SMC Networks SMC8150L2 manual 232

4 Command Line Interface

firewall. When DHCP snooping is enabled globally by this command, and enabled on a VLAN interface by the ip dhcp snooping vlan command (page 4-233),DHCP messages received on an untrusted interface (as specified by the no ip dhcp snooping trust command, page 4-234)from a device not listed in the DHCP snooping table will be dropped.

•When enabled, DHCP messages entering an untrusted interface are filtered based upon dynamic entries learned via DHCP snooping.

•Table entries are only learned for untrusted interfaces. Each entry includes a MAC address, IP address, lease time, entry type (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-addresscommand, page 4-235). 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 4-234). Note that the switch will not add a dynamic entry for itself to the binding table when it

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