Configuring UDP broadcast and IP helper parameters

Some applications rely on client requests sent as limited IP broadcasts addressed to the UDP application port. If a server for the application receives such a broadcast, the server can reply to the client. Routers do not forward subnet directed broadcasts, so the client and server must be on the same network for the broadcast to reach the server. If the client and server are on different networks (on opposite sides of a router), the client request cannot reach the server.

You can configure the Layer 3 switch to forward clients‘ requests to UDP application servers. To do so:

  • Enable forwarding support for the UDP application port, if forwarding support is not already enabled.
  • Configure a helper adders on the interface connected to the clients. Specify the helper address to be the IP address of the application server or the subnet directed broadcast address for the IP subnet the server is in. A helper address is associated with a specific interface and applies only to client requests received on that interface. The Layer 3 switch forwards client requests for any of the application ports the Layer 3 switch is enabled to forward to the helper address.

Forwarding support for the following application ports is enabled by default:

  • dns (port 53)
  • tftp (port 69)
  • time (port 37)
  • tacacs (port 65)
The application names are the names for these applications that the Layer 3 switch software recognizes, and might not match the names for these applications on some third-party devices. The numbers listed in parentheses are the UDP port numbers for the applications. The numbers come from RFC 1340.
Forwarding support for BootP/DHCP is enabled by default.

You can enable forwarding for other applications by specifying the application port number.

You also can disable forwarding for an application.

If you disable forwarding for a UDP application, forwarding of client requests received as broadcasts to helper addresses is disabled. Disabling forwarding of an application does not disable other support for the application. For example, if you disable forwarding of Telnet requests to helper addresses, other Telnet support on the Layer 3 switch is not also disabled.