Specifications

ip helper-address
15-18 IP Configuration
ip helper-address
UsethiscommandtoenableDHCP/BOOTPrelayandtheforwardingoflocalUDPbroadcasts
specifyinganewdestinationaddressonaroutinginterface.Upto3IPhelperaddressesmaybe
configuredperinterface.
Thenoformofthiscomm anddisablestheforwardingofUDPdatagramstothespecifiedaddress.
Syntax
ip helper-address address
no ip helper-address address
Parameters
Defaults
None.
Mode
Interfaceconfiguration:C3(su)‐>Router1(Config‐if(Vlan1))#
Usage
About DHCP/BOOTP Relay
DHCP/BOOTPrelayfunctionalityisappliedwiththehelpofIPbroadcastforwarding.Atypical
situationoccurswhenahostrequestsanIPaddresswithnoDHCPserverlocat e donthatsegment.
AroutercanforwardtheDHCPrequesttoaserverlocatedonanothernetworkiftheaddressof
theDHCP
serverisconfiguredasahelperaddressonthereceivinginterfaceoftherouter
forwardingtherequest,usingthiscommand(iphelper‐address).
TheDHCP/BOOTPrelayfunctionwilldetecttheDHCPrequestandmakethenecessarychanges
totheheader,replacingthedestinationaddresswiththeaddressofthe
server,andthesourcewith
itsownaddress,andsendittotheserver.Whentheresponsecomesfromtheserver,theDHCP/
BOOTPrelayfunctionsendsittothehost.
Example
ThisexampleshowshowtoforwardDHCP/BOOTPbroadcaststoaDHCP/BOOTPserverwithan
IPaddressof191.168.1.23onVLAN1:
C3(su)->router(Config)#interface vlan 1
C3(su)->router(Config-if(Vlan 1))#ip helper-address 192.168.1.28
address AddressofthehostwhereUDPbroadcastpacketsshouldbeforwarded.