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Understanding Adding AS Numbers to BGP AS Paths

 

You can expand or add one or more AS numbers to an AS sequence. The AS numbers are added before the local AS number has been added to the path. Expanding an AS path makes a shorter AS path look longer and therefore less preferable to BGP. The last AS number in the existing path is extracted and prepended n times, where n is a number from 1 through 32. This is similar to the AS path prepend action, except that the AS path expand action adds an arbitrary sequence of AS numbers.

For example, from AS 1 there are two equal paths (through AS 2 and AS 3) to reach AS 4. You might want packets from certain sources to use the path through AS 2. Therefore, you must make the path through AS 3 less preferable so that BGP chooses the path through AS 2. In AS 1, you can expand multiple AS numbers.

For routes from AS 2, this makes the route look like 1 2 2 2 2 2 when advertised, where 1 is from AS 1, the 2 from AS 2 is prepended four times, and the final 2 is the original 2 received from the neighbor router.