • Loop detection
  • Apply policy

AS-Path

  • AS 100
  • AS 300
  • AS 200
  • AS 500
  • AS 400
  • 10.0.0/16
  • 10.0.0/16
  • 10.0.0/16
  • 10.0.0/16 300 200 100
  • 10.0.0/16 300 200
  • 10.0.0/16 300 400
  • 10.0.0/16 300 200 100
  • 10.0.0/16 300 200

BGP Attributes—Next Hop

  • Next hop to reach a network
  • Usually the local net is the next hop in eBGP session
  • Third party next hop in iBGP session

eBGP and Next Hop

  • 10.0.0/16
  • 10.0.0/16
  • 10.1.1
  • 10.1.2
  • AS 100
  • AS 300
  • AS 200
  • 10.0.0/16 150.10.1.1
  • 10.0.0/16 150.10.1.1
  • A
  • B

BGP Attributes—Next Hop (Cont.)

  • Next hop of external routes not changed when announced to iBGP neighbors
  • iBGP and Next Hop
  • 10.0.0/16
  • 10.0.0/16
  • 10.1.1
  • 10.1.2
  • AS 100
  • AS 300
  • AS 200
  • 10.0.0/16 150.10.1.1
  • 10.0.0/16 150.10.1.1
  • B
  • C
  • A

BGP Attributes—Local Preference

  • Local to AS
  • Used to influence BGP path selection
  • Path with highest local preference wins
  • Local Preference

  • AS 400
  • AS 200
  • 10.0.0/16
  • AS 100
  • AS 300
  • A
  • B
  • C
  • D
  • E
  • 10.0.0/16 500 
  • > 160.10.0.0/16 800
  • 500
  • 800