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BGP loop prevention mechanisms

For visitors or readers looking for information on BGP loop prevention, and does not want to read all the complete tutorial, i will point them to these parts of the tutorial.

For BGP loop prevention within an AS, or Ibgp loop prevention based on Split-horizon mechanisms, plz read the following pages :

For BGP loop prevention between different ASs, or ISPs, which is a very important notion to understand when u are confronted to build a network with multihoming, redundancy and link failure options, plz read the following pages. As-path attribute allow ebgp loop prevention.

However the concept of BGP loop between ISPs or different ASs is very tricky since this is not something that you can experience in your own network. You will be more likely victim of it, particularly if your traffic engineering redundancy case has not been fully tested. I will even say that ebgp loop prevention is mainly related to peering agreements betweens ISPs.

This is deeply explained though the tutorial, but the four following sections are sufficient to make a clear outlook.

1)Presentation of the network implemented through the study-case

2) Peering agreement between the upstream/transit providers

3) Ebgp loop prevention mechanism based on as-path attribute

4) Observations and commentaries of the results and the redundancy scenarios.

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Update on BGP Tutorial: Practices and Multihoming techniques,regarding an ISP proposing IP transit services.

Thx for sharing your comments. As a consequence, i have made some changes and update the tutorial. Tutorial can be browsed on the site or downloaded as PDF in the download section of the site(version 0.94).

Executive Summary on this article.

The aim is to give a demonstration on how to use BGP and understand it in a world of interconnected ISPs. The contents describe practices and techniques about an ISP customer who:
– buys IP transit connectivity from different ISPs.
– needs to implement a load-balancing process.
– inserts redundancy and multihoming options for the Internet provision in case of link failure.

– The first chapter introduces the design of the network and its topology for the study case. You will find diagrams on the BGP and Static design; the traffic engineering targeted configuration, and a listing of IP-addresses and interfaces used.

Notions described:

Chapter A : The second chapter (chapter A) highlights various global notions on BGP, which are a necessity to understand. This is in order to be able to design and configure the network described in this study case, which aims to deliver IP transit services to customer, and uses multihoming connectivity to several ISPs. The general points of BGP in this chapter are related to a way of thinking within an ISP environment. People can overview it, but it is recommended to read it accurately if they do not practice BGP administration every day.

– Traffic engineering BGP tools
– peering agreement between ISPs
– relations between packets, routes announcement, ingress, inbound, egress, outbound traffic
– BGP local preference attribute,
– BGP prepend
– BGP communities, community guide, setup, and understanding
– BGP Next-hop, next-hop-self,
– BGP Recursive lookup
– BGP loop prevention, as-apth loop prevention, split-horizon mechanism
 – BGP common setup commands
– BGP basic multihoming template

Chapter B: The 3rd chapter (chapter B) targets the design directly and its configuration. Ebgp and Ibgp configurations are described and deeply explained. Peering agreements and multihoming are enlightened in a practical way.

– BGP tools and practices for return traffic: as-path prepend, LocalPreference or BGP communities
– Ebgp analysis, method discussions and setup regarding the study case
– Ibgp analysis, method discussions and setup regarding the study case
– network pools aggregation
– Ibgp and Ebgp template

Chapter C: The last chapter (chapter C) validates the configuration of ChapterB, and more generally the study case. Scenarios of links failures are presented, in order to check up if the multihoming configuration agrees the design and the traffic engineering objectives. Moreover, verifications, monitoring and debug commands are also stressed, in order to validate the setup.

– BGP monitoring tools and analysis discussion methods
– Ebgp traffic method tests and discussions
– Multihoming, redundancy traffic tests and analysis
– BGP common debug and monitoring commands for ingress and egress traffic

Chapter B BGP tutorial on site

BGP tutorial can be downloaded(in pdf), but also read on site.

Ok, it took me a while to publish ChapterB.

Chapter B aims directly to the configuration of the study-case. To summarize, ChapterB targets the design directly and its configuration.
Ebgp and Ibgp configurations are described and deeply explained. Peering agreements and multihoming are enlightened in a practical way.

As a reminder, the study-case has for for goal to give a demonstration on how to use BGP and understand it in a world of interconnected ISPs. The contents describe practices and techniques about an ISP customer who:

  • buys IP transit connectivity from different ISPs.
  • needs to implement a load-balancing process.
  • inserts redundancy and multihoming options for the Internet provision in case of link failure. 

Do not worry, u will find more than a template or configuration, each command line, and choice of the structure is described in an exhaustive way. Many diagrams are also present in order to understand why and how the traffic engineering has been built.

The notions described :

  • peering agreements
  • BGP traffic engineering tools
    • local preference
    • bgp communities
    • as-prepend
  • Ebgp
  • Ibgp
  • network aggregation
  • EBGP and IBGP (cisco) multihoming template