DHCP S
NOOPING
C
OMMANDS
23-19
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 snoopi ng is enabled globally, and also enabled 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 (as
specified by the ip dhcp snooping verify mac-address
command, page 23-21). 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 no t 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 config ured as trus ted (ip dhcp snooping trust, page 23-24).
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