394 | Dynamic Host Configuration Protocol (DHCP)
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The DHCP binding table associates addresses assigned by the DHCP servers, with the port on which the
requesting client is attached. When IP Source Address Validation is enabled on a port, the system verifies
that the source IP address is one that is associated with the incoming port. If an attacker is impostering as a
legitimate client the source address appears on the wrong ingress port, and the system drops the packet.
Likewise, if the IP address is fake, the address will not be on the list of permissible addresses for the port,
and the packet is dropped.
DHCP MAC Source Address Validation
DHCP MAC Source Address Validation (SAV) validates a DHCP packet’s source hardware address
against the client hardware address field (CHADDR) in the payload.
FTOS Release 8.2.1.1 ensures that the packet’s source MAC address is checked against the CHADDR
field in the DHCP header only for packets from snooped VLANs.
IP+MAC Source Address Validation
The following feature is available on platforms: c, s and .
IP Source Address Validation validates the IP source address of an incoming packet against the DHCP
Snooping binding table. IP+MAC Source Address Validation ensures that the IP source address and MAC
source address are a legitimate pair, rather validating each attribute individually. IP+MAC Source Address
Validation cannot be configured with IP Source Address Validation.
Task Command Syntax Command Mode
Enable IP Source Address Validation
ip dhcp source-address-validation
INTERFACE
Note: If IP Source Guard is enabled using the ip dhcp source-address-validation command and there are
187 entries or more in the current DHCP snooping binding table, Source Address Validation (SAV) may
not be applied to all entries.
To ensure that SAV is applied correctly to all entries, enable the ip dhcp source-address-validation
command before adding entries to the binding table.
Task Command Syntax Command Mode
Enable DHCP MAC Source Address Validation.
ip dhcp snooping verify mac-address
CONFIGURATION
Step Task Command Syntax Command Mode
1 Allocate at least one FP block to the
ipmacacl CAM region.
cam-acl l2acl
CONFIGURATION
2 Save the running-config to the
startup-config.
copy running-config startup-config
EXEC Privilege
3 Reload the system.
reload
EXEC Privilege
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