On start-up, each host has a copy of the graph of the Myrinet topology. The Routing Service on the HECs and SECs then determines the host address of each SEC to which control messages will be sent. From the graph of the Myrinet, each SEC determines its immediate neighbors and the host addresses of the corresponding machines acting as controllers. For a neighboring switch, these are the hosts connected to port 0 of the switches. The Routing Service also provides functions for determining the host connected to every outgoing port on a switch, determining if that host is running an HEC/SEC and other similar functions.