R2511-HP MSR Router Series Layer 3 - IP Services Configuration Guide(V5)

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Configuring UDP helper
Overview
The UDP helper function supports two modes:
• Broadcast UDP helper—Relay specified UDP broadcast packets.
• Multicast UDP helper—Relay specified UDP multicast packets.
Broadcast UDP helper
Sometimes, a host needs to forward broadcasts to obtain network configuration information or request
the names of other devices on the network. However, if the server or the device to be requested is located
in another broadcast domain, the host cannot obtain such information through broadcast.
To solve this problem, the device provides the broadcast UDP helper function to relay specified UDP
packets. In other words, UDP helper functions as a relay agent that converts UDP broadcast packets into
unicast packets and forwards them to a specified destination server.
With broadcast UDP helper enabled, if the destination port number of the received UDP broadcast
packet matches the one pre-configured on the device, the device makes a copy of the packet, modifies
the destination IP address in the IP header, and then sends the packet to the specified destination server.
Multicast UDP helper
In some networks, the intermediate device multicasts packets and the edge device broadcasts packets.
On the last hop of the multicast configure a mapping between a multicast address and a subnet
broadcast address so that a multicast packet can passed through the intermediate devices.
With multicast UDP helper enabled, if the destination port number of the received UDP multicast packet
matches the one pre-configured on the device, the device looks for the mapping between multicast
address and subnet broadcast address. If the mapping can be found, the device makes a copy of the
packet, modifies the destination IP address in the IP header to the subnet broadcast address, and then
sends the packet.
Configuring broadcast UDP helper
When you configure broadcast UDP helper, follow these guidelines:
• The broadcast UDP helper enabled device must not forward DHCP broadcast packets that use
destination port 67 or 68. Therefore, the UDP port numbers set with the udp-helper port command
must not include 67 or 68.
• By default, some devices that support directed broadcasts suppression reject all directional
broadcast packets. Therefore, execute the ip forward-broadcast command and then enable UDP
helper. For more information about the directed broadcasts suppression function, see "Configuring
IP performance optimization."
• You can specify up to 256 UDP ports for UDP helper.