ASPrependFilter is a BGPRouteFilter that prepends an AS to the AS path. This is typically used when sending a route to an EBGP peer to add our own AS number to the path.
This works on both regular EBGP and CONFEDERATION EBGP sessions
| ASPrependFilter (const AsNum &as_num, bool is_confederation_peer)
|bool filter (InternalMessage<A>& rtmsg)
Reimplemented from BGPRouteFilter.