Configuration Guide User guide

FastIron Configuration Guide 1009
53-1002494-02
Configuring IP parameters – Layer 3 Switches
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:
• bootps (port 67)
• dns (port 53)
• tftp (port 69)
• time (port 37)
• netbios-ns (port 137)
• netbios-dgm (port 138)
• tacacs (port 65)
NOTE
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.
NOTE
Forwarding support for BootP/DHCP is enabled by default. If you are configuring the Layer 3 Switch
to forward BootP/DHCP requests, refer to “BootP and DHCP relay parameter configuration” on
page 1011.
You can enable forwarding for other applications by specifying the application port number.
You also can disable forwarding for an application.
NOTE
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.