BGP Foundations · Lesson 02 of 02
Autonomous systems and AS numbers
The AS number is the identity BGP argues about. Knowing the ranges and notations keeps you out of trouble.
Written lesson
Public, private and reserved
AS numbers were originally 16-bit (0–65535). Public 16-bit ASNs (1–64495) are assigned by RIRs for use on the Internet; private ASNs (64512–65534) are for internal use and must be stripped or replaced at the Internet edge. The space then extended to 32 bits (up to 4294967295) to avoid exhaustion, with 4200000000–4294967294 reserved as private in the 32-bit range. AS 0 is reserved and must never appear in a path; 23456 (AS_TRANS) exists only so 16-bit-only speakers can represent 32-bit peers.
Written lesson
asplain, asdot and asdot+
A 32-bit ASN can be written three ways. asplain is a single decimal number: 65550. asdot+ always uses high.low: 1.14. asdot uses plain notation below 65536 and dotted above it. Cisco defaults to asplain on modern IOS. The trap is regular expressions: an AS-path regex written for asplain will not match asdot output, so pick one notation per network and configure it explicitly with bgp asnotation dot if you want dotted.
Private ASNs leak
A private ASN that reaches the Internet in an AS_PATH is a configuration error that providers may drop or that breaks loop prevention. Use neighbor x.x.x.x remove-private-as on eBGP sessions to a provider.
Ranges worth memorising
- 16-bit public: 1–64495; 16-bit private: 64512–65534.
- 32-bit reserved private: 4200000000–4294967294.
- AS 0 and AS 23456 are special and should not appear as real origins.
- Choose one AS notation network-wide before writing any AS-path regex.