Samsung Ubigate iBG2016 Configuration Manual page 150

CHAPTER 6. BGP
The BGP community attribute simplifies the configuration of complex policies.
It enables an identifier to be associated with a route. Routes that are supposed
to receive the same treatment with respect to policy can be assigned the same
identifier. The community attribute dynamically influences the routing
decisions of autonomous systems. For e.g., in a topology of two autonomous
systems AS1 and AS2, AS1 can advertise a route to AS2 and simultaneously
provide dynamic instructions that AS2 is not to advertise the route externally.
It does so by setting the COMMUNITY attribute(assigned to the advertised
route) to no-export.
Normally, whenever changes are made to the attributes or policies of a route,
the BGP TCP session with its neighbor has to be reset in order for the
modified routing behavior to take effect. When a session is reset, the routing
cache is invalidated, routes disappear and route instability increases and is
propagated throughout the Internet. To prevent this, a mechanism called soft
reconfiguration is used. It allows attribute reconfigurations to be made without
terminating an already established TCP session. This is a means of requesting
that a peer re-advertise all the prefixes learned from other peers(its Adj-RIB-
Out).
The purpose of Capability Negotiation is to facilitate the introduction of new
features in BGP by providing graceful capability negotiation without requiring
the BGP peers to be terminated. It is achieved with the addition of a new
optional parameter called 'Capabilities' in the Optional Parameters field of the
OPEN message.
Route maps are used with BGP to control and modify routing information and
to define the conditions by which routes are redistributed between routers and
routing processes.
Access lists based on AS path enables control of routing updates based on
BGP AS path information. To control updates for all routes to or from an
autonomous system, filtering based on AS path is more efficient than listing
each route individually.
BGP community list is a user defined BGP communites attribute list. BGP
community list can be used for matching or manipulating BGP communities
attribute in updates. Valid list numbers are 1-199.
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