DHCP Snooping
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the packet will only be forwarded if the client’s hardware address stored in the
DHCP packet is the same as the source MAC address in the Ethernet header.
• If the DHCP packet is not a recognizable type, it is dropped.
• If a DHCP packet from a client passes the filtering criteria above, it will only be
forwarded to trusted ports in the same VLAN.
• If a DHCP packet is from server is received on a trusted port, it will be forwarded
to both trusted and untrusted ports in the same VLAN.
If the DHCP snooping is globally disabled, all dynamic bindings are removed from
the binding table.
Additional considerations when the switch itself is a DHCP client – The port(s)
through which the switch submits a client request to the DHCP server must be
configured as trusted. Note that the switch will not add a dynamic entry for itself to
the binding table when it receives an ACK message from a DHCP server. Also,
when the switch sends out DHCP client packets for itself, no filtering takes place.
However, when the switch receives any messages from a DHCP server, any packets
received from untrusted ports are dropped.
DHCP Snooping Configuration
Command Attributes
•DHCP Snooping Status – Enables or disables DHCP snooping globally.
•DHCP Snooping MAC-Address Verification – Enables or disables MAC address
verification. DHCP packets will be dropped if the source MAC address in the
Ethernet header of the packet is not same as the client’s hardware address in the
DHCP packet.
Web – Click DHCP Snooping, Configuration.
Figure 3-114 DHCP Snooping Configuration
CLI – This example first enables DHCP Snooping, and then enables DHCP
Snooping MAC-Address Verification.
Console(config)#ip dhcp snooping 4-231
Console(config)#ip dhcp snooping verify mac-address 4-235
Console(config)#