Refer to the exhibit. An engineer is designing the network for a multihomed customer running in AS 111 AS 111 does not have any other ASs connected to it.
Which technology is more comprehensive to use in the design to make sure that the AS is not being used as a transit AS?
To ensure that the AS does not act as a transit AS, it is important to prevent routes that pass through AS 111 from being propagated to other ASes. The most comprehensive way to accomplish this is by including an AS path access list that only permits routes with AS 111 in the AS path field. This will ensure that any route advertised to neighboring ASes has AS 111 as the origin and not as a transit route. This approach is more thorough than the other options provided.
outgoing towards peer AS -> permit ^$ (empty as-path)
I do think B is correct as A still will propagate route where learnt from other BGP AS, it could potentially result AS111 as transit AS.
If the route is learnt from other AS, it should have other AS no. in the AS path, i.e. not only AS111. Hence answer A is correct.
A is correct answer. Can be achieved in few ways. Filter-list with AS PATH access-list. No-Export Community Prefix-list Filtering Distribute-list Filtering