Reference Guide

172 | Border Gateway Protocol IPv4 (BGPv4)
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• Storing Last and Bad PDUs
• Capturing PDUs
• PDU Counters
• BGP Regular Expression Optimization
• Debugging BGP
• Sample Configurations
BGP protocol standards are listed in the Chapter 58, Standards Compliance.
Protocol Overview
Border Gateway Protocol (BGP) is an external gateway protocol that transmits interdomain routing
information within and between Autonomous Systems (AS). Its primary function is to exchange network
reachability information with other BGP systems. BGP generally operates with an Internal Gateway
Protocol (IGP) such as OSPF or RIP, allowing you to communicate to external ASs smoothly. BGP adds
reliability to network connections be having multiple paths from one router to another.
Autonomous Systems (AS)
BGP Autonomous Systems (ASs) are a collection of nodes under common administration, with common
network routing policies. Each AS has a number, already assigned by an internet authority. You do not
assign the BGP number.
AS Numbers (ASNs) are important because the ASN uniquely identifies each network on the Internet. The
IANA has reserved AS numbers 64512 through 65534 to be used for private purposes. The ASNs 0 and
65535 are reserved by the IANA and should not be used in a live environment.
Autonomous Systems can be grouped into three categories, defined by their connections and operation.
A multihomed AS is one that maintains connections to more than one other AS. This allows the AS to
remain connected to the internet in the event of a complete failure of one of their connections. However,
this type of AS does not allow traffic from one AS to pass through on its way to another AS. A simple
example of this is seen in the illustration below.
A stub AS is one that is connected to only one other AS.
A transit AS is one that provides connections through itself to separate networks. For example as seen in
the illustration below, Router 1 can use Router 2 (the transit AS) to connect to Router 4. ISPs are always
transit ASs, because they provide connections from one network to another. The ISP is considered to be
“selling transit service” to the customer network, so thus the term Transit AS.
When BGP operates inside an Autonomous System (AS1 or AS2 as seen in the illustration below), it is
referred to as Internal BGP (IBGP Interior Border Gateway Protocol). When BGP operates between
Autonomous Systems (AS1 and AS2), it is called External BGP (EBGP Exterior Border Gateway
Protocol). IBGP provides routers inside the AS with the knowledge to reach routers external to the AS.
EBGP routers exchange information with other EBGP routers as well as IBGP routers to maintain
connectivity and accessibility.