How to configure BGP AS Prepending

BGP AS (Autonomous System) prepending is a technique used to influence the inbound traffic by making a particular Autonomous System path less attractive. This is achieved by adding the AS number multiple times to the AS path attribute of BGP updates sent to a neighbor. The idea is to make the AS path longer, and therefore less preferred, in order to influence incoming traffic.

Here’s a general guide on how to configure BGP AS prepending:

Cisco IOS Command-Line Configuration:

1. Access Global Configuration Mode: Enter the global configuration mode on the router:
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   router(config)# router bgp <your-AS-number>

2. Configure AS Prepending: Specify the AS path for the neighbor you want to influence. The route-map is used to apply the AS path prepend configuration.
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   router(config-router)# neighbor <neighbor-IP-address> route-map <route-map-name> out

3. Create a Route Map: Define a route map that includes the set as-path prepend command. The following example prepends the local AS number three times:
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   router(config)# route-map PREPEND-AS permit 10
router(config-route-map)# match ip address <access-list-number>
router(config-route-map)# set as-path prepend <your-AS-number> <your-AS-number> <your-AS-number>

4. Apply Access List: Create an access list to match the specific routes you want to influence:
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   router(config)# ip access-list standard <access-list-number>
router(config-std-nacl)# permit <source-IP-prefix>


   Or, you can use a prefix list for more granularity:

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   router(config)# ip prefix-list <prefix-list-name> seq 10 permit <source-IP-prefix/mask>


   Ensure that the match ip address or match ip prefix-list in the route map refers to the correct access list or prefix list.

5. Verify Configuration: Verify that the AS path has been prepended for the specific routes:
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   show ip bgp neighbor <neighbor-IP-address> advertised-routes


   Check the AS path for the routes you’ve prepended.

6. Save Configuration: Save the configuration changes:
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   write memory


Example Configuration:

Here’s an example configuration:

In this example, the AS path for routes matching access list 1 will be prepended three times before being sent to the neighbor with IP address 192.168.1.1.

Adjust the configuration according to your specific network topology, IP addressing, and BGP requirements. Always consider the impact of changes on your overall network routing before applying them.