What is bogon filtering and why should it be enabled?

• A. Filtering of unallocated/reserved prefixes; reduces spoofed advertisements and accidental leaks.
• B. Filtering of all non-AS paths.
• C. Filtering of IPv6 routes exclusively.
• D. Filtering of routes based on ASN.

Answer: (A)

Bogon filtering blocks address prefixes that should not be seen in the global Internet routing table because they are unallocated or reserved. Enabling it helps prevent bogus route advertisements from propagating, which reduces spoofed traffic, misrouting, and accidental leakage of internal or private space into the public Internet. For example, prefixes that are not allocated to any entity (and some well-known reserved ranges) should never be learned through BGP; if a router receives such a route, bogon filtering can drop it before it affects traffic. This protection also helps catch misconfigurations by peers who might accidentally advertise internal or non-routable space.

This technique applies to both IPv4 and IPv6, not just one protocol, and it differs from filtering by ASN or by requiring non-AS paths to be filtered. It also isn’t limited to IPv6-only scenarios. By focusing on unallocated or reserved prefixes, bogon filtering directly tackles the risk of obviously invalid routes entering or propagating in the Internet routing system.