Filter
Top Products
Other IP Commands
5-158 Configuring the Internet Protocol
DHCP Relay Functionality
TheDHCPRelayfunctionalityisappliedwiththehelpofIPbroadcastforwarding.Atypical
situation,asshowninFigure 5‐7,occurswhenaHostrequestsanIPaddresswithnoDHCPserver
locatedonthatsegment.
Router1canforwardtheDHCPrequest(1)totheserverlocatedonN2,
ifIPforward‐protocolis
enabledforUDP,andtheaddressoftheDHCPserverisconfiguredasahelperaddressonthe
receivinginterfaceofRouter1.TheDHCPRelayfunctionwilldetecttheDHCPrequestandmake
thenecessarychanges totheheader,replacingthedestinationaddresswith
theaddressofthe
server,andthesourcewithitsownaddress,andsenditfurther(2)totheserver.Whenthe
response(3)comesfromtheserver,theDHCPRelayfunctionsendsittothehost(4).
Figure 5-7 DHCP Functionality Example
ip helper-address
Thiscommandenablesforwardingoflocalbroadcastsspecifyingthenewdestinationaddress.Itis
oneoftwocommandsusedforUDPbroadcastforwarding.Alsorefertothe
ip forward-
protocol
commandwhichdefinestheforwardprotocolandportnumber.Youcanaddmore
thanonehelperaddressperinterface.ThecommandisalsousedtoenableBOOTPRelay.
Syntax
ip helper-address address
Syntax of the “no” Form
Thenoformdisablestheforwardingofbroadcastpacketstothespecifiedaddress:
no ip helper-address address
Mode
Interfaceconfiguration:XSR(config-if<xx>)#
Example
Inthisexample,withoneserveronnetwork191.168.1.255andthe otheronnetwork192.24.1.255,
youpermitUDPbroadcastsfromhostsoneithernetworksegmenttoreachbothservers:
Router 2
N2
Router 1
N1
Host
Destination
255.255.255.255
Global Configuration
ip forward-protocol UDP
interface ethernet 1
ip helper-address address1
1
eth 1
2
Source: 0.0.0.x
3
4
Server
addr1
INTERNET
DHCP
Relay
Function
address
Destinationbroadcastorhostaddressusedwhenforwarding.