Public vs Private AS with multiple ISPs

When an enterprise network has public IP addresses that it wants to advertise to the Internet, it will do so using the BGP protocol. Now in such a case, if the enterprise is connected to a single Internet Service Provider (ISP) , it is possible to use a private ASN. However, in the event that two or more ISPs are used to connect the Enterprise to the internet in a multihomed arrangement, then public ASNs should be used.

In a single homed connection like the following, private ASN 64512 contains the IP addresses to be advertised. It shares those with the ISP router, which in turn strips the private ASN, and advertises those IP addresses using BGP with an ASN of 54321.

bgp-single-homed-connection.png

So the Internet at large knows that to reach those IP addresses, they can use ASN 54321, and the ISP in turn knows to send them to the private ASN of 64512.

Now in a multihomed arrangement like the following, a private ASN on the enterprise will cause problems.

bgp-single-multihomed-connection-two-isps-private-asn.png

In the above, the private ASN of 64512 will be shared with both ISPs to advertise the public IP addresses of the enterprise. However, when ISP1 receives it, it will remove it and advertise those IP addresses using an ASN of 54321. Similarly, ISP2 will also remove the private ASN, and advertise those addresses using ASN 12345.

So the Internet learns about the public IP addresses of the enterprise via two different ASNs. This may cause incomplete, incorrect, and contradicting routing information being sent onto the Internet. As an enterprise you would want more control over how those two ISPs would balance incoming traffic. As it stands, this is completely out of the enterprise's control.

The best solution is to use a public ASN at the enterprise like so:

bgp-single-multihomed-connection-two-isps-public-asn.png

That way, the single public ASN of 10010 will be announced as the owner of the public IP addresses being advertised. The enterprise would have control over how the ISPs would influence incoming traffic.

https://networklessons.com/bgp/bgp-remove-private-as https://networklessons.com/bgp/bgp-private-and-public-as-range https://networklessons.com/bgp/singledual-homed-and-multi-homed-designs