Which statement is true of BGP communities?
A.
Communities are used to identify BGP routes based on the originating route’s attributes, whether it is an internal route, an external route, or unknown.
B.
Communities are added by route reflectors. A route reflector appends its own IP address and the autonomous system number separated by a colon in the community field each time a route traverses it.
C.
Communities are used to determine preference within a network; lower community values are more preferable than higher values.
D.
Communities are used as a tool for sorting and categorizing routes based on user-defined criteria.
Explanation:
Syntax
community ([ community-ids ] | no-advertise | no-export | no-export-subconfed | none);Hierarchy Level
[edit routing-instances routing-instance-name routing-options (aggregate| generate| static) (defaults| route)],
[edit routing-instances routing-instance-name routing-options rib routing-table-name (aggregate| generate| static) (defaults| route)],
[edit routing-options (aggregate| generate| static) (defaults| route)],
[edit routing-options rib routing-table-name (aggregate| generate| static) (defaults| route)],
[edit logical-systems logical-system-name routing-instances routing-instance-name routing-options (aggregate| generate| static) (defaults| route)],
[edit logical-systems logical-system-name routing-instances routing-instance-name routing-options rib routing-table-name (aggregate| generate| static) (defaults| route)],
[edit logical-systems logical-system-name routing-options (aggregate| generate| static) (defaults| route)],
[edit logical-systems logical-system-name routing-options rib routing-table-name (aggregate| generate| static) (defaults| route)]Description
Associate BGP community information with a static, aggregate, or generated route.Options
community-idsOne or more community identifiers. The community-ids format varies according to the type of attribute that you use.
The BGP community attribute format is as-number:community-value:
as-numberAS number of the community member. It can be a value from 1 through 65,535.
community-valueIdentifier of the community member. It can be a number from 0 through 65,535.
For more information about BGP community attributes, see the Configuring the Extended Communities Attribute section in the JUNOS Policy Framework Configuration Guide.
For specifying the BGP community attribute only, you also can specify community-ids as one of the following well-known community names defined in RFC 1997:
no-advertiseRoutes containing this community name are not advertised to other BGP peers.
no-exportRoutes containing this community name are not advertised outside a BGP confederation boundary.
no-export-subconfedRoutes containing this community name are not advertised to external BGP peers, including peers in other members ASs inside a BGP confederation.
noneExplicitly exclude BGP community information with a static route. Include this option when configuring an individual route in the route portion to override a community option specified in the defaults portion.Note: Extended community attributes are not supported at the [edit routing-options] hierarchy level. You must configure extended communities at the [edit policy-options] hierarchy level.
BGP Community attributes (Cisco explanation)
BGP community is a optional attribute of BGP routing protocol. It can be considered as a tag in IGP, making a set of IP addresses. The community strings often use to manipulate route, set BGP attributes for a set of ip addresses. We use access-list or prefix list to assign the community to IP addresses. Once the community attribute has been assigned, it can be changed by the route-map function in the router. By default, the community string is a decimal number. However, we can change to a new format (AA:NN) by using ip bgp-community new-formatwhere AA is Autonomous system number and NN is a value as set by ASN.
In addition, there are 3 special community values. The command name and function are as followings.
To assign BGP community string, we use set community <number>/<special community string> in the route-map command. Or, we can use ip community-list <community string> permit/deny <ACL number> as well.
