The Autonomous System (AS) number in iBGP (Interior Border Gateway Protocol) does not necessarily require registration with IANA (Internet Assigned Numbers Authority) if it is only used within a private network or in an environment where routing traffic is not exchanged with the public Internet.

Here is the detail:

Using AS Numbers in iBGP

1. iBGP and eBGP: iBGP is used for the exchange of routing information within a single Autonomous System (AS), while eBGP is used between different ASes.
2. Registered AS Number:
   • For eBGP: When you exchange routes with other ASes via eBGP, it is crucial that the AS number is officially registered to ensure uniqueness across the Internet. This avoids route conflicts and global routing problems.
   • For iBGP: If BGP routing traffic and configuration are maintained internally within a corporate or private network and do not interact with the public Internet, the AS number does not need to be globally unique.
3. Private AS Numbers:
   • IANA has reserved a range of AS numbers for private use (64512 to 65534 for 16-bit ASNs and 4200000000 to 4294967294 for 32-bit ASNs). These can be used without any registration for internal configurations such as iBGP, where there is no interaction with other external networks. These private ASes are ideal for testing, documentation, and internal networks where BGP routing does not need to be recognized outside the local network.

Considerations

• Interconnection: If you ever decide to connect your network using iBGP with external ASes or with the Internet in general, you will need an AS number registered with IANA to avoid conflicts and routing problems.
• Good practices: Although you can technically use any number in completely isolated configurations, sticking to the ranges reserved for private ASes is a good practice that prevents any confusion or errors in the future, especially in networks that may expand or connect with other networks.

In summary, AS numbers used in iBGP configurations do not need to be registered with IANA if they are only used internally and do not connect to other external networks.

For these internal applications, you can use private AS numbers without worrying about registration or global uniqueness.