Samsung Ubigate iBG3026, Configuration Manual, Page: 148 / 718

CHAPTER 15. BGP
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BGP’s basic unit of routing information is the BGP path, a route to a certain
set of CIDR prefixes. Paths are tagged with various path attributes, of which
the most important are AS_PATH and NEXT_HOP. One of BGP's most
important functions is loop detection at the Autonomous System(AS) level,
using the AS_PATH attribute. Simply put, an AS_PATH is a list of
Autonomous Systems that a route goes through to reach its destination.
The syntax of this attribute is made more complex by its need to support path
aggregation, when multiple paths are collapsed into one to simplify further
route advertisements. Loops are detected and avoided by checking for the
router’s own AS number in AS_PATHs received from its BGP peers.
Every time a BGP path advertisement crosses an AS boundary, the
NEXT_HOP attribute is changed to the IP address of the boundary router.
However, when a BGP path advertisement is passed among BGP speakers in
the same AS, the NEXT_HOP attribute is left untouched. Consequently, BGP's
NEXT_HOP is always the IP address of the first BGP router in the next
autonomous system, even though this may actually be several hops away.
The AS's interior routing protocol is responsible for computing an interior
route to reach the BGP NEXT_HOP. This leads to the distinction between
Internal BGP(IBGP) sessions(between routers in the same AS) and External
BGP(EBGP) sessions(between routers in different ASs). NEXT_HOPs are
only changed across EBGP sessions, but left intact across IBGP sessions.
Also, BGP does not relay routing traffic from one IBGP session to another,
only from an EBGP session to an IBGP session or another EBGP session.
The most important consequences of such a design are the need for interior
routing protocols to reach one hop beyond the AS boundary and the need for
BGP sessions within an AS to be fully meshed. Since the NEXT_HOP
contains the IP address of a router interface in the next autonomous system,
and this IP address is used to perform routing, the interior routing protocol
must be able to route to this address. This means that interior routing tables
must include entries one hop beyond the AS boundary. When a BGP routing
update is received from a neighboring AS, it must be relayed directly to all
other BGP speakers in the AS.
It the responsibility of the BGP implementation to select among competing
paths. RFC 1771 states only that ‘The computation be based on pre-configured
policy information. The exact nature of this policy information and the
computation involved is a local matter.’ Since the AS_PATH attribute includes
a list of Autonomous Systems used to reach the destination, it is possible to
implement primitive policy decisions such as ‘avoid all routes through AS x’.