Border Gate Protocol

Unit 5: Intradomain and Interdomain Routing Protocols 

Lesson 5-3: Border Gateway Protocol 

At a Glance 

The Border Gateway Protocol (BGP) is an interdomain routing protocol 

used in TCP/IP internetworks. BGP was created to allow organizations, 

with large networks, to control how those networks are used. This is 

particularly important to networks that sell transit service to others. 

What You Will Learn 

After completing this lesson, you will be able to do the following: 

• Describe the Border Gateway Protocol. 

• Diagram a BGP internetwork. 

• Identify the differences between inter-autonomous routing, intraautonomous 

routing, and pass-through routing. 

• Compare BGP and RIP. 

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. 

Tech Talk 

• Autonomous System (AS)—A network controlled by a single set of 

policies, for example, the Internet. 

• Distance Path Routing—An algorithm used by BGP where each AS 

adds its ID to every route that it advertises to its neighbors. 

• Intra-autonomous Routing—BGP routing that occurs within the same 

autonomous system. 

• Inter-autonomous Routing—BGP routing that occurs between two or 

more different autonomous systems. 

• Pass-through Routing—BGP routing that occurs when a router passes 

a packet, that did not originate in its own system and is not destined to 

its system, on to another autonomous system. 

Border Gateway Protocol 

The Border Gateway Protocol, BGP, was designed for a purpose different 

from the other routing protocols discussed in this unit. The other protocols 

were designed on the assumption that a group of routers would cooperate 

to provide the best possible service to each packet they encounter. BGP 

was designed with nearly the opposite assumption: that the different nodes 

have different and possibly competing purposes. 

The original Internet was composed of a single, neutral, government 

operated backbone, connecting many individual networks. This backbone 

appeared politically neutral to the organizations using it. As the Internet, 

it became impossible to continue with only a single backbone. In addition, 

the wide variety of goals among the organizations involved in the Internet 

has made the concept of "neutral" obsolete. 

These changes in the political structure of the Internet forced the 

development of a routing protocol that could cope with them. 

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Autonomous Systems 

Like other routing protocols, BGP relays information between routers. On 

the other hand it actually routes traffic between much larger entities called 

Autonomous Systems. An Autonomous System, or AS, is a network 

controlled by a single set of policies. Examples of an Autonomous System 

might be the private, Internet connected network of a large company, or 

the section of the Internet backbone operated by another company. BGP 

allows these networks to implement policies such as: 

• Traffic from Taiwan may not, under any conditions, use the Chinese 

national network for transit. 

• Traffic from the CIA to the Pentagon must never cross the public 

Internet, although any other traffic may do so. 

Autonomous Systems 

Router 

Router 

Internet 

AS4 

Router 

Router 

Router 

Router 

Border 

Router 


Router 


Router 


Router 

AS1 


Router 


Router 


Router 


Router 


Router 


Router 

Router 

Router 

Router 

AS2 

Router 

Router 

AS3 

Router 

The illustration above shows a possible network with four ASs. It is not 

necessary to run BGP on all routers within a network. Since BGP is 

responsible only for routing between ASs, it is only necessary to run it on 

border routers. 

BGP uses TCP for transport. TCP assures BGP of connection-oriented, 

error free, multi-hop connectivity between BGP routers. 

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Another requirement for BGP is that every BGP router within an AS must 

maintain a session with every other BGP router. This is an unfortunate 

requirement, since it means that each router must maintain a number of 

connections that grows arithmetically with the number of border routers in 

the AS. The requirement is necessary to assure that every border router 

presents exactly the same view of its AS to the world as every other border 

router in the AS. This allows BGP to treat AS's as single "hops" ignoring 

their internal policies. 

Check Your Understanding 

♦ The World Wide Web is an example of hundreds of autonomous 

systems linked together into a large internetwork. If the WWW 

used the Routing Information Protocol (RIP) instead of BGP, what 

do you think would be the result? 

BGP Mechanism 

BGP is a distance-vector protocol that allows the router to transmit the 

path between autonomous systems from the source to the destination. It 

does not use cost as a metric to calculate the best path; rather it uses a 

path ranking system to calculate the best path. The ranking system is an 

arbitrary metric established by the network administrator. The network 

administrator may use any of many criteria, including AS counts (similar 

to hop counts), speed and delays. 

BGP routers may be used within autonomous systems, and in between 

autonomous systems. BGP routers may also operate as a pass-through 

system. 

Within inter-autonomous systems, routing occurs between two or more 

routers in different autonomous systems. The routers must reside on the 

same physical network and maintain constant updates of the entire 

internetwork topology. The Internet is an example of BGP operating in an 

inter-autonomous system. 

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BGP routers located within the same autonomous system maintain 

updates on the system topology. BGP also is used by the routers to 

determine which router will function as the border router. Within the 

Internet, there are hundreds of intra-autonomous systems using BGP. 

When a BGP router receives a packet that did not originate from its 

system and is addressed to a destination also not within its autonomous 

system, it merely passes the packet through its own system to the next 

autonomous system. This is sometimes referred to as pass-through routing. 

Routing Tables are Exchanged Between Systems 

AS1 

Border Router 

Routing 

Table 

Internet 

Exchange 

AS2 

Border Router 

Routing 

Table 

When there is a change in a routing table, the router sends an update of 

only the portion that has changed. The router also only sends information 

indicating the best path to a network. BGP routers do not send updates at 

regular intervals as do RIP routers. 

Distance Path Routing 

BGP uses an extension of a distance-vector protocol, called distance-path 

routing. Using this algorithm, each AS adds its ID to every route that it 

advertises to its neighbors. This makes it easy to identify the shortest 

path to another AS since it is simply the route with the fewest ID's 

attached. It also makes it easy to eliminate routing loops. If an AS 

receives an advertisement with an AS path which already contains its own 

ID; it simply discards it, since it has obviously seen it before. 

This same mechanism solves the "counting to infinity" problem. 

If the link between AS1 and AS2 fails, AS1 will hear the advertisement for 

AS1 from AS3. The AS Path on that advertisement, however, will contain 

AS's 1, 2, and 3. Since AS2 has obviously heard the advertisement before, 

it discards it, and never starts counting to infinity. 

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Try It Out 

Policy Routing 

BGP was created to allow organizations with large networks, to control 

how those networks are used. This is a hot topic in the Internet today. 

This is particularly important to networks that sell transit service to 

others. Most backbone administrators use a policy descriptively called "Hot 

Potato Routing". As the name suggests, this means that they get packets 

off their own networks as quickly as possible, to minimize the amount of 

traffic they carry for other providers. 

Materials Needed: 

• Windows 95 PC 

• Internet Connection 

• Any Word Processor (e.g., MS Word) 

• Poster Paper 

• Pen/Pencil and Paper 

1. Research two network practices using routing policies. 

2. Use the Internet and any other sources you can find. 

3. Create a classroom poster that demonstrates how the two practices use 

routing policies to control how a network is used. 

4. In class discussion, explain the concepts behind your poster. 

Rubric: Suggested Evaluation Criteria and Weightings 

Criteria % Your Score 

Concise research of two routing policy practices 25 

Analysis and synthesis of information 50 

Thoughtful Poster and insightful class discussion 25 

TOTAL 100 

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Stretch Yourself 

Internet Multicast Routing Protocols 

The protocols discussed in this unit have all dealt with the delivery of a 

packet from a single source to a single destination using unicast or 

broadcast techniques. 

Internet multicast routing protocols have been developed to allow IP 

packets to be sent from one or multiple sources and delivered to multiple 

destinations. These protocols are able to overcome the limitations of RIP, 

OSPF, and BGP, and support the greater requirements of today’s new 

applications. 

There are several protocols that fall in the category of Internet multicast 

routing: 

• Internet Group Membership Protocol 

• Protocol-Independent Multicast 

• Distance-Vector Multicast Routing Protocol 

• Multicast Open Shortest Path First 



• Internet Connection (optional) 



Research these protocols and write a brief paper on the mechanisms used 

by each protocol. Compare these protocols to the protocols discussed in 

this unit. Include in your paper, no less than eight resources. 



Analysis and synthesis of information 50 

Quality paper comparing protocols 40 

Minimum of eight resources 10 

TOTAL 100 

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Network Wizards 

Visit Your Local ISP 

Internet Service Providers are the organizations that provide individuals a 

link to the WWW and the Internet via a cable or phone modem. 

Individuals are charged for the right to connect to the WWW through their 

ISP. Additionally, ISPs often will host web sites created by their clients for 

another fee. In fact, for each additional service, the ISP usually adds more 

fees. 

With such a large profitable network as the Internet and the WWW, it 

would be interesting to discover how the whole concept is funded. 


• Local ISP available for interview 



• Any Spreadsheet Program (e.g., MS Excel) 


• Color Pencils or Color Markers 

1. Make an appointment to interview the manager of your local ISP. 

2. Before your interview, make up a commercial company that sells a 

product in which you have a special interest. Your company wants to 

have a web site to advertise your product to the greatest number of 

buyers. 

3. Write up a description of your company and the product(s) that it sells. 

4. Draw a detailed illustration of your web site, including special features 

that the site has to offer (Java applets, video, sound, and CGI scripts). 

Your goal is to outline how your local ISP structures their rates to their 

customers and what fees the ISP must pay and to whom. Make sure 

you get the fees for special services such as web site hosting, including 

fees for special features you desire in your web site. 

5. Create a spreadsheet using your ISP’s rate structure and determine 

from the spreadsheet what your web site will cost to operate per month. 

Follow up your interview with a thank you note. 

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Thorough description of the student’s company 20 

Detailed illustration of the web site suitable for 

reproduction 

Accurate spreadsheet and analysis of the cost of 

the company’s web site 

40 

40 

TOTAL 100 

Summary 

In this unit, you learned the following: 

• The Border Gateway Protocol. 

• How to diagram a BGP internetwork. 

• The differences between inter-autonomous routing, intra-autonomous 

routing, and pass-through routing. 

• The comparison between BGP and RIP. 

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Review Questions 

Name_______________ 


Part A 

1. Describe the Border Gateway Protocol and identify what types of 

networks use this protocol. 

Part B 

1. Diagram a BGP internetwork, including labels. 

Part C 

Define each of the terms. 

Terms 

1. Inter-autonomous 

Routing 

Definitions 

2. Intra-autonomous 

Routing 

3. Pass –Through Routing 

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Part D 

1. Compare the Border Gateway and Routing Information Protocols. 

Scoring 



Part A: Describe the Border Gateway Protocol 30 

Part B: Diagram a BGP internetwork 25 

Part C: Identify the differences between interautonomous 

routing, intra-autonomous 

routing, and pass-through routing 

20 

Part D: Compare BGP and RIP 25 

TOTAL 100 

Try It Out 100 

Stretch Yourself 100 

Network Wizards 100 

FINAL TOTAL 400 

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Resources 

Comer, D. E. (1995). Internetworking With TCP/IP : Principles, Protocols, 

and Architecture 3rd edition, Prentice Hall, Upper Saddle River, New 

Jersey. 

Perlman, R. (1992). Interconnections: Bridges and Routers, Addison- 

Wesley Publishing Co.,Reading, MA. 

Stevens, W. R.(1994). TCP/IP Illustrated, Volume 1: The Protocols, 

Addison-Wesley Publishing Co., Reading, MA. 

Tannenbaum, A.S. (1996). Computer Networks, Prentice Hall, Upper 

Saddle River, New Jersey. 

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Border Gate Protocol

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