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