Overview of UDP Helper

Background

Hosts on a network may need to obtain the network configuration or resolve host names by sending UDP broadcast packets to the server. If the hosts and server are located in different broadcast domains, broadcast packets cannot reach the server and the hosts cannot obtain the required information from the server.

The router provides the UDP helper function to solve this problem. UDP helper can relay the UDP broadcast packets with specified destination ports. It converts the broadcast packets into unicast packets and sends the unicast packets to the specified destination servers.

As shown in Figure 6-1, HostA uses a host name to access HostB, and the NetBIOS Name Service (NetBIOS-NS) server resolves the host name of HostB. The NetBIOS-NS server and HostA are in different broadcast domains, so the UDP broadcast packet with destination port UDP 137 sent by HostA cannot reach the NetBIOS-NS server. After UDP helper is enabled on the Router, the Router can forward the packet with destination port UDP 137 to the NetBIOS-NS server through unicast so that the NetBIOS-NS server can resolve the host name of HostB.

Figure 6-1 UDP helper relays broadcast packets

Packets Forwarded by UDP Helper

UDP Helper Ports

After UDP helper is enabled on the router, the router relays the UDP packets with six specified destination ports by default. Manual configuration is required if the router needs to relay the UDP packets with other destination ports. In addition to the six default ports, another 10 destination ports can be specified.

UDP helper does not relay DHCP packets. That is, the destination port number cannot be 67 or 68. To relay DHCP packets, enable the DHCP relay function on the router. For details about DHCP relay, see Configuring a DHCP Relay Agent.