bgp path metric

Hi, I have 2 routers which perform an iBGP peer. Now, every route, which they exchange should get a special metric (e.g: received route from same ASN, route metric= route metric + 100). How am I be able to influence that? I've already tried to check from which AS a route came and preprent (I am not sure if this is the correct keyword btw) it. Here's my filter settings so far: export filter { if bgp_path ~ [= ASN =] then { bgp.path.prepend (ASN,100); accept; } }; I'd be thankful for every help.