1. Border Gateway Protocol
By: Darrell Hageman

2. Outline Introduction BGP Overview BGP Finite State Machine
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 BGP’s 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 don’t 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 < ration/finite_state_model.shtml>.
"File:BGP FSM.svg - Wikimedia Commons." Main Page - Wikimedia Commons. Web. 15 Apr <
Routing TCP IP. München: Markt Und Technik, Buch- Und Software-Verl., Print.
Solie, Karl, and Leah Lynch. CCIE Practical Studies. Indianapolis, IN: Cisco, 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 < _Studies_II&seqNum=79>.