ip helper-address
17-12 IP Configuration
Example
ThisexampleshowshowtoenableIPdirectedbroadcastsonVLAN1:
B3(su)->router(Config)#interface vlan 1
B3(su)->router(Config-if(Vlan 1))#ip directed-broadcast
ip helper-address
UsethiscommandtoenabletheDHCP/BOOTPrelayagentonaSecureStackB3routedinterface.
EnablingtherelayagentallowsforwardingofclientDHCP/BOOTPrequeststoaDHCP/BOOTP
serverthatdoesnotresideonthesamebroadcastdomainastheclient.Upto6IPhelperaddresses
maybeconfiguredper
interface.
ThenoformofthiscommanddisablestheforwardingofUDPdatagramstothespecifiedaddress.
Syntax
ip helper-address address
no ip helper-address address
Parameters
Defaults
None.
Mode
Interfaceconfiguration:B3(su)‐>Router1(Config‐if(Vlan1))#
Usage
WhenahostrequestsanIPaddress,itsendsoutaDHCPbroadcastpacket.Normally,the router
dropsallbroadcastpackets.However,byexecutingthiscommand,youenabletherouted
interfacetopassDHCPbroadcastframesthrough,sendingthemdirectlytotheremoteDHCP
server’sIPaddress.
TheDHCP/BOOTPrelayagent
willdetectDHCP/BOOTPrequestsbasedonUDPsourceand
destinationports.Itwillthenmakethenecessarychangestothepacketandsendthepackettothe
DHCPserver.Thechangesinclude:
•ReplacingthedestinationIPaddresswiththeaddressoftheDHCPserver,
•ReplacingthesourceIPaddresswithits
ownaddress(thatis,theIPaddressofthelocal
routedinterface),and
•WithintheBOOTPpartofthepacket,changingtheRelayAgentIPaddressfrom0.0.0.0tothe
addressofthelocalroutedinterface.
ThelastchangetotheBootPpacket“tells”theDHCPserverthatitneedsto
assignanIPaddress
thatisinthesamesubnetastheRelayAgentIP.Whentheresponsecomesfromtheserver,the
DHCP/BOOTPrelayagentsendsittothehost.
address AddressofthehostwhereUDPbroadcastpacketsshouldbeforwarded.
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