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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
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Classification
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Attribute
Type Value
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Description
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Origin
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Well-Known Mandatory
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1
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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.
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AS_Path
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Well-Known
Mandatory
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2
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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.
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Next_Hop
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Well-Known Mandatory
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3
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The next-hop router to be used
to reach this destination.
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Multi_Exit_Disc
(MED)
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Optional
Non-Transitive
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4
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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.)
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Local_Pref
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Well-Known Discretionary
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5
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Used in communication between
BGP speakers in the same AS to indicate the level of preference for
a particular route.
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Atomic_Aggregate
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Well-Known
Discretionary
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6
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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.
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Aggregator
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Optional Transitive
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7
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Contains the AS number and BGP
ID of the router that performed route aggregation; used for troubleshooting.
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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.
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Version 3.0 - Version Date: September 20, 2005
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