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User’s Manual of SGSD-1022 / SGSD-1022P
SGSW-2840 / SGSW-2840P
487
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 4-150). 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.
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 4-149). 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.
Example
This
Example enables DHCP snooping globally for the switch.
Console(config)#ip dhcp snooping
Console(config)#
Related Commands
ip dhcp snooping vlan
ip dhcp snooping trust