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.