BGP AS-Override Lab


Images used in lab: VIOS-ADVENTERPRISEK9-M), Version 15.9(3)M2

Topology Interfaces Only

Topology With IP Addressing

The last octet is the router number unless specified otherwise. Example: PE11's G0/0 is Access to the initial configs, EVE-NG lab file and solution is below.


If a BGP router recieves an update and sees its own AS number in the AS Path, it will not accept the prefix. This is how loop prevention works in eBGP. It works well but imagine a scenario where multiple customer sites connect to the service provider using the same AS number. When a prefix is advertised from CE1 in AS100, it goes into the provider network in a different AS and it’s then received by CE2 that also used AS100, the prefix will never be accepted into the BGP table. In this lab you’ll see this problem and fix it by using the BGP AS-Override¬†feature.


Download Lab

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Initial Configs


AS Override in is very similar to Allow-AS in. Key points to remember: "Allow-AS in" is done on the customer side. "AS Override" is done on the provider side.

Hope you enjoyed this lab. If you have questions or need any help at all please leave a comment below and I’ll get back to you.¬†


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