Brocade Communications Systems Brocate Ethernet Access Switch 6910 User Manual
Page 267
Brocade 6910 Ethernet Access Switch Configuration Guide
217
53-1002581-01
DHCP Snooping
10
Command Usage
•
Network traffic may be disrupted when malicious DHCP messages are received from an
outside source. DHCP snooping is used to filter DHCP messages received on an unsecure
interface from outside the network or fire wall. When DHCP snooping is enabled globally by this
command, and enabled on a VLAN interface by the
command, DHCP
messages received on an untrusted interface (as specified by the
command) 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 trusted interfaces. Each entry includes a MAC address, IP
address, lease time, 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 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). 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 (using
the
command). 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.