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22-12
- If DHCP snooping is enabled globally, and also enabled 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 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 22-16). 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 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; all static bindings are retained in the
binding table but will have no effect until DHCP snooping is
re-enabled.
• 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 (ip dhcp snooping trust, page 22-17).
Note that the switch will not add a dynamic entry for itself to the