Cisco IOS Cookbook, 2nd Edition

Limiting the Impact of IP Helper Addresses

Problem

After configuring your router to use IP helper addresses, you suffer from high link utilization or high CPU utilization on the DHCP server.

Solution

The ip helper-address command implicitly enables forwarding several different kinds of UDP broadcasts. You can prevent the router from forwarding the unwanted types of broadcasts with the no ip forward-protocol udp configuration command:

Router1#configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#no ip forward-protocol udp tftp
Router1(config)#no ip forward-protocol udp nameserver
Router1(config)#no ip forward-protocol udp domain
Router1(config)#no ip forward-protocol udp time
Router1(config)#no ip forward-protocol udp netbios-ns
Router1(config)#no ip forward-protocol udp netbios-dgm
Router1(config)#no ip forward-protocol udp tacacs
Router1(config)#end
Router1#

Discussion

As mentioned in Recipe 20.1, if the DHCP client and server are on different network segments, the router on the client’s segment must be configured with a helper address for DHCP to work. However, the helper address configuration forwards a variety of different UDP broadcasts, not just DHCP packets. This can cause network loading problems, as well as CPU loading problems on the DHCP server.

By default, when you configure the ip helper-address command on an interface, the router will automatically forward UDP broadcasts for all of the protocols shown in Table 20-1.

Table 20-1. Default ...