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BGP: Overview of the Available Filter Types
Page 6 | Use Route Maps and Other Filters to Filter and Alter BGP and OSPF Routes
BGP: Overview of the Available Filter Types
The following sections describe the various types of filters that can be applied to BGP
updates and the hierarchy of the filters.
Filter types
There are a number of filter types that can be applied to the BGP updates being exchanged
between BGP peers:
Distribute filters
Distribute list can filter the routing information between the routing protocol (RIP, RIPng,
OSPF, OSPFv3 or BGP) and its IP route table (routing information base - RIB). It has to make
the use of ACL, prefix list, or route map depending on which routing protocol it is configured
for. For BGP, it can only use ACL to match prefix (ip address).
These use ACLs and look at the individual prefixes within an update message. If a prefix
within the update message matches the filter criteria then that individual prefix is filtered out
or accepted depending on what action the filter entry has been configured to carry out.
Note that you cannot combine distribute filters and prefix filters.
AS Path filters
These look at the AS-Path attribute in update messages. If the AS-Path attribute in the
update matches the filter criteria then the whole update message is filtered out or accepted,
depending on what action the filter entry has been configured to carry out.
Prefix filters
These use prefix lists and look at the individual prefixes within an update message. If a prefix
within the update message matches the filter criteria then that individual prefix is filtered out
or accepted depending on what action the filter entry has been configured to carry out.
Note that you cannot combine distribute filters and prefix filters.
Route maps
These are a structured combination of match criteria and actions. They can be used to filter
out routes and also to alter the attributes in update messages.
Access Control List (ACL) filters
ACL is mainly use to limit the network traffic. It can be used by distribute list and route map
to filter the routing updates. ACLs (Access Control Lists) can be used to do a number of
different things on your network, from controlling what type of traffic passes through your
devices to deciding on what routing protocols you wish to be advertised out of your routers.