BGP Route Filter Overview

Overview

It is possible to filter when advertising and receiving route information to and from a specified neighbor with BGP. “How to identify the route information” is important in route filtering with BGP.

This section describes the different types of BGP route filters.

BGP Route Filter Types

To filter route information sent to and received from a specified neighbor with BGP, the following methods are available.

• distribute-list
• prefix-list
• filter-list
• route-map

The route map can add or change path attributes as well as simple filters.

Route Filter Points

There are two important points in the route filter.

1. Specifying the route to be filtered
2. Timing to filter

In the case of BGP, the important thing is “how to identify route information” because the timing of filtering is fixed by sending/receiving with the neighbor. In the following sections, focusing on “how to identify route information,” each filtering method will be explained.

Specifying the route to be filtered

distribute-list

The distribution-list specifies the BGP route information to be filtered by ACL. The network address of the route information is referenced by a standard ACL. Standard ACLs essentially configure conditions for the source IP address of an IP packet. However, when used in distribution-list, standard ACLs configure conditions for network addresses.

Extended ACLs are used to specify route information to be filtered by referencing not only network addresses but also subnet masks. The source IP address condition of the extended ACL is interpreted as a network address. And the destination IP address condition is interpreted as a subnet mask.

The standard/extended ACL permit/deny is the action of allowing or denying route information. Specifically, when applied in, permit means receive route information; when applied out, permit means advertise route information. Also, when applied in, deny discards route information; when applied out, deny does not advertise route information.

Related articles

See also the following article about configuration and verification commands for BGP route filter by distribute-list.

N-Study

BGP Route Filter : distribute-list | N-Study

https://www.n-study.com/en/bgp-detail/bgp-route-filter-distribute-list/

This section describes the commands for configuring and verifying the BGP route filter using distribute-list. The distribute-list identifies the BGP route information with ACL to be allowed or denied.

Here is an example of concrete configuration of BGP route filtering by distribute-list.

N-Study

BGP Route Filter : distribute-list Configuration Example | N-Study

https://www.n-study.com/en/bgp-detail/bgp-route-filter-distribute-list-configuration-example/

This is an example of BGP route filter configuration using distribute-list. BGP routes to be filtered are identified with both standard and extended ACLs.

prefix-list

The prefix list identifies the BGP route information to be filtered and determines the action to be taken. The prefix list can be configured according to the bit pattern of the network address and the range of the subnet mask.

And, permit/deny in the prefix-list is the action of allowing or denying route information.

Related articles

See also the following article about configuration and verification commands for BGP route filter by prefix-list.

N-Study

BGP Route Filter : prefix-list | N-Study

https://www.n-study.com/en/bgp-detail/bgp-route-filter-prefix-list/

This section describes the configuration and verification commands for the BGP route filter using prefix-list. The prefix-list identifies BGP routes to be filtered by network address and subnet mask.

Here is an example of concrete configuration of BGP route filtering by prefix-list.

N-Study

BGP Route Filter : prefix-list Configuration Example | N-Study

https://www.n-study.com/en/bgp-detail/bgp-route-filter-prefix-list-configuration-example/

The following is an example of configuring BGP route filter with prefix-list. Configure the same conditions with prefix-list as with distribut-list filter.

filter-list

The filter list identifies the BGP route information to be filtered by AS_PATH ACL. The AS_PATH ACL refers to the AS_PATH attribute specific to the BGP route; the string of AS numbers contained in AS_PATH is configured as a regular expression condition.

AS_PATH ACL permit/deny is the action of allowing or denying route information.

Related articles

See also the following article about configuring and verifying commands for BGP route filtering by filter-list (AS_PATH ACL).

N-Study

BGP Route Filter : filter-list(AS_PATH ACL)- | N-Study

https://www.n-study.com/en/bgp-detail/bgp-route-filter-filter-list-as-path-acl/

This section describes the commands for configuring and verifying the filter-list.The filter-list identifies the BGP routes to be filtered by AS_PATH ACL.

Here is an example of concrete configuration of BGP route filter by filter-list (AS_PATH ACL).

N-Study

BGP Route Filter : filter-list(AS_PATH ACL) Configuration Example | N-Study

https://www.n-study.com/en/bgp-detail/bgp-route-filter-filter-list-as-path-acl-configuration-example/

This is an example of BGP route filter configuration by filter list (AS_PATH ACL). It is important to know how to represent the AS_PATH attribute in a regular expression.

route-map

The route map will eventually refer to the above distribute-list/prefix-list/filter-list. Associating distribute-list/prefix-list/filter-list with a route map match condition identifies the BGP routes to be filtered.

If a route map is used, routes can also be specified using AND and OR conditions that combine distribute-list/prefix-list/filter-list.

The route-map permit/deny is the action of allowing or denying route information. Note that the combination of permit/deny of the route map and permit/deny of the distribution list/prefix list/filter list referenced in the route map can be a bit complicated. Finally, please clarify that the route-map permit/deny allows or denies route information.Then, additional path attributes can be set for the BGP routes that have been permitted.

Related articles

See also the following article about configuring and verifying commands for BGP route filtering by route-map.

N-Study

BGP Route Filter : Route-map | N-Study

https://www.n-study.com/en/bgp-detail/bgp-route-filter-route-map/

This section describes the configuration and verification commands for BGP route filtering using route-map. The route map allows you to identify BGP routes to filter by reference to standard/extended ACLs, prefix lists, AS_PATH ACLs, etc. I

Here is an example of concrete configuration of BGP route filter by route-map.

N-Study

BGP Route Filter : route-map Configuration Example | N-Study

https://www.n-study.com/en/bgp-detail/bgp-route-filter-route-map-configuration-example/

This is an example of BGP route filter configuration with route-map. Route-maps allow you to flexibly identify BGP routes to filter by referencing standard/extended ACLs, prefix-lists, and AS_PATH ACLs in the match condition.

Summary of route information specification

The following table summarizes the information referenced and how to specify the route for each filter method.

The following figure also summarizes which parts of the BGP route information are referenced by each route filter.

Route maps are the most flexible, allowing for filtering of BGP routes based on a variety of information. The route map is, to put it plainly, versatile, like a joker. But to use the route map properly, it is a prerequisite to understand the distribute-list/prefix-list/filter-list.

Summary