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Cisco Catalyst Blade Switch 3130 and 3032 for Dell Software Configuration Guide
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Chapter 39 Configuring IP Unicast Routing
Configuring BGP
Another filtering method is to specify an access list filter on both incoming and outbound updates, based
on the BGP autonomous system paths. Each filter is an access list based on regular expressions. (See the
“Regular Expressions” appendix in the
Cisco IOS Dial Technologies Command Reference, Release 12.2
for more information on forming regular expressions.) To use this method, define an
autonomous-system-path access list, and apply it to updates to and from particular neighbors.
Beginning in privileged EXEC mode, follow these steps to configure BGP autonomous-system-path
filtering:
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.
Prefix-list filtering matches 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 prefix does not match any entries in a prefix list.
•
When multiple entries of a prefix list match a 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.
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
ip as-path access-list
access-list-number
{
permit
|
deny
}
as-regular-expressions
Define a BGP-related access list.
Step 3
router bgp
autonomous-system
Enter BGP router configuration mode.
Step 4
neighbor
{
ip-address
|
peer-group name
}
filter-list
{
access-list-number
|
name
} {
in
|
out
|
weight
weight
}
Establish a BGP filter based on an access list.
Step 5
end
Return to privileged EXEC mode.
Step 6
show ip bgp neighbors
[
paths
regular-expression
]
Verify the configuration.
Step 7
copy running-config startup-config
(Optional) Save your entries in the configuration file.
