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Prefix lists are more sophisticated forms that Cisco provides for filtering BGP route advertisements. They filter on IP address just as distribute-lists do, however they are easier to read, and require fewer commands to configure. The other advantage to a distribute list is that it is easeir to add, remove and organize the statements in the manner you chose.

For example:

prefix-list xx seq 10 permit
prefix-list xx seq 20 permit
prefix-list xx seq 30 permit

While this configuration requires the same number of statements as the distribute list example, you have the option of adding ge, or le to make statements more flexible as to how you will permit blocks in that range.

For example:

prefix-list xx seq 10 permit ge 18

The statement above allows any route announcement in the range of - but that announcement must have a length greater than 18 bits in the mask. This permits you to allow announcements in the range, but not an announcement equalling the entire range (/16), or even announcements of half the range (/17). Only announcments with a length "greater than or equal to" /18 will be permitted.

If this is the power of ge, what could you do with le?

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