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Uses of Multi-Exit Discriminators
MEDs, defined in the main BGP standard, were originally intended to show the
advertising AS's preference, to another neighbor AS, the advertising AS's
preference as to which of several links, to the same AS, are preferred as the
place to which the accepting AS should transmit traffic. Another application of
MEDs is to advertise the value, typically based on delay, of multiple AS that
have presence at an IXP, that they impose to send traffic to some destination.
BGP problems and mitigation
iBGP scalability
An autonomous system with IBGP must have all of its IBGP peers connect to each
other in a full mesh (where everyone speaks to everyone directly). This
full-mesh configuration requires that each router maintain a session to every
other router. In large networks, this number of sessions may degrade performance
of routers, due either to a lack of memory, or too much CPU process
requirements.
Route reflectors and confederations both reduce the number of iBGP peers to each
router and thus reduce processing overhead. Route reflectors are a pure
performance-enhancing technique, while confederations also can be used to
implement more fine-grained policy.
Route reflectors reduce the number of connections required in an AS. A single
router (or two for redundancy) can be made a route reflector: other routers in
the AS need only be configured as peers to them.
Confederations are sets of autonomous systems. In common practice, only one of
the confederation AS numbers is seen by the Internet as a whole. Confederations
are used in very large networks where a large AS can be configured to encompass
smaller more manageable internal ASs.
Confederations can be used in conjunction with route reflectors. Confederations
allow more fine-grained policy while route reflectors are a pure scaling
technique, but either or both may be relevant to a particular situation.
Both confederations and route reflectors can be subject to persistent
oscillation, unless specific design rules, affecting both BGP and the interior
routing protocol, are followed .
However, these alternatives can introduce problems of their own, including the
following:
route oscillation,
sub-optimal routing,
increase of BGP convergence time
Additionally, route reflectors and BGP confederation were not designed to ease
BGP router's configuration. Nevertheless, these are common tools for experienced
BGP network architects. These tools may be combined, as, for example, a
hierarchy of route reflectors.
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