Reference Guide

Source Address Validation
Using the DHCP binding table, the system can perform three types of source address validation (SAV).
Table 18. Three Types of Source Address Validation
Source Address Validation Description
IP Source Address Validation Prevents IP spoofing by forwarding only IP packets that have
been validated against the DHCP binding table.
DHCP MAC Source Address Validation Verifies a DHCP packet’s source hardware address matches
the client hardware address field (CHADDR) in the payload.
IP+MAC Source Address Validation Verifies that the IP source address and MAC source address
are a legitimate pair.
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.
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Dynamic Host Configuration Protocol (DHCP)