You can't send local preference information outside of your AS. That attribute is only used within an AS to determine the preferred exit point. That being said, you can often influence the local preference of neighboring AS by using community values. By passing a specific community to a neighbor AS, you can effect how traffic to your AS will be handled. Check with ISP1 about community values you can pass to them that will influence their local preference. If they have this option, then you can announce your prefix out both ISP1 links and add community values to each that will influence the local preference within their AS to prefer link A. That should keep any traffic from going over link B unless there is a failure on link A. If you want to prefer ISP1a over ISP2 for inbound traffic, the typical way of influencing that decision is by prepending your AS to the announcement of your prefix to ISP2. This will make your ISP2 link less-desireable due to having a higher AS path count. You will also need to prepend your AS on the announcement out ISP1b, with a higher number of prepends, so that it will be less desireable than ISP2. Or you could probably use Conditional BGP advertisement. If you are receiving the default route on all links, the lack of receiving a default from both ISP1a and ISP2 will trigger the advertisement of your prefix to ISP1b.
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