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AS Path Prepending: Controlling Inbound Traffic in BGP

AS Path Prepending is a BGP feature used to make a specific path appear less preferred by artificially lengthening the AS path. This is done by adding your AS number multiple times to the AS path. It is a common method to influence inbound traffic from external networks.

  • Longer AS Path = Less preferred route.

Example Scenario:

You have two ISPs: ISP1 (through CE1) and ISP2 (through CE2). You want inbound traffic from the internet to prefer ISP1 over ISP2.

Network Topology:

  • CE1 (connected to ISP1): 10.0.1.1/30
  • CE2 (connected to ISP2): 10.0.2.1/30
  • iBGP Router (Internal) connected to both CE1 (10.0.1.2/30) and CE2 (10.0.2.2/30).

Configuration on CE2 (AS Path Prepending to Make ISP2 Less Preferred):

Create a route map to prepend your AS path multiple times for CE2:


route-map PREPEND_AS permit 10
 set as-path prepend 65001 65001 65001

Apply this route map to the neighbor in the BGP configuration for CE2:


router bgp 65001
 neighbor 10.0.2.1 remote-as 65002
 neighbor 10.0.2.1 route-map PREPEND_AS out

Configuration on CE1 (No AS Path Prepending):

No AS Path Prepending is needed for CE1:


router bgp 65001
 neighbor 10.0.1.1 remote-as 65000

Result:

Inbound traffic will prefer ISP1 (CE1) because CE2 has a longer AS path due to AS Path Prepending.

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