Border Gateway Protocol

1
Border Gateway Protocol

• By Darrell Hageman

2
Outline

• Introduction
• Background
• BGP Overview
• BGP Finite State Machine
• States
• Other General Information
• Comparisons to Similar Machines
• Conclusion
• References

3
Introduction

• BGP was initially created in three different
  implementations.
• Kirk Lougheed of Cisco systems developed a
  proprietary version of BGP for Cisco router
  equipment.
• Jeff Honig, Cornell University and Dennis
  Ferguson, University of Toronto developed a BGP
  version called gated to run on Unix computers.
• Yakov Rekhter from the IBM Thomas J. Watson
  Research Center wrote a BGP version for the
  NSFNET backbone.
• Internet standard in 1989

4
BGP Overview

• What is BGP
• The routing protocol used to exchange routing
  information across the Internet
• BGP is an inter-Autonomous System routing
  protocol.
• BGP makes it possible for ISPs to connect to each
  other and for end-users to connect to more than
  one ISP
• The primary function of a BGP is to exchange
  network reachability information with other BGP
  systems
• The network reachability information includes
  information on the list of Autonomous Systems
  (ASs) that reachability information traverses

5
BGP Overview

• The information is important to construct a graph
  of AS connectivity from which routing loops may
  be shortened
• Only protocol that can deal well with having
  multiple connections to unrelated routing domains
• Only protocol designed to deal with a network of
  the Internet's size
• One of the other important functions performed by
  the BGP algorithm is to eliminate loops from
  routing information

6
BGP Overview
7
BGP Overview

• There are four types of BGP messages
• Open - messages are sent by the router initiating
  the BGP connection, in order to identify itself
  and to begin the exchange of routing information.
  
• Update - messages are sent by BGP peers in order
  to carry the network reachability information and
  path attributes between them. These messages are
  the heart of the protocol.
• Keepalive - messages are exchanged between peers
  to continually verify peer reachability when no
  updates are sent for a period of time.
• Notification - messages are BGPs way of
  providing error messaging and control services

8
BGP Finite State Machine

• States
• Idle
• Connect
• Active
• Open Sent
• Open Confirm
• Established

9
BGP Finite State Machine

• Idle State
• Initial state
• BGP speaker is not yet ready to accept a BGP
  Connection.
• Tries to establish a TCP connection with
  configured BGP Peer
• Listens for a TCP connection from its Peer
• Changes to Connect State

10
BGP Finite State Machine

• Types of Errors
• TCP port 179 is not open.
• A random TCP port over 1023 is not open.
• Peer address configured incorrectly on either
  router.
• AS number configured incorrectly on either
  router.

11
BGP Finite State Machine

• Connect State
• Is waiting for the transport protocol connection
  to be completed
• If connection succeeds, the local system clears
  the ConnectRetry timer, completes initialization,
  sends an OPEN message to its peer, and changes
  its state to OpenSent
• If connection fails the local system restarts the
  ConnectRetry timer, continues to listen for a
  connection that may be initiated by the remote
  BGP peer, and changes its state to Active state.
• BGP does not spend much time in this state if the
  TCP session has been successfully established

12
BGP Finite State Machine

• Active State
• In active state, because it was unable to
  establish a successful TCP connection with its
  peers
• BGP speaker ignores the start event
• Resets the ConnectRetry timer
• Attempts to initiate a TCP sessions with its peer
• If successful it sends an OPEN message to its
  peer
• Sets the hold timer
• Transitions to OpenSent State
• If unsuccessful again the FSM is reset to the
  IDLE state
• Repeated failures could be a result of
• TCP port 179 is not open.
• BGP configuration error.
• Network congestion.

13
BGP Finite State Machine

• OpenSent State
• BGP peers are waiting for an OPEN message from
  its peer
• After an Open message has been received
• All fields in Open message are checked against
  local BGP configurations
• If any of the fields dont match this will cause
  an OPEN message error to occur
• Return to Idle State
• BGP peer also checks to verify that a connection
  collision has not occurred
• If there are no errors in the OPEN message
• BGP sends a KEEPALIVE message and sets a
  KeepAlive timer
• State is changed to OpenConfirm

14
BGP Finite State Machine

• OpenConfirm State
• BGP waits for a KEEPALIVE or NOTIFICATION message
• If the local system receives a KEEPALIVE message,
  it changes its state to Established
• If a timer expires before a KEEPALIVE message is
  received, or if an error condition occurs, the
  router transitions back to the IDLE state

15
BGP Finite State Machine

• Established State
• Peers reach this state after they have
  succesfully exchanged OPEN and KEEPALIVE messages
  
• Begin to UPDATE messages contianing routing
  information and KEEPALIVE messages to verify the
  TCP Connection state
• If error local peer sends a NOTIFICATION message
  with reasons back to IDLE state

16
BGP Finite State Machine
17
Other General Information

• BGP is unique because of its ability to apply
  policies to the information contained in routing
  updates and therefore accepts or rejects update
  information based on attributes of the
  information itself
• BGP uses the TCP/IP protocol on port 179 to
  establish connections
• Routing messages are the highest precedence
  traffic on the Internet, and each BGP router
  gives them first priority over all other traffic

18
Conclusion

• BGP is the glue that holds the Internet together
• Little bandwidth/processing overhead
• Suitable for indicating liveliness in both
  directions
• Appropriate for complex applications where there
  are numerous paths active at one time
• Policy-based, providing policy control over what
  is sent and received

19
Conclusion

• Not simple to configure. Certain settings must be
  coordinated with the provider
• Policy controls can require significant manual
  configuration on each router. The more policy
  controls in use, the more configuration required

20
Reference

• "BGP Finite State Model." InetDaemon.Com. Web. 15
  Apr. 2010. lthttp//www.inetdaemon.com/tutorials/in
  ternet/ip/routing/bgp/operation/finite_state_model
  .shtmlgt.
• "FileBGP FSM.svg - Wikimedia Commons." Main Page
  - Wikimedia Commons. Web. 15 Apr. 2010.
  lthttp//commons.wikimedia.org/wiki/FileBGP_FSM.sv
  ggt.
• Routing TCP IP. Munchen Markt Und Technik,
  Buch- Und Software-Verl., 2002. Print.
• Solie, Karl, and Leah Lynch. CCIE Practical
  Studies. Indianapolis, IN Cisco, 2002. Print.
•  Solie, Karl, and Leah Lynch. "InformIT CCIE
  Practical Studies, Volume II BGP Finite-State
  Machine Operation." InformIT The Trusted
  Technology Learning Source. Web. 15 Apr. 2010.
  lthttp//www.informit.com/library/content.aspx?bCC
  IE_Practical_Studies_IIseqNum79gt.