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Using Multiple Autonomous Systems Numbers (ASNs) with Vultr

Last Updated: Tue, Jul 13, 2021
Networking

Introduction

With Vultr's BGP feature, you can bring your own Autonomous System Number (ASN), or Vultr can assign you a private ASN. You can use either type of ASN to peer with Vultr Cloud servers or Bare Metal servers. This guide describes how Vultr modifies the AS_PATH in different BGP routing scenarios.

About the ASNs in this Guide

Here is a reference list of the example ASNs used in this guide.

  • AS64515 is Vultr's private ASN used when peering with our VPS servers.
  • AS20473 is Vultr's public ASN, and is also used when peering with our Bare Metal servers.
  • AS65534 is an example private ASN assigned to a customer by Vultr.
  • AS64500, AS64501, and AS64502 are example public ASNs for documentation.

For the rest of this guide, private ASNs are shown in bold for clarity.

Peering Using a Public ASN

Some customers need to advertise different prefixes with different public ASNs. To do this, they usually peer using their own public ASN and then prepend the other ASNs to the AS_PATH. For example, if a customer has AS64500, and secondaries AS64501 and AS64502, the advertised prefixes to Vultr's core network could include the following AS_PATHs:

  • 64500
  • 64500_64501
  • 64500_64502
  • 64500_64501_64502

When Vultr advertises the prefix to our transit providers, we prepend our public ASN, 20473. Working from the previous example, the corresponding AS_PATHs shown on the public internet look like this:

  • 20473_64500
  • 20473_64500_64501
  • 20473_64500_64502
  • 20473_64500_64501_64502

Because the customer is peering with Vultr using AS64500, it will always appear before AS64501 and AS64502 in the path.

Peering Using a Private ASN

Customers without a public ASN who need to advertise their own prefixes can peer with a private ASN supplied by Vultr. Using the same example ASNs as before, a customer using the private AS65534 supplied by Vultr may advertise prefixes with the following AS_PATHs:

  • 65534_64500
  • 65534_64500_64501
  • 65534_64500_64502
  • 65534_64500_64501_64502

The customer may advertise any combination of AS64500, AS64501, and AS64502 in the AS_PATH. Because Vultr removes private ASNs when advertising prefixes to our transit providers on the internet, Vultr will remove AS65534 and prepend our public AS20473. The result is:

  • 20473_64500
  • 20473_64500_64501
  • 20473_64500_64502
  • 20473_64500_64501_64502

Advantages to Using a Private ASN

Customers with a public ASN might choose to peer with Vultr using a private ASN, which can have the advantage of advertising a shorter AS_PATH on the public internet in some situations. Here's an example scenario:

If a customer advertises an address range on the internet with AS64501, the AS_PATH from Vultr will be seen as:

  • 20473_64501

Assume the same customer has a second address range to advertise with AS64502. If they are peering with Vultr with their primary AS64501, the AS_PATH on the internet for AS64502 is:

  • 20473_64501_64502

However, if the customer peers with Vultr using private AS65534, then Vultr can remove the private ASNs and advertise the prefixes as:

  • 20473_64501
  • 20473_64502

Using a private ASN, Vultr can make the AS_PATH shorter for 64502, increasing the chance to become the preferred path on the internet.

More Information

For more information about using BGP at Vultr, see the following resources:

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