Enterasys SecureStack C2 C2G170-24 - Ip Helper-Address

ip helper-address
19-14 IP Configuration
Examples
ThefollowingexamplegloballydisablesIPforwardingforUDPport69.
C2(su)->router(Config)#no ip forward-protocol udp 69
ThefollowingexampledisablesIPforwardingforUDPport69onaspecificinterfa ce.
C2(su)->router(Config)#interface vlan 10
C2(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C2routedinterface.
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:C2(su)‐>Router1(Config‐if(Vlan1))#
Usage
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.