5.2.2. Private Use ASNs

5.2.2. Private Use ASNs

The original range of Private Use ASNs [RFC6996] limited operators to 1023 unique ASNs. Since it is quite likely that the number of network devices may exceed this number, a workaround is required. One approach is to re-use the ASNs assigned to the Tier 3 devices across different clusters. For example, Private Use ASNs 65001, 65002 ... 65032 could be used within every individual cluster and assigned to Tier 3 devices.

To avoid route suppression due to the AS_PATH loop detection mechanism in BGP, upstream EBGP sessions on Tier 3 devices must be configured with the "Allowas-in" feature [ALLOWASIN] that allows accepting a device's own ASN in received route advertisements. Although this feature is not standardized, it is widely available across multiple vendors implementations. Introducing this feature does not make routing loops more likely in the design since the AS_PATH is being added to by routers at each of the topology tiers and AS_PATH length is an early tie breaker in the BGP path selection process. Further loop protection is still in place at the Tier 1 device, which will not accept routes with a path including its own ASN. Tier 2 devices do not have direct connectivity with each other.

Another solution to this problem would be to use Four-Octet ASNs ([RFC6793]), where there are additional Private Use ASNs available, see [IANA.AS]. Use of Four-Octet ASNs puts additional protocol complexity in the BGP implementation and should be balanced against the complexity of re-use when considering REQ3 and REQ4. Perhaps more importantly, they are not yet supported by all BGP implementations, which may limit vendor selection of DC equipment. When supported, ensure that deployed implementations are able to remove the Private Use ASNs when external connectivity (Section 5.2.4) to these ASNs is required.