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Configuring BGP
Configuring Basic BGP Features
IPC-306
Cisco IOS IP Configuration Guide
Configuring a Prefix List Entry
You can add entries to a prefix list individually. To configure an entry in a prefix list, use the following
command in router configuration mode:
The optional ge and le keywords can be used to specify the range of the prefix length to be matched for
prefixes that are more specific than the network/length argument. An exact match is assumed when
neither ge nor le is specified. The range is assumed to be from ge-value to 32 if only the ge attribute is
specified, and from len to le-value if only the le attribute is specified.
A specified ge-value or le-value must satisfy the following condition:
len < ge-value <= le-value <= 32
For example, to deny all prefixes matching /24 in 128.0.0.0/8, use the following command:
ip prefix-list abc deny 128.0.0.0/8 ge 24 le 24
Note You can specify sequence values for prefix list entries in any increments you want (the automatically
generated numbers are incremented in units of 5). If you specify the sequence values in increments
of 1, you cannot insert additional entries into the prefix list. If you choose very large increments, you
could run out of sequence values.
Configuring How Sequence Numbers of Prefix List Entries Are Specified
By default, the sequence numbers are automatically generated when you create a prefix list entry.
Sequence numbers can be suppressed with the no ip prefix-list sequence-number global configuration
command. Sequence values are generated in increments of 5. The first sequence value generated in a
prefix list would be 5, then 10, then 15, and so on. If you specify a value for an entry and then do not
specify values for subsequent entries, the assigned (generated) sequence values are incremented in units
of five. For example, if you specify that the first entry in the prefix list has a sequence value of 3, and
then do not specify sequence values for the other entries, the automatically generated numbers will be
8, 13, 18, and so on.
To disable the automatic generation of sequence numbers, use the following command in router
configuration mode:
To re-enable automatic generation of the sequence numbers of prefix list entries, use the ip prefix-list
sequence number command in router configuration mode:
Command Purpose
Router(config-router)# ip prefix-list list-name [seq
sequence-value] {deny | permit network/length} [ge
ge-value] [le le-value]
Creates an entry in a prefix list and assigns a sequence number
to the entry.
Command Purpose
Router(config-router)# no ip prefix-list
sequence-number
Disables the automatic generation of the sequence numbers
for prefix list entries.