Information Technology Reference
In-Depth Information
COMMUNITY
A
community
is defined as a group of prefixes that share a common property. Multiple
communities can be applied to a prefix, each community being 4 bytes. Two types of
communities exist:
•
Well-known communities
—When receiving prefixes with these communities, peers
take actions automatically based on the predefined meanings of the communities. No
additional configurations are needed. In RFC 1997, the well-known communities fall
in the range of reserved values, which are from 0xFFFF0000 through 0xFFFFFFFF.
•
Private communities
—Communities can be defined by the administrators and must
be coordinated between peers of the different autonomous systems. Actions must be
specifically configured. Private communities have values outside the reserved range.
Currently, four well-known communities are supported in Cisco IOS software:
•
NO_EXPORT
—Prefixes with this community should not be advertised to eBGP
peers but can be sent to subautonomous systems within the same confederation. The
value of this community is 0xFFFFFF01.
•
LOCAL_AS
—Do not advertise these prefixes outside the local AS. With confedera-
tion, only peers within the same sub-AS are allowed to receive these prefixes. Without
confederation, LOCAL_AS is treated the same as NO_EXPORT. In RFC 1997,
NO_EXPORT_SUBCONFED (0xFFFFFF03) is defined for this purpose.
•
NO_ADVERTISE
—Do not advertise prefixes with this community to any peer,
internal or external. The value of this community is 0xFFFFFF02.
•
INTERNET
—Advertise the prefixes to the Internet community. In other words, there
are no restrictions. This well-known community is not specifically defined in RFC
1997. In Cisco IOS software, the INTERNET community (with a value of 0) is one
that every prefix is part of.
More commonly used communities are private communities. The main objective of using
them is to attach administrative tags to prefixes so that proper policies can be created. The
private community uses the format of
AS:number
, where
AS
is the local AS number or a
peer AS and
number
is an arbitrary number administered locally or with peers to represent
a community grouping to which a policy may be applied. This user-friendly format is
enabled by
ip bgp-community new-format
in global configuration mode.
