How Dhcp Snooping Works - Dell PowerConnect B-FCXs Configuration Manual

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DHCP snooping

How DHCP snooping works

When enabled on a VLAN, DHCP snooping stands between untrusted ports (those connected to
host ports) and trusted ports (those connected to DHCP servers). A VLAN with DHCP snooping
enabled forwards DHCP request packets from clients and discards DHCP server reply packets on
untrusted ports, and it forwards DHCP server reply packets on trusted ports to DHCP clients, as
shown in the following figures
FIGURE 176
FIGURE 177
DHCP
Client
DHCP binding database
When it forwards DHCP server reply packets on trusted ports, the Dell PowerConnect device saves
the client IP-to-MAC address binding information in the DHCP binding database. This is how the
DHCP snooping binding table is populated. The information saved includes MAC address, IP
address, lease time, VLAN number, and port number.
In the Brocade device, the DHCP binding database is integrated with the enhanced ARP table,
which is used by Dynamic ARP Inspection. For more information, refer to
page 1346.
The lease time will be refreshed when the client renews its IP address with the DHCP server;
otherwise the Dell PowerConnect device removes the entry when the lease time expires.
1350
DHCP snooping at work - on an untrusted port
DHCP client
request packet
DHCP server
reply packet
DHCP snooping at work - on a trusted port
DHCP
Snooping
Switch
DHCP
Snooping
Switch
PowerConnect B-Series FCX Configuration Guide
DHCP
Server
DHCP server
reply packet
DHCP
Server
"ARP entries" on
53-1002266-01