Re: Preventing transit traffic using regular expressions
There is a very simple AS path list which will accomplish what you want:
ip as-path access-list 1 permit ^$
Use this AS path list to filter outbound to the external peers. It will permit only prefixes which originate in your AS and will not permit any transit traffic.
The logic behind it is this:
^$ permits only an empty AS path (has no AS in the path). At the point where the filter is applied your advertisements do not yet have your AS in them, so your prefixes are empty and any prefix received from any external peer will have some content in the AS path. So this AS path list is the easy way to be sure that you do not become transit.
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