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SecureStack B2 Configuration Guide 18-1
18
DHCP Snooping and
Dynamic ARP Inspection
Thischapterdescribestwosecurityfeatures:
•DHCPsnooping,whichmonitorsDHCPmessagesbetweenaDHCPclientandDHCPserver
tofilterharmfulDHCPmessagesandtobuildadatabaseofauthorizedaddressbindings
DynamicARPinspection,whichusesthebindingsdatabasecreatedbytheDHCPsnooping
featuretorejectinvalidand
maliciousARPpackets
DHCP Snooping Overview
DHCPsnoopingmonitorsDHCPmessagesbetweenDHCPclientsandDHCPserverstofilter
harmfulDHCPmessagesandtobuildabindingsdatabaseof{MACaddress,IPaddress,VLAN
ID,port}tuplesthatareconsideredauthorized.
DHCPsnoopingisdisabledgloballyandonallVLANsbydefault.Portsareuntrustedbydefault.
DHCPsnoopingmustbeenabledgloballyandonspecificVLANs.PortswithintheVLANsmust
beconfiguredastrustedoruntrusted.DHCPserversmustbereachedthroughtrustedports.
DHCPsnoopingenforcesthefollowingsecurityrules:
•DHCPpacketsfromaDHCPserver(DHCPOFFER,DHCPACK,DHCPNAK)aredroppedif
receivedonanuntrustedport.
•DHCPRELEASEandDHCPDECLINEmessagesaredroppediftheyareforaMACaddress
inthesnoopingdatabasebutthebindingʹsinterfaceinthedatabaseisdifferentfromthe
interfacewherethemessagewasreceived.
•Onuntrustedinterfaces,theswitchdropsDHCPpacketswhosesource
MACaddressdoesnot
matchtheclienthardwareaddress.Thisfeatureisaconfigurableoption.
DHCP Message Processing
ThehardwareidentifiesallincomingDHCPpacketsonportswhereDHCPsnoopingisenabled.
Onuntrustedports,thehardwaretrapsallincomingDHCPpacketstotheCPU.Ontrustedports,
For information about... Refer to page...
DHCP Snooping Overview 18-1
DHCP Snooping Commands 18-4
Dynamic ARP Inspection Overview 18-16
Dynamic ARP Inspection Commands 18-20
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