See the “Controlling Advertising and Processing in Routing Updates” section for information about the
distribute-list command.
You can use route maps on a per-neighbor basis to filter updates and to modify various attributes. A route
map can be applied to either inbound or outbound updates. Only the routes that pass the route map are sent
or accepted in updates. On both inbound and outbound updates, matching is supported based on AS path,
community, and network numbers. Autonomous system path matching requires the match as-path access-list
route-map command, community based matching requires the match community-list route-map command,
and network-based matching requires the ip access-list global configuration command.
Prefix List 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.
BGP Community Filtering
One way that BGP controls the distribution of routing information based on the value of the COMMUNITIES
attribute. The attribute is a way to groups destinations into communities and to apply routing decisions based
on the communities. This method simplifies configuration of a BGP speaker to control distribution of routing
information.
A community is a group of destinations that share some common attribute. Each destination can belong to
multiple communities. AS administrators can define to which communities a destination belongs. By default,
all destinations belong to the general Internet community. The community is identified by the COMMUNITIES
attribute, an optional, transitive, global attribute in the numerical range from 1 to 4294967200. These are some
predefined, well-known communities:
• internet—Advertise this route to the Internet community. All routers belong to it.
• no-export—Do not advertise this route to EBGP peers.
• no-advertise—Do not advertise this route to any peer (internal or external).
• local-as—Do not advertise this route to peers outside the local autonomous system.
Routing Configuration Guide, Cisco IOS XE Everest 16.6.x (Catalyst 9500 Switches)
120
Configuring IP Unicast Routing
Prefix List for BGP Filtering
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