FRR provides many very flexible filtering features. Filtering is used for both input and output of the routing information. Once filtering is defined, it can be applied in any direction.
Basic filtering is done by access-list as shown in the following example.
access-list filter deny 10.0.0.0/9
access-list filter permit 10.0.0.0/8
ip prefix-list provides the most powerful prefix based filtering mechanism. In addition to access-list functionality, ip prefix-list has prefix length range specification and sequential number specification. You can add or delete prefix based filters to arbitrary points of prefix-list using sequential number specification.
If no ip prefix-list is specified, it acts as permit. If ip prefix-list is defined, and no match is found, default deny is applied.
You can create ip prefix-list using above commands.
Less than or equal to prefix numbers and greater than or equal to prefix numbers can be used together. The order of the le and ge commands does not matter.
If a prefix list with a different sequential number but with the exact same rules as a previous list is created, an error will result. However, in the case that the sequential number and the rules are exactly similar, no error will result.
If a list with the same sequential number as a previous list is created, the new list will overwrite the old list.
Matching of IP Prefix is performed from the smaller sequential number to the larger. The matching will stop once any rule has been applied.
In the case of no le or ge command, the prefix length must match exactly the length specified in the prefix list.
Descriptions may be added to prefix lists. This command adds a description to the prefix list.
Deletes the description from a prefix list. It is possible to use the command without the full description.
With this command, the IP prefix list sequential number is displayed. This is the default behavior.
With this command, the IP prefix list sequential number is not displayed.
Display all IP prefix lists.
Show IP prefix list can be used with a prefix list name.
Show IP prefix list can be used with a prefix list name and sequential number.
If the command longer is used, all prefix lists with prefix lengths equal to or longer than the specified length will be displayed. If the command first match is used, the first prefix length match will be displayed.