BGP's Basic Components
Communities The COMMUNITIES (Type Code 8) attribute is an
optional transitive attribute that allows a given route to belong to one or
more communities. Communities are routes that share some common
property. This attribute was included in BGP to simplify the configuration
of complex BGP routing policies. For example, an academic network that
handles both academic and commercial traffic under an acceptable-use
policy might set a community attribute on the university updates; this
community attribute value would indicate that the route meets the
acceptable-use policy. More than one community can be associated with
Community attributes are optional, transitive, and variable in length.
Current communities are 32-bits long, structured as two 16-bit fields. By
convention, the first 16 bits are either zero, denoting a "well-known"
community known to the Internet, or the AS number that "owns" the com-
munity value. The second 16 bits are meaningful either as defined by the
owning AS or, in the case of well-known communities, by the IETF.
An optional non-transitive attribute may not be recognized by some imple-
mentations. These attributes are used in many private BGP-enabled net-
works. Even if the implementation of BGP does recognize the optional non-
transitive attribute of the message, it is not passed on.
If the network sees the message as an optional non-transitive attribute, say
good-bye to the message. The message is deleted and not sent to other net-
works. The following are the optional non-transitive attributes:
MED The MULTI_EXIT_DISCRIMINATOR (Type Code 4) attribute
is an optional non-transitive attribute that is used by BGP as an extensive
route-selection component. This component starts to work before the
general route-selection process begins, using a BGP attribute called multi-
exit discriminator (MED), which was originally called the Inter-AS metric
or the BGP metric. While the previous metrics inform the local AS routers
which path to select when leaving the AS, MEDs inform the neighboring
AS which link to use to receive traffic.
MED routes are used when two autonomous systems are connected by
multiple links or multiple routers. MED values are not propagated to
other autonomous systems and are considered only as part of the BGP
route-selection process. The general route-installation process never sees
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