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C H A P T E R39
Chapter Goals
• Understand the purpose of the Border Gateway Protocol.
• Explain BGP attributes and their use in route selection.
• Examine the BGP route selection process.
Border Gateway Protocol
Introduction
The Border Gateway Protocol (BGP) is an interautonomous system routing protocol. An autonomous
system is a network or group of networks under a common administration and with common routing
policies. BGP is used to exchange routing information for the Internet and is the protocol used between
Internet service providers (ISP). Customer networks, such as universities and corporations, usually
employ an Interior Gateway Protocol (IGP) such as RIP or OSPF for the exchange of routing information
within their networks. Customers connect to ISPs, and ISPs use BGP to exchange customer and ISP
routes. When BGP is used between autonomous systems (AS), the protocol is referred to as External
BGP (EBGP). If a service provider is using BGP to exchange routes within an AS, then the protocol is
referred to as Interior BGP (IBGP). Figure 39-1 illustrates this distinction.
39-1
rnetworking Technologies Handbook
Chapter 39
Border Gateway Protocol
BGP Attributes
Figure 39-1 External and Interior BGP
BGP is a very robust and scalable routing protocol, as evidenced by the fact that BGP is the routing
protocol employed on the Internet. At the time of this writing, the Internet BGP routing tables number
more than 90,000 routes. To achieve scalability at this level, BGP uses many route parameters, called
attributes, to define routing policies and maintain a stable routing environment.
In addition to BGP attributes, classless interdomain routing (CIDR) is used by BGP to reduce the size
of the Internet routing tables. For example, assume that an ISP owns the IP address block 195.10.x.x
from the traditional Class C address space. This block consists of 256 Class C address blocks, 195.10.0.x
through 195.10.255.x. Assume that the ISP assigns a Class C block to each of its customers. Wit