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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Internet Layer (OSI Network Layer) Protocols
           9  TCP/IP Routing Protocols (Gateway Protocols)
                9  TCP/IP Exterior Gateway/Routing Protocols (BGP and EGP)
                     9  TCP/IP Border Gateway Protocol (BGP/BGP-4)
                          9  BGP Fundamentals and General Operation

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BGP Route Storage and Advertisement, and BGP Routing Information Bases (RIBs)
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BGP Route Determination and the BGP Decision Process
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BGP Path Attributes and Algorithm Overview
(Page 2 of 3)

BGP Path Attribute Classes

The four formal classifications of path attributes are:

  • Well-Known Mandatory: These are the most important path attributes; they must be included in every route description in Update messages, and must be processed by each BGP device receiving them.

  • Well-Known Discretionary: These path attributes must be recognized by a BGP device if received, but they may or may not be included in an Update message. Thus, they are optional for a sender of information, but mandatory for a receiver to process.

  • Optional Transitive: These path attributes may be recognized by a BGP router and may be included in an Update message. They must be passed on to other BGP speakers when the route is advertised, even if received by a device that does not recognize the attribute.

  • Optional Non-Transitive: Optional attributes that may be recognized by a BGP device and may be included in an Update message. If received by a device that does not recognize the attribute, it is dropped and not passed on to the next router.

Note: As you might imagine, all well-known attributes are by definition transitive—they must be passed on from one BGP speaker to the next.


Key Concept: Unlike simpler routing protocols that store only limited information about how to reach a network, such as its cost and the next hop to reach it, BGP stores detailed information about complete routes to various networks. This information takes the form of path attributes that describe various characteristics of a path (route) through the ASes that connect a router to a destination network.



Previous Topic/Section
BGP Route Storage and Advertisement, and BGP Routing Information Bases (RIBs)
Previous Page
Pages in Current Topic/Section
1
2
3
Next Page
BGP Route Determination and the BGP Decision Process
Next Topic/Section

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Version 3.0 - Version Date: September 20, 2005

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