Switch User Manual

ip helper-address
19-18 IP Configuration
Examples
ThefollowingexamplegloballydisablesIPforwardingforUDPport69.
C3(su)->router(Config)#no ip forward-protocol udp 69
ThefollowingexampledisablesIPforwardingforUDPport69onaspecificinterfa ce.
C3(su)->router(Config)#interface vlan 10
C3(su)->router(Config-if(Vlan 10))#no ip forward-protocol udp 69
ip helper-address
UsethiscommandtoenabletheDHCP/BOOTPrelayagentonaSecureStackC3routedinterface
and/ortoforwardbroadcasttrafficidentifiedwiththeipforward‐protocolcommandtoaunicast
address.EnablingtherelayagentallowsforwardingofclientDHCP/BOOTPrequeststoaDHCP/
BOOTPserverthatdoesnotresideon
thesamebroadcastdomainastheclient.Upto6IPhelper
addressesmaybeconfiguredperinterface.
Thenoformofthiscommand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
TypicallyforDHCP/BootP,whenahostrequestsanIPaddress,itsendsoutaDHCPbroadcast
packet.Normally,therouterdropsallbroadcastpackets.However,byexecutingthiscommand,
youenabletheroutedinterfacetopassDHCPbroadcastframesthrough,sendingthemdirectlyto
theremoteDHCPserver’sIPaddress.
The
DHCP/BOOTPrelayagentwilldetectDHCP/BOOTPrequestsbasedonUDPsourceand
destinationports.Itwillthenmakethenecessarychangestothepacketandsendthepackettothe
DHCPserver.Thechangesinclude:
•ReplacingthedestinationIPaddresswiththeaddressoftheDHCPserver,
•ReplacingthesourceIP
addresswithitsownaddress(thatis,theIPaddressofthelocal
routedinterface),and
•WithintheBOOTPpartofthepacket,changingtheRelayAgentIPaddressfrom0.0.0.0tothe
addressofthelocalroutedinterface.
ThelastchangetotheBootPpacket“tells”theDHCPserverthat
itneedstoassignanIPaddress
thatisinthesamesubnetastheRelayAgentIP.Whentheresponsecomesfromtheserver,the
DHCP/BOOTPrelayagentsendsittothehost.
address AddressofthehostwhereUDPbroadcastpacketsshouldbeforwarded.