3-3Table 3-1 Roles of portsDevice Untrusted port Trusted port disabled fromrecording binding entriesTrusted port enabled torecord binding entriesSwitch A GigabitEthernet 1/0/1 GigabitEthernet 1/0/3 GigabitEthernet 1/0/2Switch BGigabitEthernet 1/0/3and GigabitEthernet1/0/4GigabitEthernet 1/0/1 GigabitEthernet 1/0/2Switch C GigabitEthernet 1/0/1 GigabitEthernet 1/0/3 andGigabitEthernet 1/0/4 GigabitEthernet 1/0/2DHCP Snooping Support for Option 82Option 82 records the location information of the DHCP client. The administrator can locate the DHCPclient to further implement security control and accounting. For more information, refer to Introduction toOption 82.If DHCP snooping supports Option 82, it will handle a client’s request according to the contents definedin Option 82, if any. The handling strategies are described in the table below.If a reply returned by the DHCP server contains Option 82, the DHCP snooping device will remove theOption 82 before forwarding the reply to the client. If the reply contains no Option 82, the DHCPsnooping device forwards it directly.If a client’s requestingmessage has…Handlingstrategy The DHCP snooping device will…Drop Drop the message.Keep Forward the message without changing Option 82.Option 82Replace Forward the message after replacing the original Option82 with the Option 82 padded in normal format.no Option 82 — Forward the message after adding the Option 82padded in normal format.DHCP Snooping Configuration Task ListComplete the following tasks to configure DHCP snooping:Task RemarksEnabling DHCPSnoopingRequiredBy default, DHCP snooping is disabled.Configuring DHCPSnoopingFunctions on anInterfaceRequiredSpecify an interface as trusted and configure DHCP snooping to supportOption 82.By default, an interface is untrusted and DHCP snooping does not supportOption 82.You need to specify the ports connected to the authorized DHCP servers astrusted to ensure that DHCP clients can obtain valid IP addresses. The trustedport and the port connected to the DHCP client must be in the same VLAN.
