3
Router User Guide
P
ROTOCOLS
O
VERVIEW
BGP4
Border Gateway Protocol Version 4 (also referred to as simply BGP) is an exterior routing protocol
used for the global Internet.
Once configured, BGP peers first exchange complete copies of their routing tables (including BGP
version, router ID, and keep alive hold time), which are usually very large. Thereafter, only
incremental updates (deltas) are sent as changes occur to the routing tables. BGP keeps a current
version of the routing table for all peers, keep alive packets are sent to ensure that the connection
between BGP peers, and notification packets are sent in response to problems and irregularities. This
enables longer running BGP sessions to be more efficient than shorter sessions.
BGP's basic unit of routing information is the BGP path, a route to a certain set of classless
interdomain routing prefixes. Paths are tagged with various path attributes, including an autonomous
systems (AS) path and next-hop. In fact, one of BGP's most important functions is loop detection at
the AS level, using the AS path attribute, which is a list of autonomous systems used for data
transport.
The syntax of this attribute is made more complex by its need to support path aggregation when
multiple paths are collapsed into one in order to simplify further route advertisements. A more
simplified view of an AS path is that it is a list of autonomous systems that a route goes through to
reach its destination. Loops are detected and avoided by checking for your own AS number in the AS
path's received from neighboring autonomous systems. Every time a BGP path advertisement
crosses an AS boundary, the next-hop attribute is changed on the boundary router. Conversely, as 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 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 AS's). Next-hops are only changed
across EBGP sessions, but left intact across IBGP sessions. The two most important consequences of
this design are the need for interior routing protocols to reach one hop beyond the AS boundary, and
for BGP sessions to be fully meshed within an AS.
Since the next-hop contains the IP address of a router interface in the next AS, 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.
Furthermore, since BGP does not relay routing traffic from one interior BGP session to another (only
from an exterior BGP session to an IBGP session or another EBGP session), BGP speakers must be
fully meshed.
RFC Compliance The following table provides Alcatel’s BGP RFC compliance information.
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