Below are my notes from reading the CCNP ROUTE Official Certification Guide.

What is route filtering?

Route filtering allows the engineer to filter routes advertised in an update. Filtering reduces routing tables, saves memory, improve performance and makes network more secure by limiting flow of packets.

Route filtering can be enabled using the router subcommand: distribute-list, Concept is that list refers to an ACL, prefix-list or route map. It specifies the direction, either outbound or inbound, and optionally the specific interface on which to filter updates.

Describe the concept of filtering by referencing ACLs

To filter EIGRP routes by ACL, ACL must match a route with a permit clause to then allow the route to be advertised, and match the route with a deny clause to filter the route. When referenced with the distribute-list command for the purpose of filtering, EIGRP compares ACL source-address field to the subnet number (prefix) of each EIGRP route.

Describe the concept of filtering by referencing IP Prefix Lists

IP prefix lists can examine both the prefix and the prefix length, and a range of prefixes or a range of prefix lengths. The command sets either a deny or permit action for each matched prefix/length. To use the prefix-list, use the command: prefix-list.

IP prefix lists allow matching of the prefix length, whereas the ACL used by the EIGRP distribute-list command cannot.

Describe the IP Prefix List Concepts

IP prefix lists provide mechanisms to match two components of an IP route:

• The route prefix (subnet number)
•  The prefix length (subnet mask)

Each ip prefix-list command has a sequence number to allow later deletion of individual commands and insertion of commands into a particular sequence position. Each command has a permit or deny action, because it is used only for matching routes and not for packet filtering. The permit or deny keyword just implies whether a route is matched (permit) or not (deny).

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