MegaphoneYou might have a situation where you have multiple network segments utilizing DHCP to assign IP addresses, name servers, etc. DHCP clients use network broadcasts to alert a DHCP server to their presence. Once a request for an address has been made, the DHCP server responds with another broadcast. It's obvious from all these broadcasts that the protocol is designed to work with a client and server on the same network segment. What happens when you have ten segments? That's a lot of DHCP servers.

The last thing you want to do is manage a DHCP server on each of your network segments. The obvious thing to do is use the ip helper-address interface configuration command to force your Cisco router to forward DHCP requests to a central DHCP server. One often overlooked fact is that ip helper-address will actually forward many other UDP-based broadcasts to the address specified. This may be counter to what you intended.

So, how do we only forward DHCP requests? The following configuration commands can copied and pasted directly into your router's configuration CLI.

! We want this protocol.
ip forward-protocol udp bootpc
!
! We don't want these.
no ip forward-protocol udp biff           
no ip forward-protocol udp bootps         
no ip forward-protocol udp discard        
no ip forward-protocol udp dnsix          
no ip forward-protocol udp domain         
no ip forward-protocol udp echo           
no ip forward-protocol udp isakmp         
no ip forward-protocol udp mobile-ip      
no ip forward-protocol udp nameserver     
no ip forward-protocol udp netbios-dgm    
no ip forward-protocol udp netbios-ns     
no ip forward-protocol udp netbios-ss     
no ip forward-protocol udp non500-isakmp  
no ip forward-protocol udp ntp            
no ip forward-protocol udp pim-auto-rp    
no ip forward-protocol udp rip            
no ip forward-protocol udp snmp           
no ip forward-protocol udp snmptrap       
no ip forward-protocol udp sunrpc         
no ip forward-protocol udp syslog         
no ip forward-protocol udp tacacs         
no ip forward-protocol udp talk           
no ip forward-protocol udp tftp           
no ip forward-protocol udp time           
no ip forward-protocol udp who            
no ip forward-protocol udp xdmcp

One thing you'll probably notice when you try this is that not all of the protocols are supported on your device. If you try to stop an unsupported protocol, you'll get a response like this:

SLAP(config)#no ip forward-protocol udp biff           
UDP port 512 not found to delete

That's OK. The list above is the complete list of IOS supported protocols, so your mileage may vary. What matters most is turning on bootpc and turning off everything else. And that's how we keep ip helper-address from being too forward!

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