How do you configure a filter list?

• A. Router (config-router)# neighbor {ip-address | peer-group-name} access-list-number in
• B. Router (config-router)# neighbor {ip-address | peer-group-name} filter-list number out
• C. Router (config-router)# neighbor {ip-address | peer-group-name} filter-list access-list-number {in | out}
• D. Router (config-router)# neighbor {ip-address | peer-group-name} filter ip-list access-list-number

Answer: (C)

Filtering in BGP is done by attaching an access-list to a neighbor so updates are screened either when they arrive or when they are sent. The proper syntax specifies the neighbor, then the filter-list with an access-list number, followed by the direction in or out. The command to use is router(config-router)# neighbor <ip-address | peer-group-name> filter-list {in | out}. This ties the access-list to that neighbor's updates. Inbound filtering applies to updates the router receives from that neighbor, while outbound filtering applies to updates the router sends to that neighbor. To illustrate, define an access-list that permits only the prefixes you want, then apply it with inbound to block undesired routes. The other forms are not valid here: omitting filter-list, or placing access-list-number without the filter-list keyword, or using a non-existent syntax like filter ip-list, will not configure a BGP filter-list.