Another newbie question: Creating BGP prefixes in a filter

Hi guys, (status update: no solution for the OSPFv3 <--> IOS XR yet, no idea where to look, both sides pretend to be happy but are not) So.... I need to create a prefix to be BGP-advertised to peers and transits; this would look like: if (net = xxx/32) { remove it and replace by yyy/zz; } (more or less standard prefix aggregation on the border) I could not find a hint of documentation in regard of doing a thing like this, where should I look? (We are currently forced to configure the wrong netmasks on loopback addresses, so BIRD has something to export) Yours, Elmar.