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Chapter 36 Configuring IP Unicast Routing
Configuring BGP
Configuring Prefix Lists for BGP Filtering
You can use prefix lists as an alternative to access lists in many BGP route filtering commands, including
the neighbor distribute-list router configuration command. The advantages of using prefix lists include
performance improvements in loading and lookup of large lists, incremental update support, easier CLI
configuration, and greater flexibility.
Filtering by a prefix list involves matching the prefixes of routes with those listed in the prefix list, as
when matching access lists. When there is a match, the route is used. Whether a prefix is permitted or
denied is based upon these rules:
• An empty prefix list permits all prefixes.
• An implicit deny is assumed if a given prefix does not match any entries in a prefix list.
• When multiple entries of a prefix list match a given prefix, the sequence number of a prefix list entry
identifies the entry with the lowest sequence number.
By default, sequence numbers are generated automatically and incremented in units of five. If you
disable the automatic generation of sequence numbers, you must specify the sequence number for each
entry. You can specify sequence values in any increment. If you specify increments of one, you cannot
insert additional entries into the list; if you choose very large increments, you might run out of values.
You do not need to specify a sequence number when removing a configuration entry. Show commands
include the sequence numbers in their output.
Before using a prefix list in a command, you must set up the prefix list. Beginning in privileged EXEC
mode, follow these steps to create a prefix list or to add an entry to a prefix list:
To delete a prefix list and all of its entries, use the no ip prefix-list list-name global configuration
command. To delete an entry from a prefix list, use the no ip prefix-list seq seq-value global
configuration command. To disable automatic generation of sequence numbers, use the no ip prefix-list
Command Purpose
Step 1 configure terminal Enter global configuration mode.
Step 2 ip prefix-list list-name [seq seq-value] deny |
permit network/len [ge ge-value] [le le-value]
Create a prefix list with an optional sequence number to deny or
permit access for matching conditions. You must enter at least one
permit or deny clause.
• network/len is the network number and length (in bits) of the
network mask.
• (Optional) ge and le values specify the range of the prefix length
to be matched.The specified ge-value and le-value must satisfy
this condition: len < ge-value < le-value < 32
Step 3 ip prefix-list list-name seq seq-value deny |
permit network/len [ge ge-value] [le le-value]
(Optional) Add an entry to a prefix list, and assign a sequence
number to the entry.
Step 4 end Return to privileged EXEC mode.
Step 5 show ip prefix list [detail | summary] name
[network/len] [seq seq-num] [longer]
[first-match]
Verify the configuration by displaying information about a prefix list
or prefix list entries.
Step 6 copy running-config startup-config (Optional) Save your entries in the configuration file.