Specifications

How the MSS DHCP Server Works
E-2 DHCP Server
How the MSS DHCP Server Works
WhenMSSreceivesaDHCPDiscoverpacket,theDHCPserverallocatesanaddressfromthe
configuredrangeaccordingtoRFC2131andARPstheaddresstoensurethatitisnotalready in
use.Iftheaddressisinuse,theserverallocates thenextaddressintherange,andARPs
again.The
processcontinuesuntilMSSfindsanaddressthatisnotinuse.MSSthenofferstheaddresstothe
DistributedAPorclientthatsenttheDHCPDiscover.Iftherearenounusedaddressesleftinthe
range,MSSignorestheDHCPDiscoverandgeneratesalogmessage.
IftheclientdoesnotrespondtotheDHCPOfferfromtheMSSDHCPserverwithin2minutes,the
offerbecomesinvalidandMSSreturnstheaddresstothepool.
ThesiaddrvalueintheDHCPexchangesistheIPaddressoftheVLAN.Theyiaddrvalueisan
unusedaddress
withintherangetheserverisallowedtouse.
InadditiontoanIPaddress,theOffermessagefromtheMSSDHCPserveralsocontainsthe
followingoptions:
•Option54—ServerIdentifier,whichhasthesamevalueassiaddr.
•Option51—AddressLease,whichis12hoursand cannotbeconfigured.
•Option1—SubnetMaskof
theVLAN’sIPinterface.
•Option15—DomainName.Ifthisoptionisnotsetwiththesetinterfacedhcp‐server
command’sdns‐domainoption,theMSSDHCPserverusesthevaluesetbythesetipdns
domaincommand.
•Option3—DefaultRouter.Ifthisoptionisnotsetwiththesetinterface
dhcp‐server
command’sdefault‐routeroption,theMSSDHCPservercanusethevaluesetbythesetip
routecommand.Adefaultrouteconfiguredbysetiproutecanbeusediftherouteisinthe
DHCPclient’ssubnet.Otherwise,theMSSDHCPserverdoesnotspecifyarouter
address.
•Option6—DomainNameServers.Iftheseoptionsarenotsetwiththesetinterfacedhcp‐
servercommand’sprimary‐dnsandsecondary‐dnsoptions,theMSSDHCPserverusesthe
valuessetbythe setipdnsservercommand.
Configuring the DHCP Server
YoucanconfiguretheDHCPserveronanindividualVLANbasis.Toconfiguretheserver,usethe
followingcommand:
set interface vlan-id ip dhcp-server [enable | disable] [start ip-addr1 stop ip-
addr2] [dns-domain domain-name] [primary-dns ip-addr [secondary-dns ip-addr]]
[default-router ip-addr]
Thevlan‐idcanbetheVLANnameornumber.
Thestartip‐addr1andstopip‐addr2optionsspecifythebeginningandendingaddressesofthe
addressrange(alsocalledtheaddresspool).Bydefault,alladdressesexceptthehostaddressofthe
VLAN,thenetworkbroadcastaddress,and
thesubnetbroadcastaddressareincludedinthe
range.Ifyouspecifytherange,thestartaddressmustbelowerthanthestopaddress,andall
addressesmustbeinthesamesubnet.TheIPinterfaceoftheVLANmustbewithinthesame
subnetbutisnotrequiredtobe
withintherange.
(Forinformationabouttheotheroptions,seethe.)