BGP Private ASN use cases

[Mitrixsen]

Hello, everyone.

I've recently started studying BGP and I came across the concept of AS and ASNs. I understand public ASNs and the need for them. I know that if a company has a public IP block registered, it can request a public ASN from a RIR and have it assigned so that it can participate in advertising this public block to its ISPs and the internet.

However, there also appear to be Private ASNs? Could someone please explain to me what could be some simple use cases for these? I've tried searching online but I didn't find a sufficient explanation.

Thank you in advance.

[MHM Cisco World]

If you have edge router and get public as' you can connect to ISP via that public AS

What if you want to use other AS inside network? Here you need private AS.

[MHM Cisco World]

Sure you will ask' why add more than as in my network.

The answer is ibgp need some requirements than can achieved so we need ebgp.

[M02@rt37 · Accepted solution]

Hello @Mitrixsen,

Private ASNs are used within private networks for internal routing and communication purposes. They are not globally routable or advertised on the public internet.

In large organizations, there might be multiple autonomous systems within their internal network infrastructure. Each autonomous system may use a Private ASN to facilitate internal routing between different components or locations.

Private ASNs can be used to segment networks within an organization. Different departments or divisions may be assigned separate Private ASNs to maintain routing separation and control within the network.

[David Ruess]

In addition to what others said you dont want to have a Public AS in your netowrk as it could be in use and although there are safe-gaurds in place you could cause disruptions in the internet as that AS could be in use by another company.

Its essentially the same concept as the RFC 1918 private IP address space.

-David

[Giuseppe Larosa]

Hello @Mitrixsen ,

private ASN can be allocated by an ISP to a customer for using eBGP sessions without the need to use a public ASN.

Other typical use cases are for customers in MPLS VPN services : for MPLS L3 VPN services or MPLS L2 VPN services the customer can get assigned a private ASN to be used on his CE nodes.

The ISP can override or strip the private AS number in order to build an AS path attribute that can be advertised to other providers .

override means the private AS number is changed in the public ISP AS number that can appear more then one time like in AS path prepending

remote private-as: the private AS number is removed from the AS path attribute

another possible application of private AS numbers is in data center leaf and spine to use  MP eBGP sessions for EVPN VXLAN.

Hope to help

Giuseppe