The TCP/IP Guide - BGP Path Attributes and Algorithm Overview

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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.