Enterasys b5

Enterasys B5 CLI Reference 17-1
17
DHCP Snooping and
Dynamic ARP Inspection
Thischapterdescribestwosecurityfeatures:
•DHCPsnooping,whichmonitorsDHCPmessagesbetweenaDHCPclientandDHCPserver
tofilterharmfulDHCPmessagesandtobuildadatabaseofauthorizedaddressbindings
• DynamicARPinspection,whichusesthebindingsdatabasecreatedbytheDHCPsnooping
featuretorejectinvalidand
maliciousARPpackets
DHCP Snooping Overview
DHCPsnoopingmonitorsDHCPmessagesbetweenDHCPclientsandDHCPserverstofilter
harmfulDHCPmessagesandtobuildabindingsdatabaseof{MACaddress,IPaddress,VLAN
ID,port}tuplesthatareconsideredauthorized.
DHCPsnoopingisdisabledgloballyandonallVLANsbydefault.Portsareuntrustedbydefault.

DHCPsnoopingmustbeenabledgloballyandonspecificVLANs.PortswithintheVLANsmust
beconfiguredastrustedoruntrusted.DHCPserversmustbereachedthroughtrustedports.
DHCPsnoopingenforcesthefollowingsecurityrules:
•DHCPpacketsfromaDHCPserver(DHCPOFFER,DHCPACK,DHCPNAK)aredroppedif
receivedonanuntrustedport.
•DHCPRELEASEandDHCPDECLINEmessagesaredroppediftheyareforaMACaddress
inthesnoopingdatabasebutthebindingʹsinterfaceinthedatabaseisdifferentfromthe
interfacewherethemessagewasreceived.
•Onuntrustedinterfaces,theswitchdropsDHCPpacketswhosesource
MACaddressdoesnot
matchtheclienthardwareaddress.Thisfeatureisaconfigurableoption.
DHCP Message Processing
ThehardwareidentifiesallincomingDHCPpacketsonportswhereDHCPsnoopingisenabled.
Onuntrustedports,thehardwaretrapsallincomingDHCPpacketstotheCPU.Ontrustedports,
For information about... Refer to page...
DHCP Snooping Overview 17-1
DHCP Snooping Commands 17-4
Dynamic ARP Inspection Overview 17-16
Dynamic ARP Inspection Commands 17-20