BGP Communities and Community Lists
BGP Communities and Community Lists Overview
BGP updates carry a number of path attributes. Some of these, like the AS_PATH, are mandatory and appear in every update message sent. Others are optional, and may or may not appear in any given update. Of the optional attributes, two can be specified arbitrarily by administrators to ease configuration. These two attributes are "communities" and "extended communities." Both of these attributes operate by "coloring" routes received in updates where these attributes are present; every router keeps track of the set of communities and extended communities with which a route was learned. The particular communities (or extended communities) with which a route was learned can be used to indicate that a particular set of policies should be applied to those routes.
ip community-list
Name
ip community-list - specifies a community or group of communities used in filtering or modifying community values
Syntax
ip community-list name [permit | deny] [comm-set
commset_name] [exact]? [standard | extended]?
no ip community-list name [permit | deny] [comm-set
commset_name] [exact]? [standard | extended]?
Mode
Global Configuration
Parameters
name - a string of characters that uniquely identifies this community list
permit | deny - permits or denies the entries in the community list. When creating a community list, by default, the end of the community list contains an implicit deny statement for everything.
comm-set commset_name - specify a community set or an extended community set
exact - optionally specify whether an entire community list must be matched exactly
standard - specifies that the referenced community set is a standard (non-extended) community set. standard and extended are mutually exclusive. Note: If neither standard nor extended is specified for a standard community list, then the default value is standard. If both are configured, the keyword supplied last will override the previous.
extended - specifies that the referenced community set is an extended (non-standard) community set. standard and extended are mutually exclusive. If both are configured, the keyword supplied last will override the previous.
Description
The ip community-list command is used to specify a community, an extended community, or groups of both used in filtering or modifying community values. Multiple community lists for the same name can be specified to create multiple elements in the list. This list is generated in the order in which the commands are entered.
Default
Community lists are not configured by default.
Command History
NGC 2.2 - This command was introduced.
Examples
Example 1
The following example configures a community list with two list entries. The first entry permits a community set labeled set1, and the second entry permits a community set labeled set2. Note that set1 and set2 are implicitly taken to refer to the names in the standard community set name space.
(config)# ip community-set set1 101:101
(config)# ip community-set set2 101:104
(config)# ip community-list abc permit comm-set set1 exact
(config)# ip community-list abc permit comm-set set2 exact
Example 2
The following example configures a community set, "set1", that includes AS:num 101:102. It then configures an extended community set "ext-set1", that includes Route Target AS:num 201:202. The two are then added to an extended community list, called "commlist1".
(config)# ip community-set set1 101:102
(config)# ip community-set ext-set1 extended rt 201:202
(config)# ip community-list commlist1 permit comm-set set1 extended
(config)# ip community-list commlist1 permit comm-set ext-set1 extended
ip community-set
Name
ip community-set - specifies a community or group of communities or extended communities used in filtering or modifying community values
Syntax
Standard BGP Communities
ip community-set set_id [as:num | comm-num | local-as |
no-advertise | no-export | none ] {1,n}
no ip community-set set_id [as:num | comm-num |
local-as | no-advertise | no-export | none ] {1,n}
Extended BGP Communities
ip community-set set_id extended [ none | (rt [as:num |
ip:num]) | (rt4 [as:num | ip:num]) | (soo [as:num |
ip:num]) | (soo4 [as:num | ip:num]) | (lbw as:float) (*:*) ] {1,n}
no ip community-set set_id extended [ none | (rt [as:num |
ip:num]) | (soo [as:num | ip:num]) | (lbw as:float) (*:*) ] {1,n}
Mode
Global Configuration
Parameters
Standard BGP Communities
set_id - a string of characters that uniquely identifies this community set
as:num - the autonomous system (AS) number to be concatenated with a number specified as a 16-bit number. This is used for specifying a community in the "community-split" style.
comm-num - the concatenation value of the two sixteen-bit numbers used for this arbitrary community
local-as - specifies the well-known community NO_EXPORT_SUBCONFED as defined in RFC 1997. Routes tagged with this community are not to be advertised to external peers, including those peers in other members’ autonomous systems inside of a BGP confederation.
no-advertise - specifies the well-known community NO_ADVERTISE as defined in RFC 1997. Routes tagged with this community are not to be advertised to any other peers.
no-export - specifies the well-known community NO_EXPORT as defined in RFC 1997. Routes tagged with this community are not to be exported outside of a confederation boundary when confederations are in use or if they are outside of the local AS when confederations are not configured.
none - specifies the empty set of communities. It is useful only where matches are being attempted. When used in that context, if a route has any communities associated with it, then it does not match; otherwise, it does match. Note: Empty sets of communities cannot be combined with other communities in the list.
{1,n} - this command must be used at least once, and can be used an unlimited number of times for a single community set ID
Extended BGP Communities
set_id - a string of characters that uniquely identifies this community set
none - specifies the empty set of extended communities. Note: Empty sets of extended communities cannot be combined with other communities in the list.
rt (as:num | ip:num) - specifies the Route Target extended community. This is followed by either the autonomous system (AS) number to be concatenated with num (specified as a 32-bit number), or an IPv4 address in dotted-quad format with a 16-bit number value for num.
rt4 (as:num | ip:num) - specifies a 4-bit Route Target extended community. This is followed by either the autonomous system (AS) number to be concatenated with num (specified as a 2-bit number), or an IPv4 address in dotted-quad format with a 16-bit number value for num.
soo (as:num | ip:num) - specifies the Site of Origin (Route Origin) extended community. This is followed by either the autonomous system (AS) number to be concatenated with num (specified as a 32-bit number), or an IPv4 address in dotted-quad format with a 16-bit number value for num.
soo4 (as:num | ip:num) - specifies the Site of Origin (Route Origin) extended community. This is followed by either the 4-bit autonomous system (AS) number to be concatenated with num (specified as a 2-bit number), or an IPv4 address in dotted-quad format with a 16-bit number value for num.
lbw as:float - specifies the Link Bandwidth extended community. This is followed by the autonomous system number and floating point numbering expressing the speed of a link in bytes per second.
*:* - specifies all extended communities
{1,n} - this command must be used at least once, and can be used an unlimited number of times for a single extended community ID
Description
The ip community-set command is used to specify a set of communities and extended communities that must be matched for a route to be considered a match.
BGP updates carry a number of path attributes. Some of these, like the s, are mandatory and appear in every update message sent. Others are optional, and may or may not appear in any given update. Of the optional attributes, two can be specified arbitrarily by administrators to ease configuration. These two attributes are "communities" and "extended communities." Both of these attributes operate by "coloring" routes received in updates where these attributes are present; every router keeps track of the set of communities and extended communities with which a route was learned. The particular communities (or extended communities) with which a route was learned can be used to indicate that a particular set of policies should be applied to those routes.
Notes:
- You cannot mix standard and extended communities in the same community set.
- If you configure a community set twice, the second configuration overwrites the first. The two configurations will not merge.
- Separate name spaces are used for standard and extended community sets. Thus, the same set name can be used for both a standard and an extended community set.
Default
Community sets are not configured by default.
Command History
NGC 2.2 - This command was introduced.
Examples
Example 1
The following example configures a community set named "abc" that includes AS:num 201:202.
(config)# ip community-set abc permit 201:202
Example 2
The following example configures an extended community set "ext-set1", with a Route Target AS:num 201:202 and a site of origin IP number 10.1.2.3:100.
(config)# ip community-set ext-set1 extended rt 201:202 soo 10.1.2.3:100
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