C
ONFIGURING
DHCP S
NOOPING
7-9
• When DHCP snooping is enabled, DHCP messages entering an
untrusted interface are filtered based upon dynamic entries learned via
DHCP snooping.
• Filtering rules are implemented as follows:
- If the DHCP snooping is disabled globally, all DHCP packets are
forwarded.
- If DHCP snooping is enabled globally, and also ena bled on the
VLAN where the DHCP packet is received, all DHCP packets are
forwarded for a trusted port. If the received packet is a DHCP ACK
message, a dynamic DHCP snooping entry is also added to the
binding table.
- If DHCP snooping is enabled globally, and also ena bled on the
VLAN where the DHCP packet is received, but the port is not trusted,
it is processed as follows:
* If the DHCP packet is a reply packet from a DHCP server
(including OFFER, ACK or NAK messages), the packet is
dropped.
* If the DHCP packet is from a client, such as a DECLINE or
RELEASE message, the switch forwards the packet only if the
corresponding entry is found in the binding table.
* If the DHCP packet is from client, such as a DISCOVER,
REQUEST, INFORM, DECLINE or RELEASE message, the
packet is forwarded if MAC address verification is disabled.
However, if MAC address verification is enabled, then 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 from a DHCP 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.