To build the dynamic tunnels, mGRE uses the Next Hop Resolution Protocol (NHRP) addressing service. The hub router maintains an NHRP database, acting as a route server. Spoke routers register their public IP addresses with the hub, acting as clients. The spokes query the hub database to obtain the IP addresses of the logical tunnel endpoints.
NHRP commands are described in Ciena Vyatta Network OS Services Configuration Guide.