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Configuring BGP
Configuring Basic BGP Features
IPC-304
Cisco IOS IP Configuration Guide
Configuring BGP Route Filtering by Neighbor
You can filter BGP advertisements in two ways:
• Use autonomous system path filters, as with the ip as-path access-list global configuration
command and the neighbor filter-list router configuration command
• Use access or prefix lists, as with the neighbor distribute-list router configuration command.
Filtering using prefix lists is described in the “Configuring BGP Filtering Using Prefix Lists” section.
If you want to restrict the routing information that the Cisco IOS software learns or advertises, you can
filter BGP routing updates to and from particular neighbors. You can either define an access list or a
prefix list and apply it to the updates.
Note Distribute-list filters are applied to network numbers and not autonomous system paths.
To filter BGP routing updates, use the following command in router configuration mode:
Note Although the neighbor prefix-list router configuration command can be used as an alternative to the
neighbor distribute-list command, do not use attempt to apply both the neighbor prefix-list and
neighbor distribute-list command filtering to the same neighbor in any given direction. These two
commands are mutually exclusive, and only one command (neighbor prefix-list or neighbor
distribute-list) can be applied for each inbound or outbound direction.
Configuring BGP Filtering Using Prefix Lists
Prefix lists can be used as an alternative to access lists in many BGP route filtering commands. The
section “How the System Filters Traffic by Prefix List” describes the way prefix list filtering works. The
advantages of using prefix lists are as follows:
• Significant performance improvement in loading and route lookup of large lists.
• Support for incremental updates. Filtering using extended access lists does not support incremental
updates.
Command Purpose
Router(config-router)# neighbor {ip-address |
peer-group-name} distribute-list {access-list-number
| access-list-name} {in | out}
Filters BGP routing updates to and from neighbors as
specified in an access list.
Note The neighbor prefix-list router configuration
command can be used as an alternative to the
neighbor distribute-list router configuration
command, but you cannot use both commands to
configure the same BGP peer in any specific
direction. These two commands are mutually
exclusive, and only one command (neighbor
prefix-list or neighbor distribute-list) an be applied
for each inbound or outbound direction.