
Configuring BGP Services
5-3
BGP features include
• TCP support
The neighbors communicate over a reliable transport layer connection —TCP
— so that BGP can assume that its communication with other BGP routers is
reliable. This eliminates the need to implement the update, retransmission,
acknowledgment, and sequencing that are necessary with EGP.
• Multiple path elimination
A BGP speaker can announce only routes that it actually uses. Therefore, a
border router that learns multiple paths to an external destination must choose
only one of those routes for further advertisement into the AS or to other BGP
peers.
• Authentication
BGP provides support for multiple authentication schemes. A scheme is
identified in the Open message, and each subsequent message on that TCP
connection must contain a marker field that complies with the scheme.
However, only the default authentication scheme (none) has been developed
at this time.
• AS Path attribute
Each BGP route contains a list of the autonomous systems that it has
traversed. This allows a BGP speaker to eliminate looped routes. If a BGP
speaker sees its own AS listed in a route, then there is a loop, and the route is
not used.
• Routing policy support
Each routing update contains information on which hop-by-hop policies can
be applied. For example, policies can be defined based on the information
contained in a route’s AS path attribute. BGP can favor routes based on AS
count, or the presence of a certain AS in the path. Conversely, it can also avoid
routes that contain a certain AS in the path or that originate in a certain AS.
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