Some network applications, such as the NetBIOS name service, rely on a User Datagram Protocol (UDP) broadcast to request a service or locate a server for an application. If a host is on a network, subnet segment, or VLAN that does not include a server for the service, UDP broadcasts are by default not forwarded to the server located on a different network segment or VLAN. You can resolve this problem by forwarding the broadcasts to the server through physical or virtual router interfaces.
UDP broadcast forwarding is a general mechanism for selectively forwarding limited UDP broadcasts received on an IP interface out to other router IP interfaces as a rebroadcast or to a configured IP address. If the address is that of a server, the packet is sent as a unicast packet to this address. If the address is that of an interface on the router, the frame is rebroadcast.
After a UDP broadcast is received on a router interface, it must meet the following criteria to be eligible for forwarding:
It must be a MAC-level broadcast.
It must be an IP limited broadcast.
It must be for the specified UDP protocol.
It must have a time-to-live (TTL) value of at least 2.
For each ingress interface and protocol, the policy specifies how the UDP broadcast is retransmitted: to a unicast host address or to a broadcast address.