DHCP S
NOOPING
C
OMMANDS
4-271
(Dynamic-DHCP-Binding, Static-DHCP-Binding), VLAN identifier,
and port identifier.
• When DHCP snooping is enabled, the rate limit for the number of
DHCP messages that can be processed by the switch is 100 packets
per second. Any DHCP packets in excess of this limit are dropped.
• Filtering rules are implemented as follows:
- If the global DHCP snooping is disabled, all DHCP packets are
forwarded.
- 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 4-274). 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.
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