User guide

BGP Overview Configuring BGP
page 4-6 OmniSwitch AOS Release 7 Advanced Routing Configuration Guide March 2015
BGP uses TCP as its transport protocol, eliminating the need for it to implement mechanisms for protocol
message fragmentation, retransmission, acknowledgment, and sequencing information.
Autonomous Systems (ASs)
Exterior routing protocols were created to control the expansion of routing tables and to provide a more
structured view of the Internet by segregating routing domains into separate administrations, called
Autonomous Systems (ASs). Each AS has its own routing policies and unique Interior Gateway Protocols
(IGP).
More specifically, an AS is a set of routers that has a single routing policy, runs under a single technical
administration that commonly utilizes a single IGP (though there could be several different IGPs
intermeshed to provide internal routing). To the rest of the networking world, an AS appears as a single
entity.
The diagram below demonstrates the relationship of BGP and ASs:
Each AS has a number assigned to it by an Internet Registry, much like an IP address. BGP is the standard
Exterior Gateway Protocol (EGP) used for exchanging information between ASs.
The main difference between routing within an AS (IGP) and routing outside of an AS (EGP) is that IGP
policies tend to be set due to traffic concerns and technical demands, while EGP policies are set more on
business relationships between corporate entities.
BGP 4-Octet Autonomous System Number (ASN)
An OmniSwitch with the BGP support for 4-octet ASN capability automatically advertises itself as being
capable of handling 4-octet ASNs. OmniSwitch AOS is backward-compatible with other BGP devices that
are not capable of 4-octet ASNs. The OmniSwitch supports transitive optional path attributes for
interoperability with BGP devices that do not support 4-octet ASNs.This feature provides the following:
BGP Support for 4-octet (32 bit) ASN for BGP neighbor interoperability and path attribute
interoperability as per RFC 6793.
Advertisement and discovery of 4-octet ASN capability by using the BGP Capability advertisement as
specified in RFC 5492.
Autonomous System 100 Autonomous System 200
BGP
Internal
Gateway
Protocol
Internal
Gateway
Protocol