Filtering

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.

IP Access List

access-list NAME permit IPV4-NETWORK
access-list NAME deny IPV4-NETWORK

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

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.

ip prefix-list NAME (permit|deny) PREFIX [le LEN] [ge LEN]
ip prefix-list NAME seq NUMBER (permit|deny) PREFIX [le LEN] [ge LEN]

You can create ip prefix-list using above commands.

seq
seq number can be set either automatically or manually. In the case that sequential numbers are set manually, the user may pick any number less than 4294967295. In the case that sequential number are set automatically, the sequential number will increase by a unit of five (5) per list. If a list with no specified sequential number is created after a list with a specified sequential number, the list will automatically pick the next multiple of five (5) as the list number. For example, if a list with number 2 already exists and a new list with no specified number is created, the next list will be numbered 5. If lists 2 and 7 already exist and a new list with no specified number is created, the new list will be numbered 10.
le
Specifies prefix length. The prefix list will be applied if the prefix length is less than or equal to the le prefix length.
ge
Specifies prefix length. The prefix list will be applied if the prefix length is greater than or equal to the ge prefix length.

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.

no ip prefix-list NAME

ip prefix-list description

ip prefix-list NAME description DESC

Descriptions may be added to prefix lists. This command adds a description to the prefix list.

no ip prefix-list NAME description [DESC]

Deletes the description from a prefix list. It is possible to use the command without the full description.

ip prefix-list sequential number control

ip prefix-list sequence-number

With this command, the IP prefix list sequential number is displayed. This is the default behavior.

no ip prefix-list sequence-number

With this command, the IP prefix list sequential number is not displayed.

Showing ip prefix-list

show ip prefix-list

Display all IP prefix lists.

show ip prefix-list NAME

Show IP prefix list can be used with a prefix list name.

show ip prefix-list NAME seq NUM

Show IP prefix list can be used with a prefix list name and sequential number.

show ip prefix-list NAME A.B.C.D/M

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.

show ip prefix-list NAME A.B.C.D/M longer
show ip prefix-list NAME A.B.C.D/M first-match
show ip prefix-list summary
show ip prefix-list summary NAME
show ip prefix-list detail
show ip prefix-list detail NAME

Clear counter of ip prefix-list

clear ip prefix-list [NAME [A.B.C.D/M]]

Clears the counters of all IP prefix lists. Clear IP Prefix List can be used with a specified NAME or NAME and prefix.