Configuring BGP
BGP Overview
OmniSwitch AOS Release 7 Advanced Routing Configuration Guide
March 2011
page 3-5
BGP Overview
BGP (Border Gateway Protocol) is a protocol for exchanging routing information between gateway hosts
in a network of autonomous systems. BGP is the most common protocol used between gateway hosts on
the Internet. The routing table exchanged between hosts contains a list of known routers, the addresses
they can reach, and attributes associated with the path.
BGP is a distance vector protocol, like the Routing Information Protocol (RIP). It does not require peri-
odic refresh of its entire routing table, but messages are sent between BGP peers to ensure a connection is
active. A BGP speaker must retain the current routing table of its peers during the life of a connection.
Hosts using BGP communicate using the Transmission Control Protocol (TCP) on port 179. On connec-
tion start, BGP peers exchange complete copies of their routing tables, which can be quite large. However,
only changes are exchanged after startup, which makes long running BGP sessions more efficient than
shorter ones. BGP allows administrators to control routing table exchanges based on policy statements.
BGP makes it easy to use Classless Inter-Domain Routing (CIDR), which is a way to increase addresses
within the network beyond the current Internet Protocol address assignment scheme. BGP’s basic unit of
routing information is the BGP path, which is 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 level,
using the
AS_PATH
attribute. The AS_PATH attribute is a list of ASs being 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 to simplify further route advertisements. A simplified
view of
AS_PATH
is that it is
the 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 AS_PATH
s received from neighboring
Autonomous Systems.
An OmniSwitch using BGP could be placed at the edge of an enterprise network to handle downstream
Internet traffic. An example of such a configuration would be two (2) paths to the Internet, or a dual-
homed network.
BGP is intended for use in networks with multiple autonomous systems. It is not intended to be used as an
Interior Gateway protocol (IGP), such as RIP or Open Shortest Path First (OSPF). In addition, when BGP
is used as an internal routing protocol, is best used in transit autonomous systems with multiple exit points
as it includes features that help routers decide among multiple exit paths.
Local Network
AS 1
ISP 1
AS 1
ISP 2
AS 2
OmniSwitch
