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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

Previous Topic/Section
BGP Route Storage and Advertisement, and BGP Routing Information Bases (RIBs)
Previous Page
Pages in Current Topic/Section
12
3
Next Page
BGP Route Determination and the BGP Decision Process
Next Topic/Section

BGP Path Attributes and Algorithm Overview
(Page 3 of 3)

BGP Path Attribute Characteristics

Table 135 contains a summary of the characteristics of each of the most common BGP path attributes used to describe the route to a destination, as well as the Attribute Type code assigned to each in BGP Update messages:


Table 135: Summary of BGP Path Attributes

BGP Path Attribute

Classification

Attribute Type Value

Description

Origin

Well-Known Mandatory

1

Specifies the origin of the path information. This attribute indicates whether the path came originally from an interior routing protocol, the older exterior routing protocol EGP, or some other source.

AS_Path

Well-Known Mandatory

2

A list of autonomous system numbers that describes the sequence of ASes through which this route description has passed. This is a critically important attribute, since it contains the actual path of autonomous systems to the network. It is used to calculate routes and to detect routing loops.

Next_Hop

Well-Known Mandatory

3

The next-hop router to be used to reach this destination.

Multi_Exit_Disc (MED)

Optional Non-Transitive

4

When a path includes multiple exit or entry points to an AS, this value may be used as a metric to discriminate between them (that is, choose one exit or entry point over the others.)

Local_Pref

Well-Known Discretionary

5

Used in communication between BGP speakers in the same AS to indicate the level of preference for a particular route.

Atomic_Aggregate

Well-Known Discretionary

6

In certain circumstances, a BGP speaker may receive a set of overlapping routes where one is more specific than the other. For example, consider a route to the network 34.15.67.0/24 and to the network 34.15.67.0/26. The latter network is a subset of the former, which makes it more specific. If the BGP speaker uses the less-specific route (in this case, 34.15.67.0/24) it sets this path attribute to a value of 1 to indicate that this was done.

Aggregator

Optional Transitive

7

Contains the AS number and BGP ID of the router that performed route aggregation; used for troubleshooting.


Some of these path attributes are straight-forward to understand; others are fairly cryptic and probably confusing. Delving into any more detail on the path attributes leads us into a full-blown description of detailed inter-AS route calculations. We'll look at that to some degree in the next topic.

For full details on the message format used to communicate path attributes, see the format of BGP Update messages.


Previous Topic/Section
BGP Route Storage and Advertisement, and BGP Routing Information Bases (RIBs)
Previous Page
Pages in Current Topic/Section
12
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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