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BGP: Configuring Prefix Filters
Page 22 | Use Route Maps and Other Filters to Filter and Alter BGP and OSPF Routes
BGP: Configuring Prefix Filters
Prefix filters use prefix lists to filter particular routes on the basis of their prefixes. Prefix filters
and distribute filters both filter individual routes out of BGP update packets. They are
mutually exclusive.
About prefix lists
A prefix list is a list of prefix entries. Each entry specifies a particular prefix, a mask length or
range of mask lengths, and whether or not those prefixes are deemed to explicitly match or
explicitly not match the prefix list.
A prefix list entry is created with the command:
You can choose to give an entry a sequence number by using the optional seq parameter. If
you do not, the switch assigns sequence numbers in steps of 5 (number 5, 10, 15 etc.) and
puts the new entry at the end of the list of entries.
To see entries and their numbers, use the command:
Mask length
You can specify a single prefix mask length, or you can use the ge and le parameters to
specify a range of mask lengths for the entry to match.
If you set the mask length to:
a single mask length (by specifying neither the ge nor the le parameter), then a route
matches against this entry if its prefix mask length is exactly that length.
a range of mask lengths, then a route matches against this entry if its prefix mask length is
greater than or equal to ge and less than or equal to le.
For example, to deny the IP addresses between 10.0.0.0/14 (mask of 255.252.0.0) and
10.0.0.0/22 (mask of 255.255.252.0) within the 10.0.0.0/8 (mask of 255.0.0.0) addressing
range, use the command:
The mask length (8 in this example) must be less than the value specified for the ge
parameter. This mask defines the range of subnets that the matching subnets must fall within.
awplus(config)#
ip prefix-list <list-name> [seq <number>]
{deny|permit} {any|<ipadd>/<prefix-length>}
[ge <minimum-length>] [le <maximum-length>]
awplus#
show ip prefix-list
awplus(config)#
ip prefix-list mylist deny 10.0.0.0/8 le 22 ge 14