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Internet Routing Architectures, Second Edition

behavior will eventually modify the traffic trajectories. The next chapter provides a hands-on approach to understanding the basics of setting routing policies with BGP.

Frequently Asked Questions


Does BGP send periodic updates like RIP?


No. BGP exchanges routing information once, when the BGP session is being established. After that, only network changes are exchanged between BGP peers.


Does the BGP session become "established" after all the routing updates have been exchanged between BGP neighbors?


No. It is the other way around. No routing exchange can take effect until both BGP neighbors agree on all parameters and the session becomes established.


Is the Network Layer Reachability Information (NLRI) the actual BGP routing update?


No. The NLRI is one of the elements that is carried in a BGP UPDATE message. Other elements are the attributes and the unreachable networks.


You talk about authentication as an example of the BGP optional parameters. How important is authentication?


Authentication is a means to validate the BGP peer. This is to prevent hackers from assuming the identity of one of your peers and feeding you wrong routing information. With authentication, both peers validate the connection via password mechanisms.


Where does BGP carry information about AS numbers?

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AS numbers are listed as part of the AS_PATH attribute carried in the UPDATE message.


Is BGP connection symmetrical, or does it utilize a master/slave relationship?


The BGP protocol has no master and slave roles. At the transport layer, the connection is always initiated by one side and appears as a client (with the source TCP port number greater than 2048) that connects to a server (port 179), but it does not have any influence at the protocol level.


The link to my provider has a firewall. What must be done in order for BGP to work?


The firewall must be configured to allow a TCP connection to port 179 in at least one direction (from the provider to you, or from you to the provider). Use caution, because some providers use passive BGP mode (their router does not attempt to establish the BGP connection).


1.RFC 1997, "BGP Communities Attribute," http://www.isi.edu/in-notes/rfc1997.txt

2.RFC 1966, "BGP Route Reflection: An alternative to full mesh IBGP," http://www.isi.edu/innotes/rfc1966.txt

3.RFC 1863, "A BGP/IDRP Route Server alternative to a full mesh routing," http://www.isi.edu/in-notes/rfc1863.txt

4.RFC 2283, "Multiprotocol Extensions for BGP-4," http://www.isi.edu/innotes/rfc2283.txt

5.IETF Inter-Domain Routing Working Group, http://www.ietf.org/html.charters/idrcharter.html

6.RFC 1700, "Assigned Numbers," http://www.isi.edu/in-notes/rfc1700.txt

7.Williamson, Beau. Developing IP Multicast Networks (Indianapolis, Ind.: Cisco Press, 1999)

8.RFC 2385, "Protection of BGP Sessions via the TCP MD5 Signature Option," http://www.isi.edu/in-notes/rfc2385.txt

9.RFC 1321, "The MD5 Message-Digest Algorithm," http://www.isi.edu/innotes/rfc1321.txt

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Part III: Effective Internet Routing Designs

You are now in a position to begin applying the attributes and functionality of BGP to practical routing problems. Chapter 6 begins this process by examining BGP's attribute manipulation techniques and the use of route filtering in influencing the BGP decision process. Chapter 7 introduces three fundamental design criteria—redundancy, symmetry, and load balancing—that network architects frequently must implement and balance in developing their routing policies. Chapter 8 considers how to integrate BGP with interior protocols, and Chapter 9 considers how to tap BGP's potential for managing large and growing networks. Chapter 10 takes up the problem of network stability, and increasingly challenging design goals in the wake of the ever-expanding Internet. BGP includes a number of built-in functions designed to help build stability. Part III takes an example-oriented approach, using specific topologies and scenarios to illustrate routing design concepts and applications.

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Chapter 6. Tuning BGP Capabilities

This chapter covers the following key topics:

Building peer sessions—

A walk-through of the negotiation process between BGP and its neighbors.

Sources of routing updates—

The source and method by which routes are injected into BGP have implications for the accuracy and stability of routing information.

Overlapping protocols: backdoors—

When alternative routes into and out of a network are offered by overlapping protocols, a method of ranking them by preference is available.

The routing process simplified—

The decision model by which BGP receives, filters, selects for usage, and advertises routes, as a continuous process.

Controlling BGP routes—

At the core of BGP is a collection of attributes that administrators can apply to control routing according to their networks' needs.

Route filtering and attribute manipulation—

An example-oriented, systematic look at how BGP permits or denies routes, applies filters, and manipulates attributes to define the set of routing updates that enter and exit an autonomous system.

BGP-4 aggregation—

Several specific scenarios involving different aggregation choices and how BGP-4 accommodates them.

Up to this point, this book has been concerned primarily with general definitions of interior and exterior gateway protocols and an overview of their respective and interconnected tasks. The Border Gateway Protocol (BGP) was also presented from the technical perspective of its functional elements. With this chapter, you will begin to consider more practical implementation details for BGP as part of the overall design problem in building reliable Internet connectivity. This chapter examines specific attributes of BGP and how they are applied individually and together to address this design problem. Although the terminology, attributes, and details of this chapter are specific to BGP, the general concepts and problems

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