BGP Prefix-based outbound route filtering

Every now and then you find a command that makes you think now someone is really thinking about this stuff.

BGP prefix-based outbound route filtering is one of those. If you only want to keep some of the routes from your neighbor this capability allows you to advertise the prefix list you are using to that neighbor. The neighbor then uses the prefix-list to filter the routes BEFORE it advertises them to you. This can significantly reduce the bandwidth and time taken to synchronise routes between two neighbors. The critical command is:
neighbor address capability prefixlist-orf [send | receive | both]

Once you have enterred that command (in the receive or both directions) if you enable a prefix-list to filter incoming routes your router will send the prefix list to your neighbor so that it can filter the routes for you.

The command has been around for a while (I stumbled on it when I was looking for something else and it was available in 12.2T). Some documentation is available in BGP Prefix-Based Outbound Route Filtering 12.2T