AS-Path Manipulation and Regular Expressions · Lesson 01 of 01
Shaping and matching the AS path
AS-path prepending steers inbound traffic; AS-path regex matches routes by the path they travelled. Both are everyday edge tools.
Written lesson
Prepend to be less attractive
You cannot command how other networks send traffic to you, but you can make one of your paths look worse. Prepending your own AS several times lengthens the AS_PATH, and since shorter paths win best-path, the prepended path loses to the shorter one. It is the standard tool for making a backup ISP link genuinely a backup for inbound traffic. Prepend outbound, toward the neighbour whose view you want to change.
Configuration
Prepend on the backup exit
route-map PREPEND permit 10
set as-path prepend 65001 65001 65001
!
router bgp 65001
address-family ipv4 unicast
neighbor 203.0.113.9 route-map PREPEND outWritten lesson
Matching with regex
AS-path access-lists match paths with regular expressions. ^$ matches the empty path — routes originated in your own AS (nothing traversed). _65002_ matches any path that passed through AS 65002. ^65002_ matches paths where 65002 is the first (nearest) AS. _65002$ matches paths that originated in 65002. These four patterns cover most real filtering: 'only my own routes', 'anything through this transit', 'directly from this peer', 'originated by this AS'.
The underscore
In AS-path regex the underscore matches any delimiter — start, end, space, comma, or brace. _100_ reliably matches AS 100 anywhere without also matching 1001 or 4100. Use it around every ASN you mean literally.