CCNP Routing and Switching ROUTE 300-101 Official Cert Guide

Public and Private ASNs

For the Internet to work well using BGP, IANA administers the assignment of ASNs much like it does with IP address prefixes. One key reason why ASNs must be assigned as unique values is that if ASNs are duplicated, the BGP loop-prevention process can actually prevent parts of the Internet from learning about a route. For example, consider Figure 13-7, with the same design as in the last few figures—but this time with a duplicate ASN.

Figure 13-7 Duplicate ASN (12) Preventing Route Advertisement

In this figure, both ISP1 ...