TIL: as-path prepending

Today I learned: You can prepend any AS numbers in the prepended string.

They typical method of as-path prepending is to prepend or add your autonomous system number to the AS_PATH attribute to influence inbound traffic patterns.

You can technically add any autonomous system to the AS_PATH even AS's that don't belong to you.

NOTE: This is frowned upon in production. "Just because you can doesn't mean you should!"

See the example below:

Without context or a topology this seems a little bland but the results are there. You can see from the BGP table below we have prepended a bunch of AS's that do not belong to us.

Prepeding configured out-bound from R3 --> R1:

R3#sho run | s as-path|route-map|router bgp
router bgp 200

 neighbor 155.1.13.1 remote-as 100
 neighbor 155.1.13.1 route-map AS_254 out

ip as-path access-list 254 permit ^254$

route-map AS_254 permit 10
 match as-path 254
 set as-path prepend 254 250 123

route-map AS_254 permit 20

Showing the R1 partial BGP table:

R1#sho ip bgp neighbors 155.1.13.3 routes

[ ... OUTPUT OMITTED ... ]

     Network          Next Hop            Metric LocPrf Weight Path
 *>  28.119.16.0/24   155.1.13.3                             0 200 54 i
 *>  28.119.17.0/24   155.1.13.3                             0 200 54 i
 *   51.51.51.51/32   155.1.13.3                             0 200 254 250 123 254 ?
 *   205.90.31.0      155.1.13.3                             0 200 254 250 123 254 ?
 *   220.20.3.0       155.1.13.3                             0 200 254 250 123 254 ?
 *   222.22.2.0       155.1.13.3                             0 200 254 250 123 254 ?

Credit: This was influenced by a lab from the INE workbook.