Which method is NOT typically used to filter BGP routing updates?

• A. IP prefix lists
• B. IPsec filters
• C. Route maps
• D. Filter lists

Answer: (B)

Filtering BGP routing updates relies on mechanisms that examine BGP attributes and make policy decisions about which routes to accept, advertise, or modify. Prefix lists are used to permit or deny routes based on destination prefixes, making them a straightforward way to control which updates travel in or out. Route maps add flexibility by matching on a range of BGP attributes such as AS_PATH, origin, MED, and communities, then applying actions like permit/deny or even rewriting attributes. Filter lists (distribute-list) employ ACLs to filter routes by prefix patterns and can be applied to inbound or outbound updates.

IPsec filters do not fit this role. IPsec operates at the IP layer to secure traffic between devices, providing confidentiality, integrity, and authentication of the data stream. While IPsec can protect the BGP session itself or the transport of traffic between peers, it does not filter BGP updates based on routing attributes. Therefore, IPsec filters are not the typical method used to filter BGP routing updates.