Configuring BGP4
December 2005 © Foundry Networks, Inc. 21 - 45
To use the backslash as a string character, enter two slashes. For example, to filter on AS-path strings containing
a backslash, enter the backslash portion of the regular expression as “\\”.
FESX424 Router(config-bgp-router)# as-path-filter 2 deny \\
Filtering Communities
You can filter routes received from BGP4 neighbors based on community names. Use either of the following
methods to do so.
A community is an optional attribute that identifies the route as a member of a user-defined class of routes.
Community names are arbitrary values made of two five-digit integers joined by a colon. You determine what the
name means when you create the community name as one of a route’s attributes. Each string in the community
name can be a number from 0 – 65535.
This format allows you to easily classify community names. For example, a common convention used in
community naming is to configure the first string as the local AS and the second string as the unique community
within that AS. Using this convention, communities 1:10, 1:20, and 1:30 can be easily identified as member
communities of AS 1.
The Layer 3 Switch provides the following methods for filtering on community information:
• Community filters
• Community list ACLs
NOTE: The Layer 3 Switch cannot actively support community filters and community list ACLs at the same time.
Use one method or the other but do not mix methods.
NOTE: Once you define a filter or ACL, the default action for communities that do not match a filter or ACL is
“deny”. To change the default action to “permit”, configure the last filter or ACL entry as “permit any any”.
Community filters or ACLs can be referred to by match statements in a route map.
Defining a Community Filter
To define filter 3 to permit routes that have the NO_ADVERTISE community, enter the following command:
FESX424 Router(config-bgp-router)# community-filter 3 permit no-advertise
Syntax: community-filter <num> permit | deny <num>:<num> | internet | local-as | no-advertise | no-export
The <num> parameter identifies the filter’s position in the community filter list and can be from 1 – 100. Thus, the
community filter list can contain up to 100 filters. The router applies the filters in numerical order, beginning with
the lowest-numbered filter. When a filter match is true, the router stops and does not continue applying filters from
the list.
NOTE: If the filter is referred to by a route map’s match statement, the filter is applied in the order in which the
filter is listed in the match statement.
The permit | deny parameter indicates the action the router takes if the filter match is true.
• If you specify permit, the router permits the route into the BGP4 table if the filter match is true.
• If you specify deny, the router denies the route from entering the BGP4 table if the filter match is true.
The <num>:<num> parameter indicates a specific community number to filter. Use this parameter to filter for a
private (administrator-defined) community. You can enter up to 20 community numbers with the same command.
If you want to filter for the well-known communities “LOCAL_AS”, “NO_EXPORT” or “NO_ADVERTISE”, use the
corresponding keyword (described below).
The internet keyword checks for routes that do not have the community attribute. Routes without a specific
community are considered by default to be members of the largest community, the Internet.
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