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Progress of the initiative

ASN Union

Goal

This initiative aims to simplify the process of ASN union. In today’s world there is a number of situations when several ASNs want to operate as one. The main goal of such ASN union is to get benefit from shared connectivity which includes customers, peers and upstream connections. BGP does not provide native support for such ASN unions, so engineers have to develop and support a number of ad-hoc solutions to make such networks operate properly:

  • transmission of local preference values through ASN borders;
  • ad-hoc to support “hot-potato” routing;
  • synchronize communities to avoid route leaks.

We believe that such complexity could be resolved by using BGP Roles and extended BGP Confederations.

Technical Description

First of all we propose a new border definition using BGP Roles. We define a BGP session as internal if both speakers have Internal role set in its configuration. Through such connection we permit transmission of local attributes such as local preference value, iOTC and MED.

We also propose a change in BGP decision process that replaces eBGP over iBGP comparison with External over Internal comparison . This update gives opportunity to provide native support of ASN union including resolving issues with “hot-potato” routing.

Still, if route passes through such united network, the AS path will be increased multiple times, which may affect the BGP decision process for other parties. It could become crucial if such route is announced at customer links. In order to solve this problem we propose extended ISP Confederations.

Originally AS Confederations were developed to solve the full mesh BGP problem by separating the network into smaller, more controllable parts. This standard adds two additional AS Path segment types, AS_CONFED_SEQUENCE and AS_CONFED_SET, which are used inside AS and are removed when a route leaves a confederation. While the usage of these segments protects the confederation from inner loops, for networks outside the confederation it is visible only as a single ASN.

We propose to extend AS Confederations to bring it at a multi-ASN level. This includes two changes:

  • remove limitation that AS Confederation must be represented with a single ASN;
  • add route loop check for routes learned from External links – if AS Path of a route includes segments with ASNs from confederation list, then this route should be dropped.

Together Roles and ISP Confederations can provide a native mechanism for ASN Union.

Discussion:
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