BGP Scalability and Advanced Features
still exists at the bottom of the list for the data that does not have
a match in our prefix list. However, if there are no lines in our prefix list,
instead of an implicit deny all, there is an implicit permit any.
The rule to remember when using prefix lists is that if a prefix is permitted, the
route is advertised; if a prefix is denied, the route is not advertised.
One improvement from access lists is the use of sequence numbers for
each statement in the prefix list. The statement with the smallest sequence
numbers is read first. This also allows us to modify a sequence statement
without starting over when there is a change in the network that must be
applied to our prefix list.
Configuring Prefix Lists
We create a prefix list using the prefix-list command followed by a list
name, which we will call list1. We can then optionally identify the
sequence value using the seq syntax followed by the sequence number we
wish to use. The sequence number can be any number. The lowest number
gets read first. This means that if our first sequence number is 15 and our sec-
ond is 18, then we can add 16 and 17 later if we need to modify the prefix
list with a new statement.
If we now create this prefix list, our prefix list is called ip prefix-list
list1 seq 15
; if no sequence number is identified, the number is automat-
ically assigned in increments of 5, meaning that the first would be 10 and
then 15 and so on. We now need to permit a network using the permit syn-
tax. If we do not have at least one permit statement, then we effectively deny
all the routes. It is best to start with permit statements and then move on to
selective deny statements.
If you wish to stop the incremental sequence numbers, you can use the no ip
command. To re-enable the sequence num-
bering, use the ip prefix-list sequence-number command.
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