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ASN Union

Goal

This initiative aims to simplify the process of ASN union. In today world there are 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 union, so engineers have to develop and support a number of ad-hoc solutions to make such network 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 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 External over Internal comparison with eBGP over iBGP 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 could affect BGP decision process of other parties. It could become crucial if such route is announced at customer links. To solve this problem we propose extended ISP Confederations.

Originally AS Confederations were developed to solve full mesh BGP problem by separating network to 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 using these segments protects confederation from inner loops, for networks outside confederation it is visible only as a single ASN.

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

  • remove limitation that AS Confederation must be represented with 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 filter this routes.

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

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