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Understanding Prepending AS Numbers to BGP AS Paths

You can prepend one or more autonomous system (AS) numbers at the beginning of an AS path. The AS numbers are added at the beginning of the path after the actual AS number from which the route originates has been added to the path. Prepending an AS path makes a shorter AS path look longer and therefore less preferable to BGP.

The BGP best path algorithm determines how the best path to an autonomous system (AS) is selected. The AS path length determines the best path when all of the following conditions are met:

  • There are multiple potential routes to an AS.

  • BGP has the lowest preference value (sometimes referred to as administrative distance) of the available routes.

  • The local preferences of the available routes are equal.

When these conditions are met, the AS path length is used as the tie breaker in the best path algorithm. When two or more routes exist to reach a particular prefix, BGP prefers the route with the shortest AS Path length.

If you are an enterprise that has multihoming to one or more service providers, you might prefer that incoming traffic take a particular path to reach your network. Perhaps you have two connections, but one costs less than the other. Or you might have one fast connection and another, much slower connection that you only want to use as a backup if your primary connection is down. AS path prepending is an easy method that you can use to influence inbound routing to your AS.

In Junos OS Release 9.1 and later, you can specify 4-byte AS numbers as defined in RFC 4893, BGP Support for Four-octet AS Number Space, as well as the 2-byte AS numbers that are supported in earlier releases of the Junos OS. In plain-number format, you can configure a value in the range from 1 through 4,294,967,295.

If you have a router that does not support 4-byte AS numbers in the AS path, the prependend AS number displayed in the AS path is the AS_TRANS number, AS 23456. To display the route details, use the show route command.