Foundry Networks AR3202-CL - Manual (Page 46)

June 2004 © 2004 Foundry Networks, Inc. 4 - 1
Chapter 4
Protocols Overview
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.