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.
If you are configuring both as-path-expand and
as-path-prepend policy actions in a routing policy, ensure that
you configure as-path-expand before configuring
as-path-prepend to avoid the misplacement of the AS numbers,
which can result in an incorrect AS path calculation.
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.
[edit]
policy-options {
policy-statement as-path-expand {
term expand {
from {
route-filter 192.168.0.0/16 orlonger;
route-filter 172.16.0.0/12 orlonger;
route-filter 10.0.0.0/8 orlonger;
}
then as-path-expand last-as count 4;
}
}
}
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.