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Enabling IP Source Address Validation
IP source address validation (SAV) prevents IP spoofing by forwarding only IP packets that have been
validated against the DHCP binding table.
A spoofed IP packet is one in which the IP source address is strategically chosen to disguise the attacker.
For example, using ARP spoofing, an attacker can assume a legitimate client’s identity and receive traffic
addressed to it. Then the attacker can spoof the client’s IP address to interact with other clients.
The DHCP binding table associates addresses the DHCP servers assign, with the port on which the
requesting client is attached. When you enable IP source address validation 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 is not on the list of permissible addresses for the
port and the packet is dropped.
To enable IP source address validation, use the following command.
NOTE: If you enable IP source guard using the ip dhcp source-address-validation
command and there are 187 entries or more in the current DHCP snooping binding table, 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.
• Enable IP source address validation.
INTERFACE mode
ip dhcp source-address-validation
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.
The system ensures that the packet’s source MAC address is checked against the CHADDR field in the
DHCP header only for packets from snooped VLANs.
• Enable DHCP MAC SAV.
CONFIGURATION mode
ip dhcp snooping verify mac-address
Enabling IP+MAC Source Address Validation
IP source address validation (SAV) validates the IP source address of an incoming packet against the
DHCP snooping binding table. IP+MAC SAV ensures that the IP source address and MAC source address
are a legitimate pair, rather than validating each attribute individually. You cannot configure IP+MAC SAV
with IP SAV.
1. Allocate at least one FP block to the ipmacacl CAM region.
CONFIGURATION mode
cam-acl l2acl
2. Save the running-config to the startup-config.
EXEC Privilege mode
copy running-config startup-config
Dynamic Host Configuration Protocol (DHCP)
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