Hello David
The BGP NO-ADVERTISE community is a well-known BGP community that prevents the advertisement of routes to any peer, internal or external. Using the NO-ADVERTISE community will prevent an AS from advertising specific routes to any other AS.
However, it’s important to clarify that this doesn’t directly prevent your organization from becoming a transit AS. A transit AS is an Autonomous System that allows traffic from other ASes to pass through it. Whether an AS acts as a transit AS is more a matter of its peering arrangements than of its routing advertisements.
If you want to prevent your AS from becoming a transit AS, you need to ensure that your AS doesn’t have agreements to forward traffic for other ASes. That’s where the four methods Rene mentioned in the lesson come in.
However, if you don’t want to advertise certain routes to peer ASes to reduce the chance of becoming a transit AS for those specific routes, you could use the NO-ADVERTISE community. But this is by no means a general solution for the issue, but a specific one for the particular routes that are prevented from being advertised.
I hope this has been helpful!
Laz